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Changes in characteristics of soils irrigated with processing wastewater from three New Zealand dairy factories.

Introduction

The disposal of wastewater by irrigation onto land has become the preferred method for the treatment of liquid wastes in New Zealand (Selvarajah 1996; Cameron et al. 1997; Liu and Haynes 2011a). Depending on the quality of wastewater, as well as the soil characteristics and crop type, irrigation can have benefits including increased plant and animal productivity on the irrigated area, particularly when the available soil moisture or nutrients may be limiting (Ross et al. 1982; Cameron et al. 1997; Hawke and Summers 2006; Liu and Haynes 2011a). For effective, long-term treatment of irrigated wastewaters and effluents, it is essential that the soils can accommodate the liquid volumes applied, and that the necessary breakdown or storage of waste products can occur without adverse effects on soil physical condition, accumulation of toxic materials, and health risks from pathogens (Balks et al. 1997; Cameron et al. 1997; Speir 2002; Hawke and Summers 2006; Liu and Haynes 2011a). Other methods of liquid-waste treatments are available, for example full treatment followed by controlled discharge, but because of lower cost and cultural acceptability, and the availability of suitable rural sites, application to land remains the preferred method of treatment in New Zealand.

The dairy industry is the largest contributor to New Zealand exports, earning $10.4 billion in 2009-10, comprising some 20% of the value of total goods exported and 2.8% of the total GDP (Schilling et al. 2010; Ministry of Primary Industries 2013). The bulk of dairy products are manufactured at large factories in regional centres near the sources of supply. The dairy industry in New Zealand is still expanding (Ministry of Primary Industries 2013), and it will be essential that soils irrigated with wastewater arc able to accept ongoing loadings in both volumes and composition.

Wastewaters from the New Zealand dairy industry comprise two types: (I)those generated on-farm, typically dilute, aqueous slurry of faecal material and urine from dairy shed washings, plus tank washings (Barkle et al. 2000; Hawke and Summers 2006); and (2) the wastewaters from milk processing at the large regional factories (Selvarajah 1996; Liu and Haynes 2011a). The latter is the topic of this paper. Wastewaters from the factories comprise plant wash-down water, and wastewater from dairy manufacturing processes. The wastewater typically contains lactose, protein and milk fat, is high in available carbon (C), nitrogen (N) and phosphorus (P), and can contain high levels of sodium ([Na.sup.+]) and chloride ([Cl.sup.-]) from cleaning chemicals (Degens et al. 2000; Sparling et al. 2001; Liu and Haynes 2011a, 2011 b). The wastewater is treated by irrigation onto pastures. However, components such as [Na.sup.+] in dairy-processing wastewater have the potential to deflocculate clays because of the high sodium adsorption ratio and to reduce infiltration (Lieffering and McLay 1996; Balks et al. 1997). Where required, lime or gypsum is applied to ameliorate this (Cameron et al. 2003). High levels of soluble N in wastewaters can be a concern for N leaching and water quality (Selvarajah 1996; Cameron et al. 1997; Speir 2002).

In New Zealand, irrigation of wastewaters to land is permitted after obtaining a consent administered by a Regional Authority (usually a Regional Council) under the auspices of the Resource Management Act 1991. The consent specifics conditions for the discharge, and the consent holder is required to monitor the amounts and composition of wastewater applied to the treatment area, as well as the soil and surface water characteristics, and to report these data to the Regional Authority. With the cooperation of Fonterra and Landcare Research, we present data and comment on soil condition from processing wastewater-treatment areas at the Fonterra dairy factory sites at Hautapu, Lichfield and Edgecumbe in North Island. Irrigation started at Hautapu in 1979, at Lichfield in 1995, and at Edgecumbe in 1987. Soil samples were collected at Hautapu and Lichfield during 1995-2005, and at Edgecumbe during 2000-05. To assess whether wastewater irrigation was affecting soil quality, at each site we measured chemical, biochemical and physical properties that have proved responsive to change under wastewater irrigation (Degens et al. 2000; Sparling et al. 2001; Spcir 2002). We compared the irrigated pasture with nearby matched, non-irrigated pasture under conventional dairy farming. Therefore, we had a series of measurements on the same sites through time and we were able to test for direction and rates of any changes. We report on differences and changes in these key soil properties to assess the sustainability of application to land as a method to treat dairy-factory processing wastewater.

Materials and methods

Soil quality data from a single soil sampling in February 1998 at the Hautapu and Lichfield factories have been previously reported (Degens et al. 2000; Sparling et al. 2001) along with short descriptions of sites, management details and sampling methods. A further dairy factory site with two additional soils is included here (Edgecumbe) and all sites included additional data from various sampling dates during 1995-2005 that allowed changes through time to be followed. In general, the sampling sites, soil collection and analysis methods were as previously reported, so only brief details are given here, with explanation of any differences, which were mainly in the sampling method at the Edgecumbe site.

Site descriptions and irrigation management

With the exception of the Lichfield site, irrigation had been applied to the soils for some years before the start of soil monitoring. Consequently, we had no samples of the soils before commencement of irrigation. At each site, and for each of the soils used for irrigation, an experienced pedologist surveyed dairy farms within 1 km of the irrigated sites and selected a non-irrigated sampling area with the same Soil Orders, texture, horizon depth, aspect and flat gradient. These are not 'controls' but provided examples of soils not receiving dairy factory wastewater, and were farmed and managed in a conventional manner by the land owners. We were therefor able to compare the two management systems at each sampling date, and to follow independently the trends through time.

Hautapu

The Hautapu dairy factory (37[degrees]51'40.7"S, 175[degrees]27'27.6"E) is near the township of Cambridge and processes milk into cheese and casein products. Wastewater from the processing was irrigated onto rotationally grazed pastures at the adjacent Bardowie farm. Spray irrigation of pasture commenced in 1979, and at the final time of sampling (2005), pasture had been under irrigation for 26 years. There are two soils on the irrigated areas: Horotiu silt loam and Te Kowhai silt loam (Table 1). These are classified as a Typic Orthic Allophanic Soil and a Typic Orthic Gley Soil, respectively (Hewitt 2010), and as a Hapludand and Haplaquept under the USDA classification (Soil Survey Staff 2010). The two soils were managed separately. Matched, non-irrigated sites under pasture for dairy farming were on the Andersen farm, ~ 1 km from the irrigated sites. Soils were sampled in 1995, 1996, 1998, 2001, 2002 and 2005 (Table 2).The non-irrigated pasture sites were not available in 2005 because the landowner had ploughed the land for cropping, and no other suitable matched sites could be located in the vicinity.

Wastewater was spray-irrigated onto pasture at a rate of <6mm [h.sup.-1], up to a maximum application of 25mm [day.sup.-1], applied for up to 3 consecutive days, with an irrigation rotation period of at least 14 days.

Lichfield

The Lichfield dairy factory is near the town of Lichfield, (38[degrees]08'13.9"S, 175[degrees]35.5"En) and processes milk into various cheese and whey products. Spray irrigation of wastewater to pastures commenced in 1995 and the site had been irrigated only twice before soil samples were collected. The soil on the site is Taupo shallow fine sandy loam (Table 1), an Immature Orthic Pumice Soil (Hewitt 2010), USDA classification Typic Vitrandept (Soil Survey Staff 2010). Wastewater was spray-irrigated onto pastures at a rate not exceeding 25 mm [day.sup.-1], with up to 50 mm per irrigation cycle. Irrigation rotation cycles were usually ~20 days. A matched, non-irrigated pasture site was established ~500 m to the west in a buffer zone between the factory and wastewater irrigation site (38[degrees]08' 14.2"S, 175[degrees]49'00.7"E). Samples were collected in October or November in 1996, 1998, 2000, 2002, 2003 and 2005 (Table 2). Irrigation ceased on this site in 2005.

Edgecumbe

The Edgecumbe dairy factory is near the township of Edgecumbe (37[degrees]58'43.9 S, 176[degrees]48'51.3 E) and processes milk to make protein products, cream, whey and edible fats. Wastewater from the process is irrigated onto dairy pastures (mainly ryegrass (Lolium perenne) and white clover (Trifolium repens)) for treatment. There arc two dominant soils on the site used for irrigation: Awaiti sandy loam is a Buried pumice Tephric Recent Soil (Hewitt 2010), an Aquent in USDA classification (Soil Survey Staff 2010); and Paroa silt loam on peat, an Acidic Recent Gley Soil (Hewitt 2010), an Orthent under the USDA classification (Table 1). The Awaiti sandy loam is developed from thin Tarawera tephra on fine pumiceous alluvium, chiefly derived from Kaharoa Ash and is moderately well drained. The Paroa silt loam is developed from thin Tarawera tephra overlying peat and is a poorly to imperfectly drained soil. Irrigation at Edgecumbe commenced in 1987. Wastewater was applied to Awaiti soil by fixed spray irrigators, at a rate of 6 mm [h.sup.-1], with a maximum application of 50 mm per irrigation event. Rotation cycles were at least 12 days. Wastewater was applied to the Paroa pastures by travelling irrigator at a maximum rate of 10 mm [h.sup.-1], with a maximum application of 5 mm, and a minimum 10-day rotation period. Rotation cycles were an average of 20 days (Table 2).

Matched, non-irrigated examples of the Awaiti and Paroa soils under pasture on adjacent farms used for dairying were identified as comparison sites. The irrigated Awaiti soils were sampled at Omeheu on the Brophy farm (37[degrees]57, 42.4 S, 176[degrees]46'52.8 E) and the non-irrigated Awaiti soil from the Ruiter farm (37[degrees]41'39.9 S, 176[degrees]45'54.8 E).

The irrigated Paroa soils were sampled from the Awaroa farm (37[degrees]58'27.0 S, 176[degrees]50'15.3 E) and the non-irrigated Paroa soils from Askey's farm (37[degrees]58'14.1 S, 176[degrees]51'03.1 E).

Soil samples were collected in 2000, 2003 and 2005 (Table 2).

Effluent composition

Data on effluent volumes and composition were supplied by Fonterra. Analyses were those required by the Regional Council under the conditions of the consents, and were carried out by commercial laboratories using 'Standard Methods for the Examination of Water and Wastewater' (APHA 2006). The wastewater samples were taken between 1990 and 2005 and covered the period of soil monitoring 1995-2005. A further dataset covering wastewater composition in 2013-14 was supplied by Fonterra, showing how current concentrations and volumes differ because of improvements in processing methods and cleaning chemicals. No accompanying soils data were available for 2013-14 because this was outside the monitoring period. At Lichfield, irrigation to the original pasture site ceased in 2005, and the wastewater was treated on other farmland near the site.

Soil sampling

Hautapu

Eight plots each 5 m by 5 m were established for each soil x treatment, avoiding fence lines, ruts and roads. The plots were sampled at least 3 days after an irrigation cycle, usually in November. From each plot, 20 soil cores of 0-10cm depth were taken with a 2.5-cm-diameter grass-plot sampler. These 20 cores were bulked to provide eight replicate samples for biochemical and chemical analyses. For soil physical condition, a single large core (10 cm diam. by 7.5 cm depth) was obtained from each of the eight replicate plots by pressing a stainless-steel ring into the soil and cutting around and below the ring to excavate the intact core (KJute 1986). Samples from the non-irrigated plots were collected on the same day, using the same methods, plot layout and replication.

Lichfield

Soil sampling methods were the same as at Hautapu. Irrigation on these soils ceased in 2005. The final soil sampling in October 2005 was after a 6-month period without irrigation. Sampling commenced in 1996, some 2 months after the first irrigation, which provided a reasonable baseline sample. However, a matched, non-irrigated site under conventional management was also included for comparison.

Edgecumbe

The soil-sampling method differed from those used for the Lichfield and Hautapu sites. At Edgecumbe, five 50-m transects were established in five separate paddocks, and 25 soil cores to 10 cm depth and of 2.5 cm diameter were collected every 2 m along each transect, at least 3 days after the last irrigation event. These small core samples were bulked and used for chemical and biochemical analyses. From each transect, a single large intact core of 10 cm diameter and 7.5 cm depth was collected for soil physical analyses. The same design and sampling method was used on the non-irrigated plots.

Soil analyses

Soil chemical characteristics examined were soil pH, total C and total N. T he C : N ratio was calculated from the total N and total C contents. Soil biochemical characteristics examined were anaerobically mineralisable N (AMN), soil microbial biomass C (MBC), soil respiration and denitrifying enzyme activity (DEA). Soil physical characteristics examined were dry bulk density, particle density, total porosity, macroporosity, unsaturated hydraulic conductivity, readily available water, and total available water. All soil analyses methods have been described previously (Schipper and Sparling 2000; Sparling et al. 2001) and were consistent throughout the monitored period. Brief details are provided below.

Soil from the bulked small cores was sieved to <6 mm while field-moist before measurement for AMN, MBC and DEA. Field-moist soils used to measure MBC and respiration were pre-incubated at 25[degrees]C and adjusted to -5 kPa water tension for 7 days before measurement to allow effects of sampling and sieving to subside. Analyses commenced within 7 days of collection; in the intervening period, soils were stored at 5[degrees]C. Soil respiration after pre-incubation was determined over 7 days at 25[degrees]C. Soils (20 g) were sealed into gas-tight containers (1800 mL), and the accumulation of C[O.sub.2] in the headspace was measured by gas chromatography or infrared gas analyser (Sparling and Zhu 1993). Microbial biomass was measured using the fumigation-extraction method (Vance et al. 1987; Wu et al. 1990). AMN was determined using the method of Keeney (1982). Nitrification potential was determined using the DEA method of Schmidt and Belser (1982). DEA was determined using the protocol of Tiedje et al (1989). Subsamples of the moist soils were air-dried at 22-25[degrees]C, sieved to <2 mm, and ground for analyses of total C and N by LECO FP2000 analyzer (LECO Corp., St. Joseph, Ml, USA). Water content of moist and air-dry soils was determined after drying soil at 105[degrees]C overnight, and all results are presented on a gravimetric basis of oven-dry soil. Soil pH in water was determined with a glass electrode using a 2.5: 1 water: soil ratio (Blakemore et al. 1987).

Soil physical properties were determined by the methods of Gradwell and Birrell (1979) and Claydon (1989), with the exception of unsaturated hydraulic conductivity, which was measured at -40 mm tension by the method of Cook et al. (1993). Macroporosity was determined at -5kPa tension and readily available water at - 10kPa tension (Gradwell and Birrell 1979; Klute 1986).

Statistical analyses

For each soil, and for each soil characteristic, the data were examined for evidence of differences between irrigated and non-irrigated plots. Because of the lack of pre-irrigation data on the irrigated plots, we cannot compare changes from a common baseline. In addition, the irrigated and non-irrigated plots are on separate sites, so we do not have true site-to-site replication of treatment comparisons. This means that, where there are differences between the two treatments (irrigated and non-irrigated sites), we cannot attribute the differences solely to irrigation, because there may be other factors that differ between 'paired' sites. Although we made every effort to obtain comparable sites, this issue remains a common problem in interpretation of data from paired-site studies. We have therefore tested the evidence that (a) mean levels over sampling times differ significantly between treatments, and/or that (b) simple per-replicate regression slopes over sampling times differ significantly on average. These tests have been carried out using two-sample t-tests (or equivalent one-way ANOVA), augmented by 95% simultaneous confidence intervals to allow conservative testing for non-zero slopes per treatment (Genstat Edn 15, Genstat 2012). If there are no significant results, we have no evidence of an irrigation effect for that soil-variable instance. If at least one is significant, we have assessed the implication of that finding. We also carried out paired t-tests on the treatment means per soil to test for any overall responses to effluent treatment combining all five soils. These tests, with only 4 degrees of freedom for error, have low discriminating power. Results are similar to those derived by using alternative non-parametric tests.

Results

Wastewater composition

Typical data for the period of the soil sampling 1995-2005 and for 2013 arc shown in Table 3. The data are means and standard deviation (where replicates were available). There were considerable differences in the wastewater composition from the three factories, reflecting the different products being manufactured. There were also considerable decreases in composition in 2013 compared with sampling pre-1995, most notably the N and P concentrations and annual loading at the Hautapu factory site. As a percentage of the annual rainfall, the volumes of wastewater applied were 40-57% at Hautapu, 18-35% at Lichfield, and 1-20% at Edgecumbe.

At Hautapu, the Te Kowhai soil was irrigated with the same wastewaters as the Horotiu; hence, the wastewater characteristics applied to both soils were the same, but a smaller volume was applied to the poorer draining (Table 2) Te Kowhai soil. Much less wastewater was irrigated onto the poorly draining Paroa soil than any other soil (Table 2) because of the generally higher concentrations in the wastewater (Table 3); hence, smaller volumes needed to be applied less frequently.

Characteristics of irrigated and non-irrigated soils

Hautapu

The two irrigated soils at Hautapu showed similar trends; of the 14 characteristics measured, 10 were significantly different on the irrigated Horotiu soil and nine on the irrigated Te Kowhai soil (Table 4). Total C was not significantly different on the irrigated soil, but total N and soil pH were higher. The soil C: N ratio was particularly low on the Horotiu soil. The biochemical characteristics of MBC, soil respiration, DEA and AMN were all markedly greater on the irrigated treatment. Often, there was no significant change over the monitored period (Table 4), suggesting that a new plateau had been established since irrigation started.

Soil physical characteristics (Table 4) were more variable; macroporosity of the irrigated Horotiu soil was the same as the non-irrigated, but bulk density was lower on the irrigated site, and unsaturated conductivity, total porosity, readily available water and total available water were all higher in the irrigated soil. Again, many of these measurements showed no significant increase or decrease over the monitored period. The pattern was similar on the Te Kowhai soil, with the irrigated site having greater total porosity, more readily available water and more total available water. Bulk density, unsaturated conductivity and macroporosity were lower on the irrigated soil. Again, there were many instances where no change was detected over the monitored period in the irrigated or non-irrigated soils.

Lichfield

There were fewer significant differences between the irrigated and non-irrigated sites at Lichfield, with only six characteristics showing treatment differences (Table 5). The significant difference in total N at Lichfield was caused by the concentration in the irrigated soil actually being less than that measured in the control. The C : N ratio of the irrigated soil was greater than the non-irrigated. MBC was much higher under irrigation, but no differences in soil respiration or DEA between irrigated and non-irrigated soils were detected. AMN, however, was much higher under irrigation. Of the soil physical measures, only macroporosity and readily available water showed differences, both being less in the irrigated soil.

The majority of characteristics of both the irrigated and non-irrigated soils showed no significant change over the 9-year monitoring period (Table 5). Total N on the irrigated site increased significantly, but was still less than the non-irrigated soil. MBC increased on the irrigated plots and decreased on the non-irrigated soil. AMN increased on both the irrigated and control treatments.

Edgecumbe

On the irrigated Awaiti soil, only five of the monitored characteristics differed from those on the non-irrigated soil. These were total N, which was slightly higher on the irrigated treatment (Table 6), and four biochemical indices of MBC, soil respiration, DEA and AMN, which were all markedly higher on the irrigated soil. The soil physical measures were not different between the irrigated and non-irrigated treatments. Only soil respiration showed an increase over the monitored period, which occurred in both the irrigated and non-irrigated treatments.

On the Paroa soil, there were even fewer differences between the irrigated and non-irrigated treatments. Of the three characteristics that differed, total C on the irrigated soil was less than on the non-irrigated soil, MBC of the irrigated soil was double that of the non-irrigated soil, and total available water was slightly greater under irrigation (Table

6). There were few significant changes over time; of the characteristics that differed significantly in their mean values, only total C on the irrigated sites showed an increase during the monitored period but remained less than on the non-irrigated sites.

Comparison of irrigated and non-irrigated soils across all sites

The statistical approach adopted here allowed us to test for any overall effects of wastewater irrigation. We combined the data from all five soils from both treatments at the three sites to identify any common trends. Because discriminating power was low with only 4 degrees of freedom, the following interpretation uses a value of P < 0.10 to denote significance, rather than the more conventional P < 0.05.

Overall, there were no treatment differences in total C, total N or C: N ratio, but soil pH was higher on irrigated soils (Fig. 1). Irrigated soils had greater MBC, and higher soil respiration, AMN and DEA. There were no significant differences (P> 0.10) in bulk density, unsaturated conductivity and total porosity, but macroporosity was less in the irrigated soils (Fig. 1).

Discussion

The soils under irrigation with dairy-factory processing wastewater differed in some characteristics from the equivalent, non-irrigated soils. The differences were most numerous and marked in the two soils at the Hautapu site. It is notable that the Hautapu site had received wastewater irrigation for much longer (26 years) than had Edgecumbe (18 years) or Lichfield (10 years). After 18 years of irrigation, there were very few overall differences between the irrigated and non-irrigated Paroa soil at Edgecumbe. The higher concentration of wastewater applied to this soil allowed irrigation to be less frequent and of lower volume, meaning that the loadings and number of times the soil was irrigated were less than for the Awaiti soil at the same site, with a result that the volume loading was much less at Paroa (Table 3).

Soil chemical condition

With the exception of the Paroa soil, no differences in total C were detected between the two treatments, and the total C of the irrigated Paroa soil was actually lower than the non-irrigated. This may have been due to soil variability between the irrigated and non-irrigated sites. Spatial variability in total C measures has been noted at other New Zealand site comparisons (Schipper and Sparling 2000; Speir 2002; Giltrap and Hewitt 2004). Variability makes the detection of changes difficult, particularly against high background levels. Although not detected in our study, a lower total C in irrigated Horotiu topsoil was reported by Degens et al. (2000). However, when the full soil profile was sampled to 0.75 m, it was found that the depletion of total C in the 0-10 cm depth was compensated by increased total C at 0.25-0.50 m depth, and over the whole profile there was no significant difference in total C storage compared with non-irrigated soil. It is possible that under irrigation at Hautapu there has been some leaching of C down the profile because it is known that wastewater applied at this site the past had pH values up to 11.7 (Degens et al. 2000). This would be sufficient to solubilise and leach organic C down the profile (Lieffering and McLay 1996). Current and post-1995 wastewater loadings at Hautapu are much less alkaline owing to changes in the cleaning chemicals used (Sparling et al. 2001) and were usually acidic (Table 3). The much lighter effluent loadings applied to the Taupo soil at Lichfield, and the much shorter time since irrigation started, may have been insufficient to have caused detectable change in total C. Davis and Condron (2002) suggested that after a land-use change, it would take some 10 years before a change in total soil C would be detectable.

The fate of N in soils irrigated with dairy-factory wastewater is of concern because of the possible contribution to water pollution in New Zealand (Selvarajah et al. 1994; Selvarajah 1996; Liu and Haynes 2011a). Dairy-factory wastewater applied to land contains considerable amounts of N, and maximum N loadings are specified on the consents, this often being a limiting factor for the total volume of wastewater that can be applied to a defined area of land. Both irrigated soils at Hautapu had substantially more total N than the equivalent non-irrigated soil, but the irrigated soil at Lichfield had less than the non-irrigated. At Edgecumbe, the irrigated Awaiti soil had higher total N, but Paroa did not differ significantly. Presumably, these differences in soil N contents and storage reflect the different wastewater compositions and N loadings at the three sites, because all three of them were receiving N in some form.

The amount of total N that can be stored in soil is much influenced by the total organic C content, because most N stored in soil is covalently bound to C in organic matter. In a survey of New Zealand soils, Sparling and Schipper (2004) found that dairy pastures had the highest N status, with an average C: N ratio of 11.3. That ratio is similar to that of the soils sampled here, with the exception of the lower C : N ratio of the irrigated soils at Hautapu. Batjes (1996), in a global survey, reported that the organic matter C:N ratio rarely drops below 10. This characteristic has been used to predict when soils may be reaching their maximum capacity to store further N (Schipper et al. 2004). At Hautapu, the C ; N ratio of irrigated Horotiu soil was 8.3 [+ or -] 0.05 (mean [+ or -] standard error) and markedly lower than that of the matched, non-irrigated soil (11.1 [+ or -] 0.08).

This low C; N ratio suggests that the irrigated soils at Hautapu have reached their full capacity to store N in the organic forms, implying a greater risk of N leaching as nitrates or dissolved organic N (Schipper et al. 2004). This will require careful nutrient management in the future, as plant uptake of N and subsequent removal from the site by grazing animals or cut for silage becomes an important route for balancing inputs and outputs. It is worth noting, however, that most N leached from dairy pastures derives from the high concentrations of N in urine patches deposited by the grazing animals (Haynes and Williams 1993; Di and Cameron 2002; Cichota et al. 2013). Restricting the numbers of grazing animals or nil grazing have been suggested as possible mitigation strategies to reduce nitrate leaching where lower stock numbers would be matched by decreased N loading (de Klein and Ledgard 2001). The current N loadings (2013) at the Hautapu and Edgecumbe factory sites arc decreased compared with those measured during the earlier periods of soil monitoring (Table 3).

The non-irrigated sites will also have received N inputs in the form of fertiliser, dung and urine from grazing cattle, feed supplements, and dairy shed washings, as part of normal dairy-farm management, but in general, there was no increase in total N over the monitored periods.

The soil pH was usually higher on the irrigated soils, again with the exception of the two Edgecumbe soils, where irrigated and non-irrigated did not differ. This latter result was surprising because the wastewater irrigated at Edgecumbe was alkaline, at pH 8, whereas the soil pH was 6.0 at Awaiti and 5.9 at Paroa. The wastewater pH was 5.9 at Hautapu and 5.1 at Lichfield. Nonetheless, the pH of all of the irrigated soils was close to, or above, neutral. Although base saturation of the soils was not measured, we suspect that the effluent may be increasing the base saturation of the soils. Soils normally become more acidic over time as base cations ([Ca.sup.2+], [Mg.sup.2+], [K.sup.2+]) are leached from the soil and exchange sites become dominated by acidic hydrogen and aluminium ions. The wastewater is probably increasing base cations on exchange sites (and buffering pH) from the [Na.sup.+], [Mg.sup.2+] and [Ca.sup.2+] in the effluent, and N[H.sub.4.sup.+] from mineralised organic N. This would also explain why the sandy Edgecumbe soils are not showing a response, because cation exchange capacity is probably so low in these soils that changes in base saturation would have little effect on soil pH. Increases in soil pH under wastewater irrigation are common (Speir 2002), and Schipper et al. (1996) noted an increase in soil pH even when soil was irrigated with tertiary-treated domestic effluent of very low nutrient and ionic content.

Soil biochemical characteristics

Biochemical characteristics of soils are generally more sensitive indicators of shorter term changes in soils and can be useful early predictors for more slowly changing characteristics (Powlson et al. 1987; Insam and Domsch 1988; Insam and Haselwandter 1989; Sparling 1992). MBC, soil respiration, AMN and DEA all showed significant increases on most irrigated soils, and the increases were proportionally greater than total C and N. MBC was exceptionally high on all soils. The responses reflected the response of the soil microbial population to the longer term inputs of readily metabolised C and N to the irrigated soils. Degens et al. (2000) showed that there was greater MBC and a higher rate of soil respiration in soil irrigated with wastewater from the Hautapu factory. The microbial community was particularly adapted to metabolising the soluble C compounds, particularly lactose and glucose in the wastewater. If these sources of readily available nutrient were removed (such as by stopping irrigation), then the microbial community would be expected to return to normal soil levels within 2-3 years (Sparling et al. 2001). The high measures of AMN and DEA are consistent with the input of readily mineralised N to the irrigated soils and provided more sensitive indicators of N dynamics and status than total N.

Soil physical condition

In addition to nutrient loading, a further constraint for the treatment of wastewater on land is the capacity of the soil to accept the volumes being applied while still allowing infiltration and water storage and avoiding runoff. Some wastewaters can cause surface scaling and deflocculation, and hence decrease infiltration sufficiently for this to become a limiting factor for wastewater treatment (Lieffering and McLay 1996; Balks et al. 1997). Overall, the soil physical condition of the soils irrigated with dairy factory wastewater was similar to that of the equivalent, non-irrigated soils, and soil physical condition was not a limitation to the volumes being irrigated. At Hautapu, the irrigated soils had slightly more capacity to store water and they had slightly greater porosity. This may have been caused in part by a greater mass of invertebrate soil fauna. Degens et al. (2000) reported a greater biomass of earthworms in the irrigated Horotiu and Te Kowhai soils at the Hautapu site and a greater frequency of deep burrowing worms in the surface, which could have increased the macroporosity. However, in general, macroporosity was equally low for both irrigated and non-irrigated soils. A macroporosity of >10%v/v is considered beneficial for pasture growth (Drewry et al. 2001). The low macroporosity was probably caused by treading damage from the grazing cattle, resulting in soil compaction. Low macroporosity was also found on the non-irrigated pastures, so it appears there was no connection between low macroporosity and irrigation. In a survey of New Zealand soils and land uses, compaction of pastures under dairy farming was common over a wide range of soils (Sparling and Schipper 2004).

Is land treatment of dairy factory wastewater sustainable?

For the Lichfield and Edgecumbe factories, the chemical, biochemical and physical conditions of the irrigated soils were very similar to those of the equivalent, non-irrigated soils. The soil physical condition was adequate to receive the volumes being applied and there were only modest changes in the soil chemical condition, and pasture productivity remained satisfactory. The soil biochemical measures were elevated in the irrigated soils, but these characteristics arc readily reversible, and if irrigation ceased, they would be expected to return to levels similar to those in non-irrigated soils. With the exception of the biochemical measures, there were relatively few significant changes in other soil characteristics through time, and no indication of any longer term adverse changes. At the current (2013) volumes and compositions reported here, treatment of wastewater by application to land at these sites appears to be a sustainable process with the present management.

There is some concern regarding the Hautapu site, where the soil N status was much increased in the irrigated Horotiu and Te Kowhai soils, and the low C : N ratio suggests little further capacity to store N in the soil organic matter (Schipper et al. 2004). This N has presumably accumulated during the early stages of the 26 years of irrigation, particularly pre-1995 when loadings may have been greater than during the 1995-2005 monitoring period. During that monitoring period, the total N content of the Horotiu soil continued to increase, and the C : N ratio declined.

Changes in processing methods and cleaning chemicals have greatly reduced loadings at the Hautapu site; between 2003 and 2007, the annual N loading was 356 [+ or -] 20 kg N [ha.sup.-1][year.sup.1] (J. M. Russell, unpubl. data), and by 2013 had been reduced to 290 kg [ha.sup.-1] [year.sup.1]. This represents an 18% decrease in current N loading at Hautapu compared with that in 1990-2005. Fonterra is continuing to optimise the nutrient applications to land. Overall, irrigation of dairy factory wastewaters to land as a method for the treatment remains a preferred option over direct discharges to waterways. For irrigation of wastewater to land to be effective and sustainable, the matching of loadings to the ability of the soil to process or store nutrients from the wastewater remains an essential component of this treatment method. Monitoring of soil and waters provides useful information about effectiveness of the treatment and storage potential of the soils, and can identify potential future concerns.

http://dx.doi.org/10.1071/SR14365

Acknowledgements

We acknowledge the permission from Fonterra and its former companies, and Landcare Research, for access to a series of contracted research reports, which formed the basis for this paper. Graham Sparling is a Research Associate at the School of Science, University of Waikato.

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G. P. Sparling (A,F), R. Littler (B), L. A. Schipper (A), B. Stevenson (C), L. Sherman (D), and J. M. Russell (E)

(A) School of Science, University of Waikato, Hamilton, New Zealand.

(B) Waikato Applied Statistics Unit, University of Waikato, Hamilton, New Zealand.

(C) Landcare Research, PO Box 3127, Hamilton, New Zealand.

(D) Fonterra Limited, 520 Awakeri Road, Edgecumbe, New Zealand.

(E) Fonterra Limited, Private Bag 11029, Palmerston North, New Zealand.

Corresponding author. Email: sparling@waikato.ac.nz

Table 1. Characteristics 0-20 cm depth of the Horotiu and Te Kowhai
soils (Hautapu site), Taupo soil (Lichfield site), and Awaiti and
Paroa soils (Edgecumbc site) used for disposal of dairy factory
wastewater

Data adapted from S-map online (Landcare Research 2015); n.a., not
available

                               Horotiu               Te Kowhai

Soil texture                  Silt loam              Silt loam
New Zealand soil class       Typic Orthic          Typic Orthic
                           Allophanic Soil           Gley Soil
USDA Great Soil Group         Hapludand             Haplaquept
Sand (%)                          35                    12
Silt (%)                          49                    58
Clay (%)                          17                    30
Mineralogy               Amoiphic (Allophane)   Kandic (Halloysite)
Drainage class               Well drained         Poorly drained
Annual rainfall (mm)             1264                  1264
Mean monthly temp.                14                    14
  (C[degrees])

                              Taupo              Awaiti

Soil texture                  Sand             Sandy loam
New Zealand soil class   Immature Orthic      Buried pumice
                           Pumice Soil     Tephric Recent Soil
USDA Great Soil Group      Vitrandept            Aquent
Sand (%)                       46                  73
Silt (%)                       51                  20
Clay (%)                        3                   7
Mineralogy                   Glassy               n.a.
Drainage class            Well drained        Well drained
Annual rainfall (mm)          1560                1392
Mean monthly temp.             13                 14.5
  (C[degrees])

                             Paroa

Soil texture               Silt loam
New Zealand soil class   Acidic Recent
                           Gley Soil
USDA Great Soil Group       Orthent
Sand (%)                       38
Silt (%)                       42
Clay (%)                       20
Mineralogy                    n.a.
Drainage class           Poorly drained
Annual rainfall (mm)          1392
Mean monthly temp.            14.5
  (C[degrees])

Table 2. Irrigation regime for the treatment of processing wastewater
at the Hautapu, Lichfield and Edgecumbe dairy factories and years
when soil samples were collected

Site and produce     Soils       Irrigation period   Irrigation method

Hautapu (cheese      Horotiu     1979 to present     Spray from fixed
  and casein                                           irrigators
  products)          Te Kowhai   1979 to present     Spray from fixed
                                                       irrigators
Lichfield (various   Taupo       1995-2005 (A)       Spray from fixed
  cheeses)                                             irrigators
Edgecumbe (protein   Awaiti      1987 to present     Spray from fixed
  products, cream,                                     irrigators
  whey, edible
  fats)              Paroa       1987 to present     Spray from
                                                       travelling
                                                       irrigator

Site and produce     Soils       Irrigation management

Hautapu (cheese      Horotiu     Max. 25 mm [day.sup.-1] for 3 days.
  and casein                       Rotation cycle of at least 14 days
  products)          Te Kowhai   Max. 25 mm per [day.sup.-1] for 3
                                   days. Rotation cycle of at least
                                   14 days
Lichfield (various   Taupo       Max. 25 mm per [day.sup.-1] up to 50
  cheeses)                         mm per cycle, with 14-20-day
                                   rotation
Edgecumbe (protein   Awaiti      Max. 50 mm per irrigation event at a
  products, cream,                 rate not exceeding 6 mm [h.sup.-1],
  whey, edible                     followed by a rest period of at
  fats)                            least 12 days
                     Paroa       Max. 5 mm per irrigation event
                                   applied at a max. rate of 10 mm
                                   [h.sup.-1], followed by a rest
                                   period of at least 10 days

Site and produce     Soils       Soil samples collected

Hautapu (cheese      Horotiu     1995, 1996, 1998,
  and casein                       2001, 2002, 2005
  products)          Te Kowhai   1996, 1998, 2001,
                                   2002, 2005
Lichfield (various   Taupo       1996, 1998, 2000,
  cheeses)                         2002, 2003, 2005
                                   (Oct) (A)
Edgecumbe (protein   Awaiti      2000, 2003, 2005
  products, cream,   Paroa       2000, 2003, 2005
  whey, edible
  fats)

(A) Irrigation of wastewater to pasture at Lichfield ceased May 2005.

Table 3. Typical annual volumes and composition of processing
wastewaters (mean [+ or -] standard deviation where replicate
analyses were available) irrigated onto the soils at Hautapu,
Lichfield and Edgccumbe dairy factories between 1990 and 2005, and
the most recent annual data tor 2013-14

The Te Kowhai soil received the same wastewater composition as the
Horotiu soil. BOD, Biochemical oxygen demand; COD, chemical oxygen
demand, TKN, total Kjeldahl nitrogen; SAR, sodium absorption ratio;
n.d., not determined

Average wastewater                              Hautapu
characteristics
                                          Horotiu            Te Kowhai

                                     1990-2005        2013   1990-2005

Volume applied                          720           537       502
  (mm [year.sup.-1])
Volume ([m.sup.3] [ha.sup.-1])          7200          5370     5020
BOD (g [m.sup.-3])               1261 [+ or -] 181    1052
TKN (g [m.sup.-3])                74 [+ or -] 5.5      51
Nitrate-N (g [m.sup.-3])          3.2 [+ or -] 0.5    2.9
Total P (g [m.sup.-3])           1261 [+ or -] 181     54
pH                               5.87 [+ or -] 0.21   6.1
Na (g [m.sup.-3])                 426 [+ or -] 57     294
K (g [m.sup.-3])                  7.1 [+ or -] 0.7     50
Ca (g [m.sup.-3])                99.7 [+ or -] 18.7    31
Mg (g [m.sup.-3])                6.84 [+ or -] 6.9    6.4
SAR                              11.1 [+ or -] 5.0    13.1
Annual N load (kg [ha.sup.-1]           1120          290       760
  [year.sup.-1])
Total P (kg [ha.sup.-1])                560           293       380
COD load (kg [ha.sup.-1])              36300          5700    25 200

Average wastewater                      Lichfield
characteristics
                                          Taupo

                                     1998-2005       2013

Volume applied                    273 [+ or -] 51    542
  (mm [year.sup.-1])
Volume ([m.sup.3] [ha.sup.-1])         2470          5418
BOD (g [m.sup.-3])                     n.d.          1082
TKN (g [m.sup.-3])                93 [+ or -] 11      64
Nitrate-N (g [m.sup.-3])         3.6 [+ or -] 2.9    1.72
Total P (g [m.sup.-3])                 n.d.           91
pH                               5.1 [+ or -] 0.56   5.91
Na (g [m.sup.-3])                436 [+ or -] 140    366
K (g [m.sup.-3])                  136 [+ or -] 45     77
Ca (g [m.sup.-3])                 58 [+ or -] 21     122
Mg (g [m.sup.-3])                8.1 [+ or -] 4.3     7
SAR                              15.8 [+ or -] 4.3    12
Annual N load (kg [ha.sup.-1]     254 [+ or -] 54    361
  [year.sup.-1])
Total P (kg [ha.sup.-1])          86 [+ or -] 11     435
COD load (kg [ha.sup.-1])        2224 [+ or -] 285   5740

Average wastewater                           Edgecumbe
characteristics
                                     Awaiti             Paroa

                                 2004-05   2013    2004-05   2013

Volume applied                     205      278     15.7     16.9
  (mm [year.sup.-1])
Volume ([m.sup.3] [ha.sup.-1])    2048     2778      157      169
BOD (g [m.sup.-3])                5103     4054     19475    12033
TKN (g [m.sup.-3])                 230      142      286      212
Nitrate-N (g [m.sup.-3])           1.6      17      0.88      3.7
Total P (g [m.sup.-3])             164      98       416      518
pH                                  4        5       4.6      5.2
Na (g [m.sup.-3])                  226      252      772      653
K (g [m.sup.-3])                   282      153     1156      845
Ca (g [m.sup.-3])                  341      161      690      953
Mg (g [m.sup.-3])                  23       12       71       67
SAR                                3.2      5.2      7.5      5.2
Annual N load (kg [ha.sup.-1]      500      440      45       37
  [year.sup.-1])
Total P (kg [ha.sup.-1])           335      272      70       88
COD load (kg [ha.sup.-1])         22429    14900    4792     2852

Table 4. Mean values, significance and change over time of key soil
characteristics of Horotiu and Te Kowhai soils irrigated with
wastewaters from Hautapu dairy factory compared with matched
non-irrigatcd 'control' soils

Measurements made between 1995 and 2005. DEA, Denitrifying enzyme
activity; AMN, anaerobically mineralised nitrogen. * P<0.05; **
P<0.01; *** P<0.001; n.s., not significant; d.f., degrees of freedom;
s.e.m., standard error of the mean

                                                Comparison of means

Characteristic                    Treatment    Mean   s.e.m.      P

Horotiu soil

Total C (%w/w)                    Control      11.4     0.18
                                  Irrigated    11.2     0.21     0.71
Total N (%w/w)                    Control      1.02      0.2
                                  Irrigated    1.36     0.03    <0.000
C: N ratio                        Control      11.1     0.08
                                  Irrigated     8.3     0.05    <0.001
pH                                Control      5.95     0.05
                                  Irrigated    7.23     0.01    <0.000
Microbial C ([micro]g C           Control      1920       63
  [g.sup.-1] soil)                Irrigated    7707      302    <0.000
Soil respiration ([micro]g C      Control      2.05     0.05
  [g.sup.-1] [h.sup.-1])          Irrigated    4.07     0.27    <0.000
DEA (nmol N [g.sup.-1]            Control        62      3.5
  [h.sup.-1])                     Irrigated    1994      223    <0.000
AMN ([micro]g N [g.sup.-1])       Control       173      5.7
                                  Irrigated     491       21    <0.000
Bulk density (Mg [m.sup.-3])      Control     0.713    0.014
                                  Irrigated   0.541    0.008    <0.000
Unsaturated conductivity          Control      22.3      1.8
  (mm [h.sup.-1])                 Irrigated    37.9      3.2    0.002
Total porosity (%v/v)             Control      67.9     0.46
                                  Irrigated    77.2     0.31    <0.000
Macroporosity (%v/v)              Control      7.53     0.52
                                  Irrigated    7.83     0.33     0.63
Readily available water (%v/v)    Control       9.7     0.32
                                  Irrigated    11.9     0.45    0.002
Total available water (%v/v)      Control      32.7     0.41
                                  Irrigated    39.6      0.4    <0.000

Te Kowhai soil

Total C (%w/w)                    Control      6.24    0.085
                                  Irrigated    5.92     0.21    0.191
Total N (%w/w)                    Control     0.563   0.0085
                                  Irrigated   0.634    0.022    0.018
C: N ratio                        Control      11.1      0.1
                                  Irrigated     9.4     0.04    <0.001
pH                                Control      5.93    0.038
                                  Irrigated    7.02    0.014    <0.000
Microbial C ([micro]g C           Control      2041       73
  [g.sup.-1] soil)                Irrigated    3619      138    <0.000
Soil respiration ([micro]g C      Control      1.99     0.04
  [g.sup.-1] [h.sup.-1])          Irrigated    2.63     0.07    <0.000
DEA (nmol N [g.sup.-1]            Control      39.5        5
  [h.sup.-1])                     Irrigated     547      183    <0.000
AMN ([micro]g N [g.sup.-1])       Control       169      8.4
                                  Irrigated     220      5.9    <0.000
Bulk density (Mg [m.sup.-3])      Control     0.838    0.015
                                  Irrigated   0.822    0.012    0.421
Unsaturated conductivity          Control      25.3      2.8
  (mm [h.sup.-1])                 Irrigated    33.1      3.9    0.127
Total porosity (%v/v)             Control      63.2     0.62
                                  Irrigated    64.9      0.5    0.045
Macroporosity (%v/v)              Control      7.66     0.92
                                  Irrigated    5.12     0.58    0.039
Readily available water (%v/v)    Control       9.7     0.32
                                  Irrigated   1 1.9     0.45    0.002
Total available water (%v/v)      Control      31.5     0.62
                                  Irrigated    35.2     0.69    0.002

                                              Comparison of means

Characteristic                    Treatment   d.f.   Significance

Horotiu soil

Total C (%w/w)                    Control
                                  Irrigated    13        n.s.
Total N (%w/w)                    Control
                                  Irrigated    12        ***
C: N ratio                        Control
                                  Irrigated    11        ***
pH                                Control
                                  Irrigated     7        ***
Microbial C ([micro]g C           Control
  [g.sup.-1] soil)                Irrigated     7        ***
Soil respiration ([micro]g C      Control
  [g.sup.-1] [h.sup.-1])          Irrigated     7        ***
DEA (nmol N [g.sup.-1]            Control
  [h.sup.-1])                     Irrigated     7        ***
AMN ([micro]g N [g.sup.-1])       Control
                                  Irrigated     8        ***
Bulk density (Mg [m.sup.-3])      Control
                                  Irrigated    10        ***
Unsaturated conductivity          Control
  (mm [h.sup.-1])                 Irrigated    10        ***
Total porosity (%v/v)             Control
                                  Irrigated    12        ***
Macroporosity (%v/v)              Control
                                  Irrigated    11        n.s.
Readily available water (%v/v)    Control
                                  Irrigated    12         **
Total available water (%v/v)      Control
                                  Irrigated    13

Te Kowhai soil

Total C (%w/w)                    Control
                                  Irrigated     9        n.s.
Total N (%w/w)                    Control
                                  Irrigated     8         *
C: N ratio                        Control
                                  Irrigated    12        ***
pH                                Control
                                  Irrigated     8        ***
Microbial C ([micro]g C           Control
  [g.sup.-1] soil)                Irrigated    10        ***
Soil respiration ([micro]g C      Control
  [g.sup.-1] [h.sup.-1])          Irrigated    10        ***
DEA (nmol N [g.sup.-1]            Control
  [h.sup.-1])                     Irrigated     7        ***
AMN ([micro]g N [g.sup.-1])       Control
                                  Irrigated    12        ***
Bulk density (Mg [m.sup.-3])      Control
                                  Irrigated    13        n.s.
Unsaturated conductivity          Control
  (mm [h.sup.-1])                 Irrigated    12        n.s.
Total porosity (%v/v)             Control
                                  Irrigated    13         *
Macroporosity (%v/v)              Control
                                  Irrigated    11         *
Readily available water (%v/v)    Control
                                  Irrigated    12        ***
Total available water (%v/v)      Control
                                  Irrigated    13        ***

                                                  Change over time
                                                     (1995 2005)

Characteristic                    Treatment   Mean slope   Significance

Horotiu soil

Total C (%w/w)                    Control       -0.18         P<0.05
                                  Irrigated      0.129         n.s.
Total N (%w/w)                    Control        0.003         n.s.
                                  Irrigated      0.0295       P<0.05
C: N ratio                        Control       -0.001         n.s.
                                  Irrigated     -0.179        P<0.05
pH                                Control        0.0852       P<0.05
                                  Irrigated     -0.0358       P<0.05
Microbial C ([micro]g C           Control        6             n.s.
  [g.sup.-1] soil)                Irrigated    -18             n.s.
Soil respiration ([micro]g C      Control        0.055        P<0.05
  [g.sup.-1] [h.sup.-1])          Irrigated     -0.013         n.s.
DEA (nmol N [g.sup.-1]            Control       -5             n.s.
  [h.sup.-1])                     Irrigated    379            P<0.05
AMN ([micro]g N [g.sup.-1])       Control        1.8           n.s.
                                  Irrigated      4.6           n.s.
Bulk density (Mg [m.sup.-3])      Control       -0.0037        n.s.
                                  Irrigated     -0.0108       p<0.05
Unsaturated conductivity          Control        1.04          n.s.
  (mm [h.sup.-1])                 Irrigated      1.23          n.s.
Total porosity (%v/v)             Control        0.227        P<0.05
                                  Irrigated     -0.1           n.s.
Macroporosity (%v/v)              Control       -0.29         P<0.05
                                  Irrigated     -0.37         P<0.05
Readily available water (%v/v)    Control        0.304        P<0.05
                                  Irrigated     -0.017         n.s.
Total available water (%v/v)      Control        0.787        P<0.05
                                  Irrigated     -0.319        P<0.05

Te Kowhai soil

Total C (%w/w)                    Control       -0.112         n.s.
                                  Irrigated      0.106         n.s.
Total N (%w/w)                    Control        0.0046        n.s.
                                  Irrigated      0.0151        n.s.
C: N ratio                        Control       -0.107        P<0.05
                                  Irrigated     -0.048         n.s.
pH                                Control        0.129        P<0.05
                                  Irrigated     -0.0456       P<0.05
Microbial C ([micro]g C           Control     -110            P<0.05
  [g.sup.-1] soil)                Irrigated   -126            P<0.05
Soil respiration ([micro]g C      Control        0.111        P<0.05
  [g.sup.-1] [h.sup.-1])          Irrigated      0.02          n.s.
DEA (nmol N [g.sup.-1]            Control      -12             n.s.
  [h.sup.-1])                     Irrigated    219            P<0.05
AMN ([micro]g N [g.sup.-1])       Control        8.1          P<0.05
                                  Irrigated     -5.5          P<0.05
Bulk density (Mg [m.sup.-3])      Control        0.0047        n.s.
                                  Irrigated     -0.0156       P<0.05
Unsaturated conductivity          Control        3             n.s.
  (mm [h.sup.-1])                 Irrigated      9            P<0.05
Total porosity (%v/v)             Control       -0.1           n.s.
                                  Irrigated      0.64         P<0.05
Macroporosity (%v/v)              Control        1.48         P<0.05
                                  Irrigated      0.15          n.s.
Readily available water (%v/v)    Control       -0.15          n.s.
                                  Irrigated      0.382        P<0.05
Total available water (%v/v)      Control        0.8          P<0.05
                                  Irrigated      0.23          n.s.

                                               Change over time
                                                  (1995 2005)

Characteristic                    Treatment   Direction of change

Horotiu soil

Total C (%w/w)                    Control     Decrease
                                  Irrigated   None detected
Total N (%w/w)                    Control     None detected
                                  Irrigated   Increase
C: N ratio                        Control     None detected
                                  Irrigated   Decrease
pH                                Control     Increase
                                  Irrigated   Decrease
Microbial C ([micro]g C           Control     None detected
  [g.sup.-1] soil)                Irrigated   None detected
Soil respiration ([micro]g C      Control     Increase
  [g.sup.-1] [h.sup.-1])          Irrigated   None detected
DEA (nmol N [g.sup.-1]            Control     None detected
  [h.sup.-1])                     Irrigated   Increase
AMN ([micro]g N [g.sup.-1])       Control     None detected
                                  Irrigated   None detected
Bulk density (Mg [m.sup.-3])      Control     None detected
                                  Irrigated   Decrease
Unsaturated conductivity          Control     None detected
  (mm [h.sup.-1])                 Irrigated   None detected
Total porosity (%v/v)             Control     Increase
                                  Irrigated   None detected
Macroporosity (%v/v)              Control     Decrease
                                  Irrigated   Decrease
Readily available water (%v/v)    Control     Increase
                                  Irrigated   None detected
Total available water (%v/v)      Control     Increase
                                  Irrigated   Decrease

Te Kowhai soil

Total C (%w/w)                    Control     None detected
                                  Irrigated   None detected
Total N (%w/w)                    Control     None detected
                                  Irrigated   None detected
C: N ratio                        Control     Decrease
                                  Irrigated   None detected
pH                                Control     Increase
                                  Irrigated   Decrease
Microbial C ([micro]g C           Control     Decrease
  [g.sup.-1] soil)                Irrigated   Decrease
Soil respiration ([micro]g C      Control     Increase
  [g.sup.-1] [h.sup.-1])          Irrigated   None detected
DEA (nmol N [g.sup.-1]            Control     None detected
  [h.sup.-1])                     Irrigated   Increase
AMN ([micro]g N [g.sup.-1])       Control     Increase
                                  Irrigated   Decrease
Bulk density (Mg [m.sup.-3])      Control     None detected
                                  Irrigated   Decrease
Unsaturated conductivity          Control     None detected
  (mm [h.sup.-1])                 Irrigated   Increase
Total porosity (%v/v)             Control     None detected
                                  Irrigated   Increase
Macroporosity (%v/v)              Control     Decrease
                                  Irrigated   None detected
Readily available water (%v/v)    Control     None detected
                                  Irrigated
Total available water (%v/v)      Control     Increase
                                  Irrigated   None detected

Table 5. Mean values, significance and change over time of key soil
characteristics of Taupo Soil irrigated with wastewater from
Lichfield dairy factory compared with non-irrigated 'control' soil

Measurements made between 1996 and 2005. DEA, Denitrifying enzyme
activity; AMN, anaerobically mineralised nitrogen. *P<0.05; **
p<0.01, *** p<0.001; n.s., not significant; d.f., degrees of freedom;
s.e.m., standard error of the mean

                                                Comparison of means
Taupo soil
Characteristic                    Treatment    Mean   s.e.m.      P

Total C (%w/w)                    Control      9.04     0.15
                                  Irrigated    9.28     0.14     0.249
Total N (%w/w)                    Control     0.793    0.012
                                  Irrigated   0.753    0.011     0.033
C:N ratio                         Control      11.4     0.07
                                  Irrigated    12.4     0.07    <0.001
PH                                Control      5.53    0.027
                                  Irrigated    6.39    0.019    <0.000
Microbial C ([micro]g C           Control      1682       33
  [g.sup.-1] soil)                Irrigated    2103       57    <0.000
Soil respiration ([micro]g C      Control      2.98    0.054
  [g.sup.-1] [h.sup.-1])          Irrigated    3.19     0.13     0.159
DEA (nmol N [g.sup.-1]            Control      53.1       23
  [h.sup.-1])                     Irrigated    66.8      6.6     0.577
AMN ([micro]g N [g.sup.-1])       Control       169      3.2
                                  Irrigated     219      7.3    <0.000
Bulk density (Mg [m.sup.-3])      Control     0.671    0.008
                                  Irrigated   0.681    0.007     0.353
Unsaturated conductivity          Control      15.2      1.2
  (mm [h.sup.-1])                 Irrigated    12.3      1.1     0.101
Total porosity (%v/v)             Control      69.5     0.29
                                  Irrigated    69.3     0.32     0.712
Macroporosity (%v/v)              Control       7.8     0.37
                                  Irrigated     5.7     0.56     0.008
Readily available water (%v/v)    Control      11.5     0.19
                                  Irrigated    10.9     0.19     0.043
Total available water (%v/v)      Control      40.3      0.3
                                  Irrigated    41.4     0.55     0.096

                                              Comparison of means
Taupo soil
Characteristic                    Treatment   d.f.   Significance

Total C (%w/w)                    Control
                                  Irrigated    13        n.s.
Total N (%w/w)                    Control
                                  Irrigated    13         *
C:N ratio                         Control
                                  Irrigated    13        ***
PH                                Control
                                  Irrigated    12        ***
Microbial C ([micro]g C           Control
  [g.sup.-1] soil)                Irrigated    11        ***
Soil respiration ([micro]g C      Control
  [g.sup.-1] [h.sup.-1])          Irrigated     9        n.s.
DEA (nmol N [g.sup.-1]            Control
  [h.sup.-1])                     Irrigated     8        n.s.
AMN ([micro]g N [g.sup.-1])       Control
                                  Irrigated     9        ***
Bulk density (Mg [m.sup.-3])      Control
                                  Irrigated    13        n.s.
Unsaturated conductivity          Control
  (mm [h.sup.-1])                 Irrigated    13        n.s.
Total porosity (%v/v)             Control
                                  Irrigated    13        n.s.
Macroporosity (%v/v)              Control
                                  Irrigated    12         **
Readily available water (%v/v)    Control
                                  Irrigated    13         *
Total available water (%v/v)      Control
                                  Irrigated    10        n.s.

                                                  Change over time
                                                     (1996-2005)
Taupo soil
Characteristic                    Treatment   Mean slope   Significance

Total C (%w/w)                    Control      -0.041          n.s.
                                  Irrigated    -0.03           n.s.
Total N (%w/w)                    Control       0.0006         n.s.
                                  Irrigated     0.0086        P<0.05
C:N ratio                         Control      -0.02           n.s.
                                  Irrigated    -0.137         P<0.05
PH                                Control      -0.0317        P<0.05
                                  Irrigated     0.0161         n.s.
Microbial C ([micro]g C           Control     -35.7           P<0.05
  [g.sup.-1] soil)                Irrigated    30.1           P<0.05
Soil respiration ([micro]g C      Control       0.129         P<0.05
  [g.sup.-1] [h.sup.-1])          Irrigated     0.138         P<0.05
DEA (nmol N [g.sup.-1]            Control       4.7            n.s.
  [h.sup.-1])                     Irrigated    -0.4            n.s.
AMN ([micro]g N [g.sup.-1])       Control       6.2           P<0.05
                                  Irrigated     8.8           P<0.05
Bulk density (Mg [m.sup.-3])      Control       0.003          n.s.
                                  Irrigated     0.009         P<0.05
Unsaturated conductivity          Control       2.01          P<0.05
  (mm [h.sup.-1])                 Irrigated    -0.15           n.s.
Total porosity (%v/v)             Control      -0.061          n.s.
                                  Irrigated    -0.275         P<0.05
Macroporosity (%v/v)              Control      -0.516         P<0.05
                                  Irrigated    -0.54          P<0.05
Readily available water (%v/v)    Control       0.078          n.s.
                                  Irrigated     0.247         P<0.05
Total available water (%v/v)      Control       0.151          n.s.
                                  Irrigated    -0.173          n.s.

                                               Change over time
                                                  (1996-2005)
Taupo soil
Characteristic                    Treatment   Direction ol change

Total C (%w/w)                    Control     None detected
                                  Irrigated   None detected
Total N (%w/w)                    Control     None detected
                                  Irrigated   Increase
C:N ratio                         Control     None detected
                                  Irrigated   Decrease
PH                                Control     Decrease
                                  Irrigated   None detected
Microbial C ([micro]g C           Control     Decrease
  [g.sup.-1] soil)                Irrigated   Increase
Soil respiration ([micro]g C      Control     Increase
  [g.sup.-1] [h.sup.-1])          Irrigated   Increase
DEA (nmol N [g.sup.-1]            Control     None detected
  [h.sup.-1])                     Irrigated   None detected
AMN ([micro]g N [g.sup.-1])       Control     Increase
                                  Irrigated   Increase
Bulk density (Mg [m.sup.-3])      Control     None detected
                                  Irrigated   Increase
Unsaturated conductivity          Control     Increase
  (mm [h.sup.-1])                 Irrigated   None detected
Total porosity (%v/v)             Control     None detected
                                  Irrigated   Decrease
Macroporosity (%v/v)              Control     Decrease
                                  Irrigated   Decrease
Readily available water (%v/v)    Control     None detected
                                  Irrigated   Increase
Total available water (%v/v)      Control     None detected
                                  Irrigated   None detected

Table 6. Mean values, significance and change over time of key soil
characteristics of Awaiti and Paroa soils irrigated with wastewaters
from Edgecumbe dairy factory compared with matched non-irrigated
control soils

Measurements made between 2000 and 2005. DEA, Denitrifying enzyme
activity; AMN, anaerobically mineralised nitrogen. * P<0.05; **
P<0.01; *** P< 0.001; n.s, not significant; d.f., degrees of freedom;
s.e.m., standard error of the mean

                                                Comparison of means

Characteristic                   Treatment     Mean   s.e.m.      P

A waiti soil

Total C (%w/w)                   Control       6.56     0.25
                                 Irrigated     8.07     0.58     0.061
Total N (%w/w)                   Control      0.605     0.02
                                 Irrigated    0.824     0.06     0.019
C: N ratio                       Control       10.8     0.05
                                 Irrigated      9.8     0.05     0.008
pH                               Control       5.98     0.03
                                 Irrigated     6.04     0.02     0.16
Microbial C ([micro]g C          Control       1217       55
  [g.sup.-1] soil)               Irrigated     3539      308     0.002
Soil respiration ([micro]g C     Control        3.2     0.15
  [g.sup.-1] [h.sup.-1])         Irrigated      6.3     0.18    <0.000
DEA (nmol N [g.sup.-1]           Control       64.4       23
  [h.sup.-1])                    Irrigated     1362      262     0.008
AMN ([micro]g N [g.sup.-1])      Control        204       14
                                 Irrigated      412       22    <0.000
Bulk density (Mg [m.sup.-3])     Control      0.712    0.023
                                 Irrigated    0.711    0.018     0.215
Unsaturated conductivity         Control         42      8.1
 (mm [h.sup.-1])                 Irrigated       20      9.1     0.118
Total porosity (%v/v)            Control       67.1     0.88
                                 Irrigated     68.6     0.58     0.213
Macroporosity (%v/v)             Control        7.1      1.2
                                 Irrigated      4.8      1.1     0.213
Readily available water (%v/v)   Control       13.6     0.75
                                 Irrigated     12.5     0.35     0.25
Total available water (%v/v)     Control       39.2        1
                                 Irrigated     42.4      1.2     0.082

Paroa soil

Total C (%w/w)                   Control       8.82     0.26
                                 Irrigated      7.1      0.4     0.011
Total N (%w/w)                   Control       0.81    0.029
                                 Irrigated      0.7    0.046     0.079
C: N ratio                       Control       10.9     0.12
                                 Irrigated     10.3     0.03     0.008
PH                               Control       5.95    0.038
                                 Irrigated      5.9     0.04     0.409
Microbial C ([micro]g C          Control       1219       45
  [g.sup.-1] soil)               Irrigated     2265      196     0.007
Soil respiration ([micro]g C     Control       3.07     0.37
  [g.sup.-1] [h.sup.-1])         Irrigated     3.84     0.35     0.17
DEA (nmol N [g.sup.-1]           Control         59      8.9
  [h.sup.-1])                    Irrigated      344      142     0.115
AMN ([micro]g N [g.sup.-1])      Control        234      7.8
                                 Irrigated      318       32     0.061
Bulk density (Mg [m.sup.-3])     Control      0.808    0.016
                                 Irrigated    0.785    0.029     0.524
Unsaturated conductivity         Control       22.1        3
  (mm [h.sup.-1])                Irrigated     19.3      5.7     0.672
Total porosity (%v/v)            Control       64.9     0.47
                                 Irrigated     67.2      1.1     0.117
Macroporosity (%v/v)             Control        5.3      0.4
                                 Irrigated      4.7        1     0.64
Readily available water (%v/v)   Control        9.5     0.22
                                 Irrigated      9.3     0.19     0.59
Total available water (%v/v)     Control       33.6      1.4
                                 Irrigated     38.3      1.1     0.032

                                                Comparison
                                                 of means

Characteristic                   Treatment    d.f.   Signif.

A waiti soil

Total C (%w/w)                   Control
                                 Irrigated     5      n.s.
Total N (%w/w)                   Control
                                 Irrigated     5        *
C: N ratio                       Control
                                 Irrigated     7      n.s.
pH                               Control
                                 Irrigated     6      n.s.
Microbial C ([micro]g C          Control
  [g.sup.-1] soil)               Irrigated     4       **
Soil respiration ([micro]g C     Control
  [g.sup.-1] [h.sup.-1])         Irrigated     7       ***
DEA (nmol N [g.sup.-1]           Control
  [h.sup.-1])                    Irrigated     4       **
AMN ([micro]g N [g.sup.-1])      Control
                                 Irrigated     6       ***
Bulk density (Mg [m.sup.-3])     Control
                                 Irrigated     7      n.s.
Unsaturated conductivity         Control
 (mm [h.sup.-1])                 Irrigated     7      n.s.
Total porosity (%v/v)            Control
                                 Irrigated     6      n.s.
Macroporosity (%v/v)             Control
                                 Irrigated     7      n.s.
Readily available water (%v/v)   Control
                                 Irrigated     5      n.s.
Total available water (%v/v)     Control
                                 Irrigated     7      n.s.

Paroa soil

Total C (%w/w)                   Control
                                 Irrigated     6        *
Total N (%w/w)                   Control
                                 Irrigated     6      n.s.
C: N ratio                       Control
                                 Irrigated     7       **
PH                               Control
                                 Irrigated     7      n.s.
Microbial C ([micro]g C          Control
  [g.sup.-1] soil)               Irrigated     4       **
Soil respiration ([micro]g C     Control
  [g.sup.-1] [h.sup.-1])         Irrigated     7      n.s.
DEA (nmol N [g.sup.-1]           Control
  [h.sup.-1])                    Irrigated     4      n.s.
AMN ([micro]g N [g.sup.-1])      Control
                                 Irrigated     4      n.s.
Bulk density (Mg [m.sup.-3])     Control
                                 Irrigated     6      n.s.
Unsaturated conductivity         Control
  (mm [h.sup.-1])                Irrigated     6      n.s.
Total porosity (%v/v)            Control
                                 Irrigated     5      n.s.
Macroporosity (%v/v)             Control
                                 Irrigated     4      n.s.
Readily available water (%v/v)   Control
                                 Irrigated     7      n.s.
Total available water (%v/v)     Control
                                 Irrigated     7        *

                                                  Change over time
                                                      (2000-05)

Characteristic                   Treatment    Mean slope   Significance

A waiti soil

Total C (%w/w)                   Control         0.179        P<0.05
                                 Irrigated       0.111         n.s.
Total N (%w/w)                   Control         0.0169       P<0.05
                                 Irrigated       0.011         n.s.
C: N ratio                       Control        -0.188        P<0.05
                                 Irrigated      -0.325         N.S
pH                               Control         0.011         n.s.
                                 Irrigated      -0.0025        n.s.
Microbial C ([micro]g C          Control        55             n.s.
  [g.sup.-1] soil)               Irrigated    -211            P<0.05
Soil respiration ([micro]g C     Control         0.57         P<0.05
  [g.sup.-1] [h.sup.-1])         Irrigated       0.887        P<0.05
DEA (nmol N [g.sup.-1]           Control        31             n.s.
  [h.sup.-1])                    Irrigated     735            P<0.05
AMN ([micro]g N [g.sup.-1])      Control       -20.6          P<0.05
                                 Irrigated     -25.1          P<0.05
Bulk density (Mg [m.sup.-3])     Control        -0.0143        n.s.
                                 Irrigated       0.03         P<0.05
Unsaturated conductivity         Control        -6.1 1        P<0.05
 (mm [h.sup.-1])                 Irrigated      -0.22          n.s.
Total porosity (%v/v)            Control         0.6           n.s.
                                 Irrigated      -0.93          n.s.
Macroporosity (%v/v)             Control        -1.06         P<0.05
                                 Irrigated       0.05          n.s.
Readily available water (%v/v)   Control         0.99          n.s.
                                 Irrigated       0.19          n.s.
Total available water (%v/v)     Control         1.81         P<0.05
                                 Irrigated      -0.26          n.s.

Paroa soil

Total C (%w/w)                   Control         0.037         n.s.
                                 Irrigated       0.146        P<0.05
Total N (%w/w)                   Control         0.0039        n.s.
                                 Irrigated       0.0151       P<0.05
C: N ratio                       Control        -0.113        P<0.05
                                 Irrigated      -0.045         n.s.
PH                               Control        -0.053        P<0.05
                                 Irrigated      -0.013         n.s.
Microbial C ([micro]g C          Control        73             n.s.
  [g.sup.-1] soil)               Irrigated     182             n.s.
Soil respiration ([micro]g C     Control         0.42          n.s.
  [g.sup.-1] [h.sup.-1])         Irrigated       0.69          n.s.
DEA (nmol N [g.sup.-1]           Control        14             n.s.
  [h.sup.-1])                    Irrigated     187            P<0.05
AMN ([micro]g N [g.sup.-1])      Control        11.6           n.s.
                                 Irrigated       1.4           n.s.
Bulk density (Mg [m.sup.-3])     Control         0.013         n.s.
                                 Irrigated       0.0171       P<0.05
Unsaturated conductivity         Control        -3.7           n.s.
  (mm [h.sup.-1])                Irrigated       0.2           n.s.
Total porosity (%v/v)            Control        -0.55          n.s.
                                 Irrigated      -0.61          n.s.
Macroporosity (%v/v)             Control        -0.24          n.s.
                                 Irrigated      -0.36          n.s.
Readily available water (%v/v)   Control         0.49          n.s.
                                 Irrigated       0.88         P<0.05
Total available water (%v/v)     Control        -1.25          n.s.
                                 Irrigated      -0.32          n.s.

                                               Change over time
                                                   (2000-05)

Characteristic                   Treatment    Direction of change

A waiti soil

Total C (%w/w)                   Control      Increase
                                 Irrigated    None detected
Total N (%w/w)                   Control      Increase
                                 Irrigated    None detected
C: N ratio                       Control      Decrease
                                 Irrigated    None detected
pH                               Control      None detected
                                 Irrigated    None detected
Microbial C ([micro]g C          Control      None detected
  [g.sup.-1] soil)               Irrigated    Decrease
Soil respiration ([micro]g C     Control      Increase
  [g.sup.-1] [h.sup.-1])         Irrigated    Increase
DEA (nmol N [g.sup.-1]           Control      None detected
  [h.sup.-1])                    Irrigated    Increase
AMN ([micro]g N [g.sup.-1])      Control      Decrease
                                 Irrigated    Decrease
Bulk density (Mg [m.sup.-3])     Control      None detected
                                 Irrigated    Increase
Unsaturated conductivity         Control      Decrease
 (mm [h.sup.-1])                 Irrigated    None detected
Total porosity (%v/v)            Control      None detected
                                 Irrigated    None detected
Macroporosity (%v/v)             Control      Decrease
                                 Irrigated    None detected
Readily available water (%v/v)   Control      None detected
                                 Irrigated    None detected
Total available water (%v/v)     Control      Increase
                                 Irrigated    None detected

Paroa soil

Total C (%w/w)                   Control      None detected
                                 Irrigated    Increase
Total N (%w/w)                   Control      None detected
                                 Irrigated    Increase
C: N ratio                       Control      Decrease
                                 Irrigated    None detected
PH                               Control      Decrease
                                 Irrigated    None detected
Microbial C ([micro]g C          Control      None detected
  [g.sup.-1] soil)               Irrigated    None detected
Soil respiration ([micro]g C     Control      None detected
  [g.sup.-1] [h.sup.-1])         Irrigated    None detected
DEA (nmol N [g.sup.-1]           Control      None detected
  [h.sup.-1])                    Irrigated    Increase
AMN ([micro]g N [g.sup.-1])      Control      None detected
                                 Irrigated    None detected
Bulk density (Mg [m.sup.-3])     Control      None detected
                                 Irrigated    Increase
Unsaturated conductivity         Control      None detected
  (mm [h.sup.-1])                Irrigated    None detected
Total porosity (%v/v)            Control      None detected
                                 Irrigated    None detected
Macroporosity (%v/v)             Control      None detected
                                 Irrigated    None detected
Readily available water (%v/v)   Control      None detected
                                 Irrigated    Increase
Total available water (%v/v)     Control      None detected
                                 Irrigated    None detected
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Article Details
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Author:Sparling, G.P.; Littler, R.; Schipper, L.A.; Stevenson, B.; Sherman, L.; Russell, J.M.
Publication:Soil Research
Article Type:Report
Date:Jul 1, 2015
Words:12641
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