Effects of tillage practices on soil and water phosphorus and nitrogen fractions in a Chromosol at Rutherglen in Victoria, Australia.
Excessive nutrient concentrations are a major problem for many of the streams draining agricultural catchments in Australia. In the south-eastern State of Victoria, the grains industry is one of many potential contributors to the excessive phosphorus (P) and nitrogen (N) concentrations in rivers and lakes (The State of Victoria 2002; Mathers et al. 2007). Water quality problems are often a consequence of land use changes and land-based nutrient management (The State of Victoria 2002), particularly in high rainfall zones (HRZ) where mean annual rainfall is >500 mm (Zhang et al. 2006b). Consequently, many authorities have introduced nutrient management strategies to decrease nutrient exports from agricultural land into waterways (Holmes 2002; Corangamite Catchment Management Authority 2003).
Annual cropping has been expanding in the HRZ of southern Australia (Zhang et al. 2006b) where grazing industries previously dominated (Poole et al. 2002). Nutrient exports from cropping areas of the HRZ can be greater than those from uncultivated areas, as fertilisers, sparingly used in extensive grazing industries, are used to enhance crop production. In the HRZ of south-eastern Australia 2.74 Mha is used for crop production (Zhang et al. 2006b). The potential for nutrient exports from cropping systems in the HRZ, where there is significant variation in environmental variables and management strategies, is likely to be a complex function of soil type, agronomic management practice (such as rates and timing of fertiliser use and tillage practice), and seasonal conditions.
In Australia, nutrient exports from cropping systems have not been investigated to the same extent as exports from other agricultural sectors, such as dairy systems (Nash and Halliwell 1999; Barlow et al. 2005; Dougherty et al. 2006), and as a result it is difficult to assess what impact these systems and the different agronomic management practices associated with them may have on nutrient exports. Conservation cropping systems that use direct drilling of the seed and fertiliser, together with stubble retention, have been widely promoted throughout the HRZ (Kirkegaard 1995). These systems are designed to mitigate soil erosion while sustaining soil fertility and crop productivity (Kirkegaard 1995; Newton 2001). However, management strategies aimed at preventing the export of soil-bound nutrients may not alleviate, and may even increase, dissolved nutrient exports, especially in surface runoff (i.e. overland flow) (Sharpley 1995; Schreiber 1999; Penn et al. 2006). Dissolved nutrients (<0.45 [micro]m) are often more environmentally damaging than particulate nutrients as they are likely to be transported further into the water system and tend to be more readily bioavailable (Peters 1981; McDowell and Koopmans 2006). In conservation cropping systems, dissolved nutrients can form a larger proportion of nutrient exports than in conventional systems with deep tillage (>250mm depth) and little residue retention (McDowell and McGregor 1984; Sharpley and Menzel 1987; Sharpley et al. 1992; Sharpley et al. 1993), as both organic and inorganic nutrients often accumulate at the soil surface (i.e. 0-20mm) from where dissolved nutrients are mobilised (Ahuja and Lehman 1983).
Stubble burning and cultivation are often regarded as a cause of soil organic matter decline and farmers are encouraged to retain stubble for soil organic matter improvement and erosion control (Chan et al. 1992). While direct drilling and stubble retention can remediate this decline in soil organic matter, they can also lead to the stratification of soil nutrients (i.e. greater nutrient concentrations in the surface soil, decreasing with depth), thereby increasing the risk of dissolved nutrient exports, particularly P exports, in surface runoff (Sharpley 2003; Dougherty et al. 2006). Residue retention can also assist in maintaining or improving soil fertility, structural stability, water infiltration characteristics, and water availability (Carter and Steed 1992; Heenan et al. 2004). In doing so, residue retention may also enhance nutrient exports through subsurface pathways (Buczko and Kuchenbuch 2007). Despite having low nutrient concentrations compared with that of plant biomass (Whitbread et al. 2000), stubble itself may be a source of nutrients exported from conservation cropping systems, especially where stubble is not incorporated into soil but left on the surface (Bunemann et al. 2006; Mathers et al. 2007). In these situations, nutrients released or leached from stubble may enter runoff directly without any interaction with the soil matrix (Mathers et al. 2007).
The first objective of this study was to compare the changes in surface soil chemical characteristics relevant to nutrient exports under 2 tillage and 2 stubble retention regimes. The second objective was to measure potential P and N concentrations and loads in surface runoff from cropped areas in the HRZ under different tillage and stubble management regimes using a rainfall simulator.
Materials and methods
This study was conducted on the long-term agro-ecological experimental site (SR1, stubble retention trial 1) at the Rutherglen Research Institute (36[degrees]11'S, 146[degrees]28'E) in northeastern Victoria. The climate is characterised by hot, dry summers (December-March) and cold, wet winters (June-September). Rutherglen is situated in the HRZ of south-eastern Australia and has a long-term mean annual rainfall of 589mm. However, in 2006 when this study was undertaken, a total of just 240mm rainfall was received. Monthly temperatures and rainfall for 2006 are shown in Fig. 1. The mean annual maximum temperature for Rutherglen is 220C and the mean annual minimum temperature is 7.4[degrees]C. Rutherglen is 175m above sea level and the soil at the trial site has been described as a Chromosol (Isbell 2002), Dy 3.33 (Northcote 1979) or an Ultic Haploxeralf (Soil Survey Staff 2006). Selected soil chemical properties before commencement of the trial in 1981 are presented in Table 1.
The SR1 trial began in 1981 with wheat crops grown until 1986. Since then, the plots have been continuously cropped on a 2-year, wheat-grain legume rotation split from 3 different tillage/stubble retention treatments: (i) conventional cultivation with stubble burning (CCb), (ii) direct drill with stubble burning (DDb), and (iii) direct drill with stubble retained (DDr). The CCb treatment comprised 2 passes with a 'Konskilder' triple-K tine cultivator with 9 tines per rack by 4 racks deep with a nominal spacing of 420 mm. It is of note that this is not a particularly disruptive cultivation process.
[FIGURE 1 OMITTED]
Treatments were laid out in a randomised complete block design with a plot size of 4.5 by 20 m with 4 replicates, but only 3 of those replicates were sampled for each treatment. In 2004 the trial was sown with wheat (Triticum aestivum cv. Dollarbird), followed by faba beans in 2005 (Vicia faba L.) and wheat again in 2006. All plots received the same quantity of fertiliser (20kg P/ha. year as single superphosphate) banded adjacent with the seed. This wheat-grain legume cropping system has the benefits that carryover N from the legume phase of the rotation supplies sufficient N for the subsequent cereal crop, there is a break in the disease cycle through growing different host species, and moisture is conserved (Reeves et al. 1984; Newton 2001). No N from external sources was applied to the trial plots.
Soil samples were first collected in February 2006 from the 0-20, 20-50, and 50-150 mm depths using a trailer-mounted, 44-mm-diameter, hydraulic percussion soil coring machine. In August 2006, a 40-mm-diameter hand-auger was used to sample the same soils. The 0-20 mm surface sampling depth, rather than 0-50 mm, was chosen for this study as nutrients mobilised in runoff typically originate from this zone (Ahuja and Lehman 1983). In February, around 10 cores were collected from 3 random points in each replicate plot and bulked, equalling 9 bulked samples (3 bulked samples from each of the 3 plots) in total for each treatment. In August, around 10-15 cores were collected from 4 points in each replicate plot and bulked, equalling 12 bulked samples (4 bulked samples from each of the 3 plots) in total for each treatment. Soil samples were stored for no more than 24 h at 4[degrees]C in polyurethane bags, before oven drying at 40[degrees]C. Subsamples of dried soils were sent to the Department of Primary Industries--Werribee Centre, where they were sieved to <2 mm prior to chemical analysis.
The rainfall simulator used in the study is well described elsewhere (Humphry et al. 2002; Sharpley and Kleinman 2003), and a low intensity nozzle 30 WSQ (Spraying Systems Inc., USA) positioned 3m above ground level was used to deliver rainfall at an intensity of 25 mm/h. This intensity of rainfall was chosen to simulate dissolution processes in gentle storms rather than erosion processes that would occur in a 1-in-50-year storm of 100 mm/h over a short period. During August 2006, 4 simulations were conducted in each replicate treatment plot, equalling 12 simulations for each treatment. At 2 locations within each treatment plot, duplicate simulation plots were established by inserting 3-sided steel frames into the ground (0.75 m wide by 2 m long and 40 mm above- and below-ground). Surface runoff was collected in steel gutters on the downslope side of the plots, and transported through polyurethane tubes to 5-L plastic collection bottles (Robertson and Nash 2008).
Bore water collected from Chiltern (36[degrees]10'S, 146[degrees]37'E) was used as rainfall. For each simulation, the rainfall simulator was centred over the plots and rainfall was applied until 30 min after surface runoff began. The simulation was ceased if surface runoff had not started after 240 min. Surface runoff was collected for 30 min, with the collection bottles being changed every 5 min. All bottles were weighed and the surface runoff from each plot was mixed in a 60-L container, from which a subsample was taken for further analyses. Samples of bore water used for the simulations were taken for analysis at the beginning of each simulation. Mean concentrations of total N and total P for the bore water were 0.61 and 0.08mg/kg, respectively. Mean electrical conductivity (EC) was 170 [micro]S/cm and pH was 7.77.
Soil moisture content was estimated by weighing samples before and after drying to a constant weight at 105[degrees]C. Soils were analysed for pH (in both water and Ca[Cl.sub.2]) and EC in a 1 : 5 soil to water extract. Total P (TP), Olsen P, total N (TN), ammonium-N (N[H.sub.4.sup.+]-N), nitrate-N (N[O.sub.3.sup.-]-N), and soil organic carbon (SOC) were measured using standard methods (Rayment and Higginson 1992). Ca[Cl.sub.2]-extractable P (Ca[Cl.sub.2]-P) was measured after a 1-h extraction using a 1 : 10 ratio of soil to 0.01 M Ca[Cl.sub.2] solution, followed by filtration through a Whatman No. 2 filter paper and analysed using the standard colourimetric method (Murphy and Riley 1962). Soil extractions with 0.01 M Ca[Cl.sub.2] are designed to reflect soil solution on the basis that cation exchange is minimised by the use of calcium and chloride at this concentration (Schofield 1955; McDowell and Sharpley 2001).
Soil water was extracted from the post-rain 0-20 mm depth soil samples by centrifugation of 200 g of moist soil at ~1500G for 15 min. (Toifl et al. 2003). Centrifugation was completed within 12 h of collection and all the extracted soil water was filtered immediately through 0.45 [micro]m membrane filters (Sartorius Minisart[R]). Concentrations of P and N were determined using a LaChat QuikChem flow injection analyser (LaChat Instruments, Milwaukee, WI). All soil water samples were analysed for total dissolved P (TDP, QuikChem method 10-115-01-1-E), dissolved reactive P (DRP, QuikChem method 10-115-01-1-A), total dissolved N (TDN, QuikChem method 10-107-04-1-A), N[H.sub.4.sup.+]-N (QuikChem method 10-107-06-l-A), and N[O.sub.3.sup.-]-N (QuikChem method 13-107-04-1-B).
Surface runoff was filtered through 0.45 [micro]m membrane filters immediately following collection. The filtered and unfiltered samples were stored at 4[degrees]C before being analysed using standard methods (Rayment and Higginson 1992). Concentrations of P and N were determined using the same methods as previously described for soil water. All samples (bore water, filtered and unfiltered surface runoff) were analysed for TP (QuikChem method 10-115-01-1-E) and TN (QuikChem method 10-107-04-1-A). The filtered surface runoff samples were also analysed for DRP, N[H.sub.4.sup.+]-N, and N[O.sub.3.sup.-]-N, including nitrite-N (N[O.sub.2.sup.-]-N). Dissolved reactive P was analysed within 24 h of collection and filtration. The unfiltered surface runoff samples were also analysed for pH and EC using standard methods (Rayment and Higginson 1992).
In surface runoff samples, TP was defined as the concentration of total P in the unfiltered samples, and TDP as the concentration of total P in the filtered samples. Particulate P (PP) was calculated as the difference between TP and TDP, and dissolved unreactive P (DUP) was calculated as the difference between TDP and DRP. Similarly, TN was the concentration of total N in the unfiltered samples, and TDN was the concentration of total N in the filtered samples. Particulate N (PN) was calculated as the difference between TN and TDN; dissolved inorganic N (DIN) was the sum of N[H.sub.4.sup.+]-N, N[O.sub.3.sup.-]-N, and N[O.sub.2.sup.-]-N; and dissolved organic N (DON) was calculated as the difference between TDN and DIN. Total solids (TS) were the concentration of solids in the unfiltered samples, and total dissolved solids (TDS) were the concentration of solids in the filtered samples. Total suspended solids (TSS) were calculated as the difference between TS and TDS. All sample analyte concentrations were corrected for background concentrations in the source bore water.
Treatment effects and depth effects on soil nutrients were assessed using the General Analysis of Variance (ANOVA) procedure of GENSTAT V 8 (GENSTAT Committee 2005) and means were compared using least significant difference. The blocking structure was that of a split-plot, with replicate split for plot split for core on each depth increment. There were 2 treatment structures as comparisons: conventional cultivation v. direct drilling (CCb v. DDb), and stubble burning v. retention of stubble (DDb v. DDr). Data from both sampling dates were analysed through a single model. Residual values were examined graphically to check distribution normality and constant variance, with only N[H.sub.4.sup.+]-N data being log-transformed before analysis to establish normal distribution. Unless otherwise specified, data are presented as means of the relevant variable combinations.
Treatment effects were tested by the ANOVA and Linear Regression procedures in GENSTAT (GENSTAT Release 9.1, Copyright 2006, Lawes Agricultural Trust). Data transformations were performed where necessary to conform to requirements of normality or homogeneity of variances. Individual data are included in the figures and tables to show the variation. There were 2 treatment structures as comparisons: conventional cultivation v. direct drilling (CCb v. DDb), and stubble burning v. retention of stubble (DDb v. DDr). Unless otherwise stated, significance was assessed at the 95% confidence level (P=0.05). Where no runoff was collected, 'missing values' were inserted in the treatment structure.
Soil moisture during summer (February; Table 2) was least in the surface 20 mm soil depth (0.2-0.5%) and greatest (2.2-3.1%) in the 50-150mm soil depth, most probably due to the low rainfall in January/February (Fig. 1) and to the surface soil drying out at the height of summer. During winter (August), when rainfall was greater (Fig. 1), soil moisture (Table 3) was similar at all depths. Soil moisture was greater under DDr than either of the burnt treatments at all depths in both February and August.
Soil pH in both Ca[Cl.sub.2] and water was greater in the topsoil (0-20mm) than lower in the profile (P<0.05), and the relationship with depth depended on both cultivation technique (i.e. direct drill or conventional cultivation) and stubble management (i.e. burning or retention). For example, overall mean soil pH in Ca[Cl.sub.2] (regardless of treatment or sampling date) decreased from 5.1 to 4.7 and 4.7 for the 0-20, 20-50, and 50-150 mm soil depths, respectively.
Changes in pH in Ca[Cl.sub.2] with depth were lower for CCb than direct drilling (P = 0.008). In the February sampling (Table 2) at the 0-20, 20-50, and 50-150 mm depths in the CCb treatment, pH was 5.0, 4.9, and 4.9. The equivalent data for the direct drilled plots were 5.2, 4.7, and 4.8. These data are consistent with the shallow cultivation having mixed surface and near-surface topsoil, as both tillage treatments were otherwise treated similarly, including being planted using the same machinery.
Stubble management in the direct drill treatments also affected soil pH (P=0.016). Again, in the February sampling (Table 2) when variation with depth was greatest, pH in Ca[Cl.sub.2] for the 0-20, 20-50, and 50-150mm depths was 5.4, 5.0, and 4.9 for the DDb, and 5.0, 4.5, and 4.7 for the DDr treatments, respectively.
Concentrations of SOC decreased with depth (Table 4) in all treatments (P < 0.001) and these concentrations varied with the time of sampling (P = 0.015). A plot of SOC for the 3 treatment combinations is presented in Fig. 2. At each depth, SOC was not significantly different between the 3 treatments when presented together (Table 4) or at each sampling date (Fig. 2).
The TN data were generally consistent with the SOC data (Table 4). Total N concentrations (averaged across treatments) decreased (P < 0.001) with depth from 1.12 g/kg at the surface to 0.87 and 0.49g/kg in the 20-50 and 50-150mm sampling depths, respectively. However, TN did not vary with sampling date or management practice. At the surface, TN in the DDr treatment was higher than in the CCb treatment (P<0.05). This perhaps explains the depth x treatment interaction (P = 0.11) and is consistent with increased retention of N in stubble and decreased soil mixing in the direct drilling treatments.
Nitrate-N decreased with depth (P=0.002), although the trend varied depending on sampling date (P < 0.001), cultivation (P=0.009), and stubble management (P=0.011) (Fig. 3). For example, in February the mean N[O.sub.3.sup.-]-N concentrations for the 0-20, 20-50, and 50-150 mm depths, regardless of treatment, were 12.9, 7.23, and 3.08mg/kg, respectively, while the equivalent data for August were 7.55, 6.34, and 7.35 mg/kg.
Ammonium-N also varied with depth (P<0.001) and sampling date (P=0.044). Irrespective of the stubble management regime there were differences in N[H.sub.4.sup.+]-N concentrations between the cultivated and direct drill treatments (P=0.036), and the depth x time x tillage practice interaction suggests that changes in N[H.sub.4.sup.+]-N concentration with depth between the 2 sampling dates depended on the tillage practice in place (P=0.05). For example, in the February sampling the N[H.sub.4.sup.+]-N concentrations in the 0-20, 20-50, and 50-150mm depths were 3.97, 2.02, and 0.98mg/kg, respectively, for CCb and 4.75, 2.04, and 1.06mg/kg for DDb (Fig. 4). The equivalent data for the August sampling were 14.0, 5.67, and 0.94mg/kg for CCb and 4.93, 2.04, and 2.60 mg/kg for DDb (Fig. 4). These results presumably reflect N mineralisation under the moister conditions in August.
Olsen P (P=0.003) and Ca[Cl.sub.2]-P (P= 0.003) concentrations all decreased with depth (Fig. 5, Table 4). Ca[Cl.sub.2]-P concentrations in the CCb treatment averaged across sampling dates were 0.50, 0.49, and 0.14 mg/kg for the 0-20, 20-50, and 50-150mm soil depths, respectively. In the direct drill treatments these values were 0.64, 0.42, and 0.18mg/kg for DDb and 0.76, 0.42, and 0.21 mg/kg for DDr. There was a marginal effect of cultivation on the decrease in Olsen P with depth (P = 0.053).
[FIGURE 2 OMITTED]
Total dissolved nutrient concentrations in soil water were 0.9-1.5 mg P/L (Fig. 6a) and 7.8-11.8 mg N/L (Fig. 6b). The DDr treatment had greater soil water nutrient concentrations than either CCb or DDb (Fig. 6), although this was not statistically significant (P>0.05). The inorganic P fraction (DRP) was the minor fraction of dissolved P in soil water for all 3 treatments, with DRP being 45, 47, and 40% of TDP for CCb, DDb, and DDr, respectively. The DIN fraction was the major fraction of dissolved nutrients in soil water for CCb (52% of TDN) and DDb (54% of TDN), but was the minor fraction for DDr (44% of TDN). The DIN fraction contained more N[O.sub.3.sup.-]-N (mean 82%) than N[H.sub.4.sup.+]-N (mean 18%) in all treatments.
No surface runoff was produced in >50% of the simulations on the direct drill treatments after 4 h of applied rainfall. Mean surface runoff volumes were significantly different (P<0.01) between treatments (Table 5), with CCb (2.10L)>DDb (0.57 L) > DDr (0.10 L). There were no significant differences between treatments in the concentrations of TS, TDS, and TSS in surface runoff. For TS the order of treatments was: DDr < CCb < DDb; for TDS it was DDr < DDb < CCb; and for TSS it was DDr < CCb < DDb (Table 5).
[FIGURE 3 OMITTED]
Nutrient concentrations in the surface runoff were not significantly different (P > 0.05). However, the large differences in runoff volumes will have commensurate effects on the total nutrients transported from these plots (i.e. loads) as presented and discussed below.
Mean TP concentrations in runoff were 0.10-0.2 mg P/L (Table 6), but there were no significant differences between treatments in either TP or concentrations of its constituent P fractions. The DDr treatment recorded the lowest values for all P fractions (Table 6). DRP was the dominant form of dissolved P in surface runoff from all treatments.
The runoff concentrations were multiplied by the volume of surface runoff collected over 30 min to calculate the mass of P fractions carried by surface runoff water (referred to as 'loads'). There were no significant differences between CCb and DDb in loads of any P fractions transported by runoff (Table 7). However, for all P fractions, DDr recorded significantly lower loads than CCb, but only in the case of TP and PP was DDr significantly greater than DDb (Table 7). Total P contained a greater proportion of PP in the DDr treatment than either the DDb or CCb treatments. Particulate P accounted for 75%, 67%, and 83% of TP for CCb, DDb, and DDr, respectively.
[FIGURE 4 OMITTED]
Mean concentrations of TN in surface runoff varied between 0.44 and 0.73 mg N/L (Table 8). There were no significant differences between treatments for any N fractions. Concentrations of N fractions were in the order DDr < DDb < CCb, with the exceptions N[H.sub.4.sup.+]-N where the order was DDb < DDr < CCb and PN where the order was DDr < CCb < DDb (Table 8). Nitrogen in surface runoff was mostly in the dissolved form and dominated by inorganic N rather than organic N.
The pattern of significant differences (DDr significantly lower than CCb and DDb) in TN loads between treatments (Table 9) was the same as that for TP loads (Table 7), suggesting similar detachment and entrainment mechanisms may have been operating. This pattern was the same for TDN, DIN, and N[O.sub.3.sup.-]-N, but for N[H.sub.4.sup.+]-N and DON there were no significant differences. For PN load, DDr results were significantly lower than CCb but neither of these was significantly different from DDb.
Many Australian soils have low organic C contents and under conventional cropping this is often decreased further (Dalai and Mayer 1986; Chan et al. 1992). Direct drilling and stubble retention can maintain a higher level of SOC than cultivation and stubble burning (Heenan et al. 2004). Many studies report that SOC concentrations can increase in surface soils with the retention of stubble, minimum or zero tillage and the application of fertilisers that stimulate biomass production, leading to the stratification of soil nutrients (Chan et al. 1992; Franzluebbers 2002; Zhang et al. 2007).
In this study all nutrient concentrations decreased with depth, reflecting the role of plants acting as biological pumps taking nutrients up into their roots and depositing them on the soil surface as detrital material. It may, therefore, have been expected that physical disturbance of the soil and decreased organic matter additions as a result of burning stubble would have resulted in less obvious SOC changes with depth in the CCb treatment. However, there were no significant differences in SOC concentrations between conventional cultivation and direct drill or between the stubble burnt and stubble retained treatments at any depth, even though the pH data are consistent with increased organic matter turnover, which would be expected to lower soil pH where stubble was retained with no addition of lime (Pocknee and Sumner 1997; Curtin et al. 1998). This may be a reflection of the general variability in SOC that can be observed in plot studies of this type, and the low level of soil disturbance created by this particular conventional cultivation system. These results may also reflect the relatively low rainfall during 2006 (Fig. 1). Lower soil moisture may have decreased microbial activity and the subsequent mineralisation of stubble residues, leading this treatment to show similar effects to those treatments where stubble was burnt. This can increase the between-plot variation and decrease the ability of the statistical tests to discriminate treatment effects. Such an explanation is consistent with the variation in SOC between sampling dates.
While most nutrient concentrations decreased with depth, the apparent lack of treatment effects in the SOC data contrasts with the data for most of the other nutrients. Stratification of soil TN and N[H.sub.4.sup.+]-N with depth occurred in all treatments, and direct drilling increased stratification in the top 50 mm soil depth for TN relative to conventional cultivation. Haines and Uren (1990) also reported stratification of TN with depth at the same experimental site in 1987, and similar stratification has been observed in other studies (Wright et al. 2007). Total N was significantly different between conventional cultivation and direct drilling treatments when stubble was burnt in February. These results differ from those of another study, also in northeastern Victoria (Carter and Mele 1992), where no significant tillage and stubble management effects on TN were reported. This may be a result of sampling for N at only one point in time, as our study suggests that tillage and stubble treatments can induce differences in N mineralisation at different times of the year. Interestingly, N[H.sub.4.sup.+]-N concentrations were higher in the surface soil of the conventionally cultivated treatment than the direct-drilled treatments and may indicate that stubble burning combined with cultivation has lead to a decrease in soil microbial biomass and hence a slower rate of mineralisation (Powlson et al. 1987; Franzluebbers et al. 1995). The slower rate of mineralisation before harvest can produce a greater accumulation of N[H.sub.4.sup.+]-N in the surface soils as less N[H.sub.4.sup.+]-N is converted into N[O.sub.3.sup.-]-N, leaving the remaining attached to soil clay particles. Higher ammonium concentrations have also been reported in soil when wheat stubble was burnt compared with not burning and this was attributed to higher clay contents and insufficient aeration of the soil (Coskan et al. 2007).
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Nitrate-N was highly stratified in all treatments during February (Fig. 3a) but was not stratified with depth during August (Fig. 3b). During February, N[O.sub.3.sup.-]-N was greater in the surface 0-20mm soil depth under the DDr and DDb treatments, being 45% and 18% greater, respectively, than the CCb treatment. Haines and Uren (1990) reported that mineral N was 62% greater under direct drill with stubble retained than under conventional cultivation with stubble burnt in the 0-25 mm depth in 1987. The increased percentage difference in that study could have been a result of adding N[H.sub.4.sup.+]-N and N[O.sub.3.sup.-]-N concentrations together to estimate mineral N. Nitrate-N also decreased in the 0-20 mm depth in August under all 3 treatments compared with February. Presumably these data reflect the leaching of N[O.sub.3.sup.-]-N in the limited rain that fell in late autumn and early winter (Fig. 1), but this distribution could also have been a result of denitrification from the moist surface soil layer (Dalal et al. 2003). Given that there was no similar trend in TN, this suggests that N[O.sub.3.sup.-]-N concentrations were not reflecting the incorporation of N into biomass.
Direct drilling increased stratification in the top 50 mm soil depth for TP and Ca[Cl.sub.2]-P compared with conventional cultivation. Ca[Cl.sub.2]-P extraction of surface soil is related to dissolved reactive P (DRP) in surface runoff (McDowell and Sharpley 2001), as its low ionic strength simulates the dissolution and desorption of P from soil particles that would be expected to occur in runoff water and leachates (Bloesch and Rayment 2006). Stratification of P in the top 100mm of soil increases the likely mobilisation of P in surface runoff compared with when P is evenly distributed throughout the topsoil (Sharpley 2003; Dougherty et al. 2006; Nash et al. 2007b). Sharpley (2003) has suggested that tillage will distribute and mix high-P surface soil with low-P subsoil. However, in an Australian long-term field trial, Bunemann et al. (2006) concluded that the effects of stubble management and tillage intensity on the dynamics of inorganic P in the top 200 mm soil depth were not significant, and while TP had increased over 24 years, there were no detectable differences between treatments in a single year. Our results indicate that stratification was occurring for TP and Ca[Cl.sub.2]-P, but not Olsen P, in the top 50 mm soil depth under conventional cultivation even though only 2 passes were made as part of the cultivation regime. In summary, our data suggest that most soil P, especially that most vulnerable to mobilisation, was located in the top 50 mm soil depth where it is potentially available for mobilisation into surface runoff. But does this mean nutrient exports are greater from conservation tillage systems?
Conservation tillage has been shown to improve soil structure and stability, therein decreasing the risk of runoff and pollution of surface waters with sediment and nutrients (Holland 2004; Peigne et al. 2007). While this is true for the majority of studies, some studies have shown runoff from zero tillage and reduced tillage systems similar to or greater than from conventional cropping systems (Richardson and King 1995; Kimmell et al. 2001; Peigne et al. 2007). These results may reflect differing soil properties as it would be expected that conservation tillage and residue retention practices would improve soil structural stability and soil porosity (Vogeler et al. 2006). In doing so, this would decrease slaking and increase water infiltration leading to decreased runoff volumes. While accumulation of nutrients in the surface of the conservation tillage treatments implies nutrient concentrations in surface runoff would be higher from these treatments, the overall effects on receiving waters may be lower due to there being less surface runoff leaving these plots.
Phosphorus and N exports in surface runoff from agricultural systems have been widely attributed to fertiliser application (Nash and Halliwell 1999; Zhang et al. 2006a; Tian et al. 2007) and tillage practices (Andraski et al. 1985b; Bundy et al. 2001; Zhang et al. 2007). However, minimum tillage does not always decrease nutrient exports in surface runoff (Bundy et al. 2001; Franklin et al. 2007). In the current study, fertiliser application rate was identical for all 3 tillage treatments, indicating that any differences observed in soil nutrient concentrations, runoff volumes, and nutrient loads would be directly related to the tillage practice and/or stubble management system.
Zhang et al. (2007) reported that under simulated rainfall (100 mm/h), both runoff and soil loss were significantly greater under conventional cultivation with burnt stubble than direct drilling with stubble retained. In our study, the time to runoff (data not shown) and the volume of surface runoff generated from the simulations were extremely variable. Mean runoff volumes (Table 6) were highest from the conventional cultivation with stubble burnt treatment (2.10 L), decreasing in the direct drill with stubble burnt treatment (0.57 L) and were least from the direct drill with stubble retained treatment (0.10 L). Nutrient concentrations were in the reverse order, although the effects of treatment were not significant, presumably as a result of the simulator not allowing dissolution processes to proceed to any great extent (Nash et al. 2002). It would be expected that more nutrients would be lost from the conventional cultivation with stubble burning treatments over time based on the number of times runoff was produced from these plots and the cumulative volume of surface runoff leaving the plots. Previous rainfall simulation studies have also reported that total runoff volumes were consistently lower for conservation tillage than for conventional cultivation (Andraski et al. 1985a; Zhang et al. 2007). This has been attributed to the structural degradation of tilled soil and greater macroporosity in soil from direct drill systems with stubble retained, which contributes to greater infiltration, increased wetting depths, and hence less runoff under conservation tillage systems (Carter and Steed 1992; Connolly et al. 1997; Zhang et al. 2007).
Twenty-five years after the start of these tillage treatments, soil nutrient concentrations in the 0-20mm soil depth were consistently greater in the DDr treatment, with the exception of N[H.sub.4.sup+]-N. There was little difference in SOC concentrations between stubble burning and stubble retention under direct drilling, indicating that the decrease in SOC under the CCb treatment was more affected by cultivation than stubble burning, which supports the results of previous studies (Carter and Steed 1992; Chan et al. 2002; Wright et al. 2007).
In a study of surface soil P tests that could be used to assess P loss from agricultural systems in runoff, organic P and the environmental soil P test, Ca[Cl.sub.2]-P, best explained the variations in soil TP (Nash et al. 2007a). Although there was no difference between treatments in soil TP concentrations in this study, Ca[Cl.sub.2]-P in the 0-20mm depth was significantly greater in the DDr treatment than both of the burnt treatments prior to rainfall simulation (Table 5). Ca[Cl.sub.2]-P has been reported to be directly related to DRP concentrations in surface runoff (McDowell and Sharpley 2001) as the low ionic strength of the Ca[Cl.sub.2] extraction simulates the dissolution and desorption of P from soil particles in surface runoff and leachates (Bloesch and Rayment 2006). The greater Ca[Cl.sub.2]-P concentrations in soil under DDr suggest that DRP concentrations in surface runoff would be greater from this treatment. However, soil structure, hydraulic conductivities, and infiltration rates would also influence the mobilisation of DRP into surface runoff.
McDowell et al. (2004) indicate that wetter soils will have a greater potential than dry soils for surface runoff and thus P movement. Even though the DDr soils were slightly wetter than the 2 burnt soils they had a greater percentage of PP in surface runoff TP than did the burnt treatments (Tables 6 and 7). This suggests that P attached to entrained particles is contributing to the PP fraction in surface runoff from the DDr treatment as there was less TSS mobilised into surface runoff from the DDr treatment than from either of the burnt treatments (Table 5). The concentrations of TP in surface runoff were relatively low compared with other studies using simulated rainfall on cropped soils under different tillage systems (Andraski et al. 1985b; Sharpley and Kleinman 2003; Zhang et al. 2006a). However, in all 3 treatments in this study, TP concentrations were still greater than the default trigger value of 0.05 mg P/L for physical and chemical stressors in lowland rivers for south-east Australia (ANZECC 2000).
Nitrogen can be exported in many forms including nitrate, nitrite, and ammonium, dissolved organic components, and colloidal material, with N[O.sub.3.sup.-]-N often being the most important species in the dissolved fraction (Mathers et al. 2007). In previous studies, minimum tillage has also increased soil TN relative to conventional tillage in the surface 0-50 mm soil depth (Franzluebbers 2002; Wright et al. 2007). In this study, soil TN increased significantly in the 0-20 mm soil depth with reduced tillage in the order DDr > DDb > CCb, and although mineral N (as N[O.sub.3.sup.-]-N) was always greater in soil under the DDr treatment it was not significantly different from the burnt treatments. Greater N[O.sub.3.sup.-]-N concentrations in the surface soil under DDr suggest that increased concentrations of soil N[O.sub.3.sup.-]-N could be mobilised into surface runoff from this treatment compared with the DDb and CCb treatments. The decrease in soil N[O.sub.3.sup.-]-N after rainfall simulation may reflect export in surface runoff or losses due to leaching or denitrification, particularly in wet, fine-textured soils (Peigne et al. 2007).
The concentrations of TN in surface runoff were quite low in all treatments compared with those reported by Tian et al. (2007) for wheat. However, the concentrations in the 2 burnt treatments were greater than the default trigger value of 0.5 mg N/L for lowland rivers for south-eastern Australia (ANZECC 2000). Burning of stubble may therefore contribute to increased exports of N, regardless of the tillage practice in place. The majority of N in surface runoff from all 3 treatments was in the dissolved form, mostly as N[O.sub.3.sup.-]-N. These concentrations were much less than those from both conventional and no-tillage systems in the Southern Plains region of the USA (Sharpley and Smith 1994). Although N[O.sub.3.sup.-]-N concentrations were greater in surface soil from the 2 direct drill treatments in this study, the load of N[O.sub.3.sup.-]-N being mobilised into surface runoff was much less from these treatments than that from the CCb treatment. Conventional cultivation also appears to have higher PN exports than reduced tillage systems, and on a silt loam soil of 5% slope under corn, the exports of PN were higher from conventional cultivation than from zero tillage systems (McDowell and McGregor 1984). However, as a proportion of TN, PN loads were greater for the DDr treatment than either of the burnt treatments. Like PP, this suggests that a higher proportion of N attached to particles is being mobilised into the surface runoff from the DDr plots. McDowell and McGregor (1984) reported that PN exports were greater than dissolved N exports in both conventional and reduced tillage systems. In this study, dissolved N concentrations and loads in surface runoff from all 3 treatments were greater than PN concentrations or loads.
Retention of stubble and direct drilling decreased the concentrations and loads of both TP and TN in surface runoff compared to the 2 burnt systems in the current study. However, retaining stubble in reduced tillage systems can decrease the export of biologically available nutrient forms such as DRP and N[O.sub.3.sup.-]-N, but may increase the proportion of particulate P and N. In terms of the availability of nutrients to aquatic organisms, DRP is generally considered fully available while only a portion of PP is available (Biggs and Close 1989; McDowell et al. 2004). Nitrate-N is also highly available to aquatic organisms and the combination of increased loads of both DRP and N[O.sub.3.sup.-]-N being exported from conventional tillage systems can have significant implications for catchment water quality.
Increased nutrient concentrations in the surface of undisturbed soils can potentially increase nutrient exports. In this study, it was expected that by facilitating mixing of surface soils, conventional cultivation would decrease nutrient stratification and this occurred for total P, total N, nitrate-N, Olsen P, and Ca[Cl.sub.2]-P in the 0-50 mm soil depth. The 0-20 mm soil depth is the layer from which most nutrients are mobilised. The large quantities of N[O.sub.3.sup.-]-N and Ca[Cl.sub.2]-P available in the surface 20mm of all treatments, but particularly the direct drill treatments, suggests that they could easily become mobilised into surface runoff if a heavy rain event were to occur. This is particularly important for Ca[Cl.sub.2]-P as it is directly related to dissolved reactive P, which is immediately bioavailable to aquatic biota; yet nutrient loads exported in surface runoff were less from the DDr treatment than from CCb or DDb systems. This would indicate that direct drill systems with the retention of stubble have increased soil infiltration properties and retained nutrients within the soil-plant system. Therefore, direct drilling with stubble retention in the HRZ cropping areas of north-eastern Victoria is more likely to retain nutrients on-site and improve soil fertility than burning stubble and cultivating the soil.
From the view of water quality, TP and TN concentrations in surface runoff from all 3 tillage treatments exceeded those recommended by Australian authorities for lowland rivers in south-east Australia, with the exception of TN from the DDr treatment. Even though the proportion of bioavailable DRP in surface runoff TP was less than PP, DRP concentrations were also greater than the default trigger values for P in the 2 burnt treatments. Nitrate-N concentrations were well below the default trigger value for N, even though TDN concentrations and loads in surface runoff were greater than those for PN. These results indicate that P, particularly the non-dominant but highly bioavailable form of DRP, exported from these systems is more likely to detrimentally affect catchment water quality than N exports. While concentrations appear to be low compared with other parts of the world, they are still well above the recommended guidelines for the lowlands of southeastern Australia. This study also suggests that leaching of N[O.sub.3.sup.-]-N from all treatments to the lower soil depths during winter may be important in the mobilisation of nutrients to groundwater.
It would be easy to conclude from this study that, because there are more nutrients in the surface soil under direct drill with stubble retention, they would be more readily mobilised from this treatment. Management alternatives to address nutrient stratification, for example intensive cultivation 1 year in every 10, may be useful to redistribute surface soil nutrients through the root-zone of cropping systems and decrease the dissolved and particulate nutrient concentrations in surface runoff. However, equally if not more important than the concentration of nutrients in runoff is the actual runoff volume. Given the improved soil physical properties that often result after changing from conventional cultivation to conservation tillage, the potential to retain nutrients within the system is increased and the actual off-site impact of the system is decreased if lower volumes of runoff are leaving the system.
The authors would like to acknowledge the Victorian Department of Primary Industries (DPI) and Grains Research and Development Corporation (GRDC) who together provided funds for this research. The authors would also like to thank Philip Newton, Ken Wilson, Miranda Green, Craig Butler, Mark Agnew, and Philomena Gangaiya for their assistance with data, field sampling and laboratory analysis. Murray Hannah is acknowledged for his help and guidance with statistical analyses and Dr Aldo Bagnara, Dr Roger Armstrong, Dr Kirsten Barlow, Dr Michael Crawford, and several anonymous reviewers are acknowledged for their comments and suggestions on earlier drafts of the manuscript.
Manuscript received 1 May 2008, accepted 2 December 2008
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Nicole J. Mathers (A,B) and David M. Nash (A)
(A) Future Farming Systems Research Division, Department of Primary Industries--Ellinbank Centre, 1301 Hazeldean Rd, Ellinbank, Vic. 3821, Australia.
(B) Corresponding author. Email: Nicole.Mathers@dpi.vic.gov.au
Table 1. Initial soil chemical properties (before cultivation) in the 0-150 mm depth of the long-term agro-ecological experimental site located at Rutherglen in north-eastern Victoria, Australia Chemical property 0-150mm pH(water) 5.5 pH(Ca[Cl.sub.2]) 4.8 EC (dS/m) 0.12 Exchang. cations (cmol(+)/kg): Ca 4.5 Mg 1.0 K 0.98 Na 0.12 Coarse sand (%) 13 Fine sand (%) 47 Silt (%) 15 Clay (%) 19 http://www.dpi.vic.gov.au/dpi/vro/neregn.nsf/pages/ ne_soil_detailed_rutherglen_index Table 2. February soil properties (meant [+ or -] s.e.) of the 3 treatments in the 0-20, 20-50, and 50-150 mm depths of the long-term agro-ecological experimental site located at Rutherglen in north-eastern Victoria, Australia (n=9) CCb, Conventional cultivation, stubble burnt; DDb, direct drill, stubble burnt; DDr, direct drill, stubble retained Treatment % Moisture pH(water) 0-20 mm CCb 0.40 [+ or -] 0.26 5.63 [+ or -] 0.06 DDb 0.20 [+ or -] 0.05 5.94 [+ or -] 0.06 DDr 0.46 [+ or -] 0.06 5.48 [+ or -] 0.04 20-50 mm CCb 0.65 [+ or -] 0.06 5.51 [+ or -] 0.08 DDb 0.70 [+ or -] 0.07 5.52 [+ or -] 0.06 DDr 1.17 [+ or -] 0.08 5.22 [+ or -] 0.07 50-150 mm CCb 2.24 [+ or -] 0.12 5.61 [+ or -] 0.11 DDb 2.22 [+ or -] 0.11 5.58 [+ or -] 0.11 DDr 3.05 [+ or -] 0.10 5.31 [+ or -] 0.08 EC (dS/ Treatment pH(Ca[Cl.sub.2]) [m.sup.3]) 0-20 mm CCb 4.99 [+ or -] 0.08 0.08 DDb 5.41 [+ or -] 0.12 0.08 DDr 4.91 [+ or -] 0.04 0.09 20-50 mm CCb 4.83 [+ or -] 0.07 0.05 DDb 4.91 [+ or -] 0.11 0.04 DDr 4.54 [+ or -] 0.06 0.02 50-150 mm CCb 4.84 [+ or -] 0.07 0.01 DDb 4.89 [+ or -] 0.12 0.01 DDr 4.59 [+ or -] 0.07 0.01 Table 3. August soil properties (mean [+ or -] s.e.) of the 3 treatments in the 0-20, 20-50, and 50-150 mm depths of the long-term agro-ecological experimental site located at Rutherglen in north-eastern Victoria, Australia (n=12) CCb, Conventional cultivation, stubble burnt; DDb, direct drill, stubble burnt; DDr, direct drill, stubble retained Treatment % Moisture pH(water) 0-20 mm CCb 15.2 [+ or -] 0.73 5.86 [+ or -] 0.06 DDb 14.4 [+ or -] 0.67 5.75 [+ or -] 0.07 DDr 18.7 [+ or -] 0.55 5.53 [+ or -] 0.05 20-50 mm CCb 15.6 [+ or -] 0.43 5.53 [+ or -] 0.07 DDb 14.2 [+ or -] 0.48 5.32 [+ or -] 0.07 DDr 15.9 [+ or -] 1.01 5.16 [+ or -] 0.06 50-150 mm CCb 14.4 [+ or -] 0.35 5.49 [+ or -] 0.06 DDb 13.5 [+ or -] 0.37 5.21 [+ or -] 0.06 DDr 16.9 [+ or -] 0.97 5.30 [+ or -] 0.09 EC (dS/ Treatment pH(Ca[C1.sub.2]) [m.sup.3]) 0-20 mm CCb 5.10 [+ or -] 0.04 0.06 DDb 5.02 [+ or -] 0.07 0.06 DDr 4.81 [+ or -] 0.05 0.07 20-50 mm CCb 4.79 [+ or -] 0.05 0.05 DDb 4.58 [+ or -] 0.06 0.05 DDr 4.48 [+ or -] 0.06 0.05 50-150 mm CCb 4.78 [+ or -] 0.04 0.04 DDb 4.53 [+ or -] 0.06 0.05 DDr 4.61 [+ or -] 0.09 0.06 Table 4. Mean soil nutrient concentrations in 2006 for the 3 treatments in the 0-20,20-50, and 50-150 mm depths of the long-term agro-ecological site located at Rutherglen in north-eastern Victoria, Australia (n=24) CCb, Conventional cultivation, stubble burnt; DDb, direct drill, stubble burnt; DDr, direct drill, stubble retained. Standard errors are in parentheses and values in columns followed by the same letter are not significantly different Total P Ca[Cl.sub.2]-P Treatment (mfg) (mg/kg) CCb 403 (7.90)a 0.50 (0.04)ac DDb 386 (6.92)ab 0.64 (0.06)ab DDr 392 (7.18)a 0.76 (0.06)b CCb 395 (8.53)a 0.49 (0.06)ac DDb 361 (7.83 b 0.42 (0.05)c DDr 360 (6.50)b 0.42 (0.05)c CCb 282 (6.19)c 0.14 (0.05)d DDb 279 (9.14)c 0.18 (0.04)d DDr 270 (6.04)c 0.21 (0.03)d Treatment Olsen P Total N (mg/kg) (g/kg) 0-20 mm CCb 21.4 (0.90)a 1.00 (0.02)ac DDb 20.5 (0.97)a 1.12 (0.04)ab DDr 22.8 (0.90)a 1.23 (0.02)b 20-50 mm CCb 22.5 (1.18)a 0.88 (0.02)c DDb 20.2 (1.08)a 0.87 (0.03)c DDr 23.2 (1.23)a 0.86 (0.02)c 50-150 mm CCb 12.9 (1.13)b 0.49 (0.04)d DDb 15.2 (1.32)b 0.47 (0.06)d DDr 15.4 (0.99)b 0.51 (0.03)d Treatment N[H.sub.4.sup.+] N[O.sub.3.sup.-] Soil organic C -N -N (g/kg) (mg/kg) CCb 9.71 (2.07)a 8.63 (0.79)a 15.4 (0.64)a DDb 4.85 (0.87)bc 9.49 (1.03)ab 16.2 (0.59)a DDr 6.46) (1.32)b 12.6 (1.93)b 15.7 (0.34)a CCb 4.11 (0.85)c 7.01 (0.58)ac 12.9 (0.05)b DDb 2.04 (0.18)d 6.25 (0.57)ac 13.0 (0.05)b DDr 2.83 (0.60)d 7.10 (1.12)ac 11.8 (0.04)b CCb 0.96 (0.07)e 4.50 (0.35)c 8.14 (0.38)c DDb 1.94 (0.99)e 5.83 (0.86)ac 8.10 (0.46)c DDr 1.39 (0.27)c 5.31 (1.23)c 7.58 (0.24)c Table 5. Physical properties in simulated runoff from the 3 treatments at the long-term agro-ecological experimental site located at Rutherglen in north-eastern Victoria, Australia in August 2006 CCb, Conventional cultivation, stubble burnt; DDb, direct drill, stubble burnt; DDr, direct drill, stubble retained. Standard errors are in parentheses and values in columns followed by the same letter are not significantly different Treatment Volume of Total solids Total Total runoff (L) (mg/L) dissolved suspended solids solids (mg/L) (mg/L) CCb 2.10 (0.66)a 245 (66)a 83 (22)a 162 (45)a DDb 0.57 (0.26)b 273 (128)a 52 (22)a 221 (108)a DDr 0.10(0.07)b 104(76)a 25 (17)a 79 (61)a Table 6. Phosphorus concentrations (mgtL) in simulated runoff from the 3 treatments at the long-term agro-ecological experimental site located at Rutherglen in north-eastern Victoria, Australia in August 2006 (n=12) CCb, Conventional cultivation, stubble burnt; DDb, direct drill, stubble burnt; DDr, direct drill, stubble retained. Standard errors are in parentheses and values in columns followed by the same letter are not significantly different Treatment Total P Total Dissolved dissolved P reactive P CCb 0.28 (0.09)a 0.07 (0.02)a 0.05 (0.02)a DDb 0.24 (0.09)a 0.08 (0.03)a 0.06 (0.03)a DDr 0.10 (0.07)a 0.03 (0.02)a 0.02 (0.01)a Treatment Dissolved Particulate P unreactive P CCb 0.013 (0.004)a 0.21 (0.08)a DDb 0.018 (0.007)a 0.17 (0.07)a DDr 0.006 (0.004)a 0.09 (0.06)a Table 7. Phosphorus loads (mg) in simulated runoff from the 3 treatments at the long-term agro-ecological experimental site located at Rutherglen in north-eastern Victoria, Australia in August 2006 (n=12) CCb, Conventional cultivation, stubble burnt; DDb, direct drill, stubble burnt; DDr, direct drill, stubble retained. Standard errors are in parentheses and values in columns followed by the same letter are not significantly different Treatment Total P Total Dissolved dissolved P reactive P CCb 1.09 (0.45)a 0.27 (0.11)a 0.210 (0.088)a DDb 0.30 (0.17)a 0.09 (0.04)ab 0.075 (0.037)ab DDr 0.06 (0.04)b 0.01 (0.01)b 0.010 (0.007)b Treatment Dissolved Particulate P unreactive P CCb 0.056 (0.021)a 0.82 (0.39)a DDb 0.020 (0.008)ab 0.20 (0.08)a DDr 0.003 (0.002)b 0.05 (0.04)b Table 8. Nitrogen concentrations (mgtL) in simulated runoff from the 3 treatments at the long-term agro-ecological experimental site located at Rutherglen in north-eastern Victoria, Australia in August 2006 (n=12) CCb, Conventional cultivation, stubble burnt; DDb, direct drill, stubble burnt; DDr, direct drill, stubble retained. Standard errors are in parentheses and values in columns followed by the same letter are not significantly different Treatment Total N Total Dissolved dissolved N inorganic N CCb 0.73 (0.22)a 0.54 (0.15)a 0.40 (0.13)a DDb 0.68 (0.25)a 0.42 (0.18)a 0.30 (0.12)a DDr 0.44 (0.30)a 0.39 (0.22)a 0.16 (0.09)a Treatment N[0.sub.3.sup.-]-N N[H.sub.4.sup.+]-N CCb 0.29 (0.08)a 0.11 (0.09)a DDb 0.29 (0.12)a 0.01 (0.01)a DDr 0.09 (0.05)a 0.07 (0.04)a Treatment Dissolved Particulate N organic N CCb 0.14 (0.10)a 0.19 (0.10)a DDb 0.12 (0.11)a 0.25 (0.11)a DDr 0.09 (0.07)a 0.05 (0.27)a Table 9. Nitrogen loads (mg) in simulated runoff from the 3 treatments at the long-term agro-ecological experimental site located at Rutherglen in north-eastern Victoria, Australia in August 2006 (n=12) CCb, Conventional cultivation, stubble burnt; DDb, direct drill, stubble burnt; DDr, direct drill, stubble retained. Standard errors are in parentheses and values in columns followed by the same letter are not significantly different Treatment Total N Total Dissolved dissolved N inorganic N CCb 2.77 (0.97)a 1.95 (0.62)a 1.56 (0.64)a DDb 0.94 (0.43)a 0.67 (0.36)a 0.44 (0.22)a DDr 0.26 (0.20)b 0.13 (0.10)b 0.07 (0.06)b Treatment N[0.sub.3.sup.-]-N N[0.sub.3.sup.-]-N CCb 1.01 (0.31)a 1.01 (0.31)a DDb 0.43 (0.22)a 0.43 (0.22)a DDr 0.05 (0.04)b 0.05 (0.04)b Treatment Dissolved Particulate N organic N CCb 0.38 (0.45)a 0.83 (0.49)a DDb 0.23 (0.15)a 0.27 (0.11)ab DDr 0.06 (0.04)a 0.13 (0.10)b
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|Author:||Mathers, Nicole J.; Nash, David M.|
|Publication:||Australian Journal of Soil Research|
|Date:||Feb 1, 2009|
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