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The distribution of net nitrogen mineralisation within surface soil. 1. Field studies under a wheat crop.

Introduction

In Australian soils, nitrogen (N) supply to cereal crops depends on the release of N from the soil organic matter via mineralisation and supplementation from fertiliser. Nitrogen mineralisation is a biological process, the rate of which depends on the substrate concentration [organic carbon (C) and N] and environmental factors (soil moisture, aeration, temperature, pH, and availability of other nutrients). Both substrate concentration and environment vary with soil depth.

Studies in which soils were sampled to depths of [is greater than or equal to] 1 m have demonstrated a decrease in the concentration of total C and N with depth (Hadas et al. 1989; Weier and MacRae 1993; Ajwa et al. 1998). As a consequence, field work (Hadas et al. 1989) and laboratory work (Hadas et al. 1986; Weier and McCrae 1993) have confirmed that N mineralisation decreased through depths of [is greater than] 1 m.

However, field and laboratory studies of N mineralisation commonly use soil sampled from the surface 0-10 cm (Stanford and Smith 1972; Stein et al. 1987; Xu et al. 1996). There are few studies that investigate the distribution of soil properties within the 0-10 cm interval. In field studies of 2 soils under forest, Raison et al. (1987) found that 26% and 17% of the N mineralised in the depth of 0-15 cm was produced in the surface 2.5 cm. Laboratory research in the USA (Woods 1989) and south-eastern Australia (Young et al. 1995) demonstrated that total C and N concentrations decreased by a factor of 2 between depths of 0-2 and 2-4 cm under undisturbed pasture. This was reflected in the finding that 40-70% of net N mineralised in the surface 10 cm of soil originated in the surface 2 cm. In contrast, where soils were mixed by cultivation, both total C and N concentrations as well as N mineralisation were distributed uniformly within the surface 10 cm (Woods 1989). No work has been done in reduced tillage systems.

At Wagga Wagga, Storrier (1965a) reported that mean daily temperatures decreased between depths of 2.5 and 7.5 cm by l [degrees] C and 7 [degrees] C in winter and summer, respectively, in bare soil. Variations with depth were less under a wheat crop. In addition, diurnal variations in soil temperature of [is greater than] 20 [degrees] C occur at a depth of 1 cm but are [is less than] 5 [degrees] C at 10 cm (Alston and Fischer 1986). Soil pH has been found to decrease with depth within the surface 0-10 cm of soil subjected to reduced tillage. Groffman et al. (1987), Haynes and Knight (1989), and Chan et al. (1992) observed that in soils under crop, the pH at a depth of 0-5 cm was significantly higher than in soil from a depth of 5-20 cm.

No fieldwork has been done to identify the depth distribution of net N mineralisation in the surface 10 cm when reduced cultivation techniques are adopted. It is hypothesised that net N mineralisation will be concentrated in the surface few centimetres of soil as with pasture soils because of less mixing of soil during the tillage operations. We measured the distribution of net N mineralisation with depth through the surface 20 cm of soil during the growth of wheat on a Red Kandosol in south-western New South Wales, Australia. The crop was established using minimum tillage.

Materials and methods

Site

The experiment was conducted on the Charles Sturt University (CSU) farm at Wagga Wagga. The soil in this paddock was a Red Kandosol (Isbell 1996) or Dr2.12 (Northcote 1979). Since 1988, the rotation was 1 year peas-2 years wheat, with both crops being established using reduced tillage. The paddock was top-dressed with lime (3000 kg/ha) in 1991. The area was scarified once to a depth of 10 cm and sprayed with glyphosate before sowing to wheat on 6 June 1993. Nitrogen fertiliser was not applied within 10 m of the perimeter of the paddock. The central area of the field was used for experiments reported by Poss et al. (1995). They applied 0 and 100 kg/ha of diammonium phosphate at sowing.

Selected soil properties

Soil samples were collected from depths of 0-2, 24, 4-6, 6-8, 8-10, 10-15, and 15-20 cm from 10 sites within the area of the paddock used for the present field experiment. Soil from each layer was bulked, dried at 40 [degrees] C, mixed, ground to [is less than] 2 mm, and stored prior to analysis. A 2-g soil sample was placed in a centrifuge tube and shaken with 20 mL of 0.1 m Ba[Cl.sub.2]/0.1 M [NH.sub.4]Cl for 2 h (Gillman and Sumpter 1986). The tubes were centrifuged and the supernatant analysed for Ca, Mg, K, Na, Al, and Mn using atomic absorption spectroscopy to enable calculation of the exchangeable cation concentrations. Effective cation exchange capacity was calculated as the sum of exchangeable cations. For soil pH, 5 g of [is less than] 2 mm soil was shaken for 1 h with 25 mL 0.01 M Ca[Cl.sub.2] prior to determination with a combination glass electrode. Other analytical procedures used were: organic C concentration (Yeomans and Bremner 1988), total N concentration by a Kjeldahl

procedure (Bremner and Mulvaney 1982), and particle size analysis using a hydrometer method following destruction of organic matter with [H.sub.2][O.sub.2] (Piper 1942).

Experimental procedures

In the unfertilised perimeter area of the field used for the present experiment, 10 sites were selected for net N mineralisation measurements. Net N mineralisation was estimated using a modification of the method described by Raison et al. (1987). This method involved sampling every 3 weeks from 4 May to 17 November 1993. To measure water, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentrations at the start of each 3-weekly sampling interval ([T.sub.o]), 5 of the 10 sites were sampled to a depth of 20 cm using PVC tubes (i.d. 10 cm) driven into the ground using a hydraulic ram. The remaining 5 sites were only sampled to a depth of 10 cm. Therefore, there were 10 and 5 replicates for the 0-10 cm and 10-20 cm depths, respectively. Another set of 10 PVC tubes was inserted at the same time, capped, and left in position in the field for 3 weeks ([T.sub.f]) before being sampled.

The soil cores were excavated with a spade and put in a cold room (4 [degrees] C) within 3 h of sampling. The following day, the soil in the tubes was sampled. The soil was pushed out from the tubes using a hydraulic ram and cut into 2-cm intervals for the 0-10 cm depth and into 5-cm intervals for the 10-20 cm depth. For each depth interval, soil was mixed and subsamples were analysed for gravimetric water, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. The [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentrations were determined following extraction of approximately 40 g of moist soil in 200 mL of 2 m KCl for 1 h (Bremner 1965). The [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentration in the extract was measured colorimetrically on an autoanalyser (Crooke and Simpson 1971). The [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentration in the extract was measured after reducing the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] to [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] using a copperised cadmium column (Henriksen and Selmer-Olsen 1970).

Net N mineralisation for each sampling time and depth interval was calculated by subtracting the initial inorganic N concentration from that present after 3 weeks in the capped tubes. This calculation assumes that inorganic N, particularly that in the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] form, did not move between the layers and that denitrification was negligible. There were changes in the water content in the capped tubes, confirming water movement. The observed net N mineralisation in each layer was corrected for water movement using mass balance. For downward flow, that is, the water content decreased between [T.sub.o] and [T.sub.f]:

(1) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is corrected net N mineralisation (kg N/ha) in depth interval j; [N.sub.j] is uncorrected net N mineralisation (kg N/ha); [C.sub.j] is [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentration in solution (kg N/L) in the depth interval j; [Delta][V.sub.j] is the change in the concentration of water (L/ha) in layer j between the times TO and [T.sub.f]; and j, j-l, and j+ 1 are the depth intervals in which the correction was made, the depth above, and the one below, respectively. For upward water flow:

(2) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

Soil temperature at depths of 1 and 9 cm, both inside and outside the tubes, was monitored during the field sampling period. One criticism of the Raison et al. (1987) method is that temperature in the tubes is altered. No previous attempts have been reported to compare the soil temperatures inside and outside the tubes under crops. The soil temperature was monitored hourly using thermocouple wires connected to a data logger. The mean daily soil temperature throughout the growing season can be seen in Fig. 1. Rainfall data were obtained from a weather station located within 1 km of the field.

[Figure 1 ILLUSTRATION OMITTED]

Results and discussion

Soil properties

Organic C and N concentrations decreased with soil depth (Table 1). The higher concentration of organic C and N near the soil surface is attributed to the addition of plant residues to the soil surface and less mixing of soil during minimum tillage with typed implements. Blevins et al. (1977), Haynes and Knight (1989), and Wood et al. (1990) have reported similar trends.

Table 1. Chemical and physical properties of soil at the commencement of the field experiment

See Materials and methods for full details of procedures
Depth Exchangeable cations ECEC(A)
(cm) Ca Mg K Na Al Mn
 (cmol(+)/kg)

 0-2 5.9 0.9 1.1 0.03 0.02 0.36 8.3
 2-4 5.9 0.9 1.0 0.03 0.03 0.38 8.2
 4-6 4.5 0.7 0.8 0.03 0.05 0.36 6.4
 6-8 3.5 0.6 0.6 0.02 0.18 0.36 5.3
8-10 3.9 0.7 0.5 0.02 0.26 0.37 5.8
10-15 4.1 1.1 0.5 0.03 0.11 0.20 6.0
15-20 4.3 1.5 0.4 0.02 0.03 0.09 6.3

Depth Organic Total pH Particle size
(cm) C N Clay Silt
 (%) (%)

 0-2 2.13 0.13 5.69 39 15
 2-4 1.91 0.12 5.40 39 12
 4-6 1.86 0.12 5.00 40 15
 6-8 1.61 0.09 4.54 40 13
8-10 1.40 0.08 4.25 41 13
10-15 1.11 0.06 4.45 47 12
15-20 1.06 0.06 5.06 52 10


(A) Effective cation exchange capacity.

Stratification or rapid change in soil pH with depth was evident. The pH was 5.69, 4.25, and 5.06 in layers at depths of 0-2, 8-10, and 15-20 cm, respectively (Table 1). Over the period of crop growth there was no significant change in soil pH at any depth (Purnomo 1996). This pH stratification in cropped soils has been found in New Zealand (Haynes and Knight 1989) and Australia (Chan et al. 1992; Evans et al. 1998). In most of the work prior to this study, sampling depth intervals [is greater than or equal to]5 cm were used. The present study shows that stratification can occur in much smaller depth increments. The stratification of soil pH at this scale is commonly found in pastoral soils (Williams 1980; Conyers and Scott 1989; McLaughlin et al. 1990; Pumomo and Black 1994; Young et al. 1995).

Rainfall

The distribution of rainfall throughout the growing season with rainfall occurring during most of the sampling periods is shown in Fig. 1. The total rainfall during the experiment (May-Nov. 1993) was 441 mm. The 100-year average for the same period was 336 mm. The maximum rainfall during the experiment occurred in the sampling period of 12 July-2 August, which was 56 mm, and the minimum rainfall occurred at the beginning of the experiment (4-25 May).

Soil temperature

The diurnal patterns of soil temperature were similar both inside and outside the tubes at both depths (Fig. 2). The maximum temperature outside the tube at a depth of 1 cm was higher by 1-4 [degrees] C for a period of 1-2 h/day than inside the tube, whereas the minimum temperature outside the tube was slightly lower than that inside. Laboratory incubations for 4 weeks at a water content equivalent to field capacity using a range of constant temperatures showed that a constant increase in temperature of 4 [degrees] C elevated net N mineralisation by 16% (Purnomo 1996). The short time each day when the temperatures were different may influence mineralisation but it is suggested that the effect would be small. In the 8-10 cm soil layer, temperatures were the same inside and outside the tube for both periods. These data suggest that given the conditions during the growth of wheat, the enclosures are unlikely to cause significant errors in measurements of net mineralisation due to modification of soil temperature.

[Figure 2 ILLUSTRATION OMITTED]

Distribution of soil water, ammonium, and nitrate

The distribution of water, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is shown for 5 of the 8 sampling periods which represent the main period of plant N uptake: 12 July-2 August, 2-23 August, 23 August-13 September, 13 September-3 October, and 27 October-17 November (Fig. 3).

[Figure 3 ILLUSTRATION OMITTED]

Soil water

The average gravimetric water content for the soil at a depth of 0-20 cm at field capacity was 18%. The average change in the volumetric water content in soil to a depth of 20 cm inside the tubes for the sampling periods 4-25 May, 21 June-12 July, 12 July-2 August, 2-23 August, 23 August-13 September, 13 September-3 October, 3-27 October, and 27 October-17 November was 1.2, 2.1, 0.4, -2.2, 1.5, -1.6, -3.5, and -0.9% respectively. Thus for 7 of the 8 periods the change in water content was relatively small, being less than one-sixth of that at field capacity. For 4 sampling periods, there was a net increase in the water content between [T.sub.o] (at the beginning) and [T.sub.f] (at the end) inside the tubes. This was attributed to upward water movement in the soil inside the tubes from wet soil outside. For the remaining 4 sampling periods, there was a net decrease in the soil water content due to drainage between the soil depths during the period the tubes were in the ground. The effect was greatest from 3 to 27 October, as the tubes were inserted immediately after a period of rainfall. This redistribution of soil water inside the tube may influence the distribution of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and will be discussed below.

At [T.sub.j], the water content outside the tubes varied relative to that inside the tubes (Fig. 3a-d). The greatest differences in soil water content occurred in the 0-2 cm depth interval where the value changed by an average of 3% over the growing season but by as much as 6% for the sampling period of 13 September-3 October (Fig. 3d). At depths of 2-10 cm, water content changed by a seasonal average of [is less than] 1.5%. These differences reflect the changes in the water balance induced by the enclosed tubes at a single time, that is, only at the end of the 3-week sampling period. The short-term differences during the 3-week sampling period could not be estimated. The effect of these short-term variations in soil water on the reliability of the estimates of net mineralisation using the Raison et al. (1987) method is not known. However, effects are likely to be small as incubation of soil samples from a depth of 0-2 cm soil at 30 [degrees] C showed that net N mineralisation was reduced by [is less than] 10% for a 3% change in water content (Purnomo 1996).

Ammonium

Throughout the season, the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentration was always low ([is less than] 5 mg N/kg) (Fig. 3f-j). Because the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentration was lower than the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentration, this suggests that most of the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] produced by mineralisation was nitrified. There is no consistent pattern in the distribution of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] with depth for the 5 selected periods.

Nitrate

Accumulation of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] between [T.sub.o] and [T.sub.f] was measured in soil inside the tubes (Fig. 3k-o). The largest changes occurred in the 0-2 cm layer and decreased with depth.

Distribution of net nitrogen mineralisation

The use of the Raison et al. (1987) method to estimate net N mineralisation assumes that [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] does not enter or leave the tube with water. However, in the present experiment, there was some water movement inside the tubes during the 3-week periods. This was less than one-sixth of the water in the soil. Fig. 4 compares the estimate of net N mineralisation over the growing season when uncorrected or corrected for water movement using Eqns 1 and 2. Correction of mineralisation for [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] movement in this study did not change the distribution of net N mineralisation with depth.

[Figure 4 ILLUSTRATION OMITTED]

Over the whole growing season, stratification of net N mineralisation was observed at the experimental site (Fig. 4). On average, over all sampling periods, 32% of the net N mineralised in the 0-20 cm depth interval originated from the surface 0-2 cm, with 72% being from the 0-6 cm layer and only 13% being from below 10 cm. No previous studies have demonstrated this marked stratification of net N mineralisation in soil under crops. In the previous study of surface soils used for cropping in the USA, Woods (1989) did observe uniform distribution of mineralisation with depth. Recently, in a field study of sandy soil in Western Australia, Murphy et al. (1998) reported that approximately 50% of microbial biomass C and N in the surface 10 cm was located in the surface 2.5 cm. This distribution is similar to that found in net N mineralisation in the present study.

The distribution of net N mineralisation with depth to 20 cm is correlated with organic C ([r.sup.2] = 0.84, P [is less than] 0.05) and total N ([r.sup.2] = 0.83, P [is less than] 0.05) concentrations. The C : N ratio did not vary greatly with soil depth, ranging from 15.5 to 17.6 (Table 1). The distribution of C and N through the surface 20 cm is attributed to return of organic residues to the surface and the lack of mixing associated with minimal cultivation.

Rate of net nitrogen mineralisation through the season in soil from a depth of O-20 cm

The rate of net N mineralisation (RNM) in the soil from a depth of 0-20 cm, soil temperature, and soil water inside the tubes throughout the growing season are given in Fig. 5. The RNM ranged from 0 to 0.67 kg N/ha.day. With the exception of the first (before autumn rains) and the last sampling period, the soil water content was near field capacity (Fig. 5). Stein et al. (1987) had previously studied the soil N supply of wheat in relation to the method of cultivation in a similar climate to that experienced in the present study. In 1983, their soil water content was constantly high throughout the season. They found that the RNM ranged from about 0.25 to 0.70 for a conventionally tilled plot and from 0.30 to 0.65 for a direct-drilled plot. These rates are similar to those in the present study.

[Figure 5 ILLUSTRATION OMITTED]

The minimum RNM occurred during the period of 12 July-2 August, not in the sampling period of 21 June-12 July when average soil temperature was at its lowest. The daily soil temperature pattern showed that during the last 10 days of the sampling period of 21 June-12 July, the soil temperature increased substantially (Fig. 1). This temperature increase is believed to have increased the net N mineralisation despite the low average temperature for the sampling interval. This seasonal pattern of RNM was comparable with that reported by Stein et al. (1987) who observed that during 2 growing seasons, minimum RNM did not occur when the air temperature was at a minimum.

The maximum RNM was observed during the sampling periods of 2-23 August and 3-27 October when temperatures were rising and soil water was near field capacity.

N uptake by wheat

Generally, plant N uptake increased until the last sampling time when the crop had taken up 76 kg N/ha (Poss et al. 1995).

The N available for plant uptake was estimated as being equivalent to available N in the soil at sowing plus that released by mineralisation (0-20 cm) during the growing season (Fig. 6). The apparent recovery of N available for plant uptake ranged from 9% to 54% at the 3-leaf and flowering growth stages, respectively. The results near maturity are comparable to the results of Stein et al. (1987) who found that the recovery of fertiliser N ranged from 38% to 70%. The high apparent recovery at flowering is attributed to good growth conditions and negligible losses by leaching of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and biological denitrification. Storrier (1965b) and Poss et al. (1995) reported that most [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] which was leached from the surface remained within the root-zone of wheat and was available for uptake later in the season. At the site during the same season, Smith et al. (1998) found that only 4.2 kg N/ha was leached below a depth of 90 cm. On a similar soil east of Wagga Wagga, Smith and Bond (1999) found that denitrification only made a small contribution to N loss. They attributed this outcome to the free-draining nature of the soil.

[Figure 6 ILLUSTRATION OMITTED]

Conclusions

In this soil where the crop was established using techniques which result in minimal soil disturbance, most of the plant-available N was released by mineralisation from the surface few centimetres of soil. This result is consistent with earlier work in the region where the depth distribution of net mineralisation in soils under pasture was investigated (Young et al. 1995). This finding indicates that loss of the surface few centimetres of soil by wind or water erosion will decrease the supply of N to crops and pastures. These data contribute to an understanding of the observations of Aveyard (1983) who found that removal of the top few millimetres of soil at Wagga Wagga resulted in an average reduction in grain yield of 47%.

These surface layers of soil are exposed to extremes of drying and temperature, each of which reduces the rate of mineralisation in this soil (Paul et al. 1999). It seems plausible that the quantity of N mineralised under these conditions would be less than that where the source of the plant-available N was more evenly spread through the surface 10 cm of soil.

In situ incubation methods to estimate net N mineralisation in 0-10 cm of soil have been used over the past decade (Raison et al. 1987; Stein et al. 1987; Adams et al. 1989; Subler et al. 1995). Because this method uses a relatively short incubation period of about 3 weeks, the effects on environmental factors which may influence net mineralisation have been assumed to be small. The present paper has quantified the effects of soil temperature during the growth period of a wheat crop. There were decreases in temperature during the day and increases in temperature at night inside the tubes relative to outside. These effects were small, so it is suggested that errors due to temperature artefacts on mineralisation are small. Effects of the enclosures on soil moisture appear to induce small changes in mineralisation. This is the first time that this method has been used to estimate net mineralisation in small depth intervals under a crop. In this situation where the moisture conditions inside and outside the tubes varied, there was some redistribution of water. The extent of water movement was insufficient to influence the estimate of net N mineralisation in each depth interval.

Acknowledgments

We thank the Farrer Centre at Charles Sturt University for funding the research, and the Indonesian Government for providing a scholarship to E.P. through the SUDR-ADB project.

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Manuscript received 28 May 1999, accepted 4 October 1999

Erry Purnomo(AB), A. S. Black(AE), C. J. Smith(C), and M. K. Conyers(D)

(A) School of Agriculture, Charles Sturt University, PO Box 588, Wagga Wagga, NSW 2678, Australia.

(B) Present address: Division of Soil, Faculty of Agriculture, Lambung Mangkurat University, PO Box 1028, Banjarbaru 70714, South Kalimantan, Indonesia.

(C) CSIRO Land and Water, Canberra, ACT 2601, Australia.

(D) Agricultural Institute, PMB, Wagga Wagga, NSW 2650, Australia.

(E) Corresponding author; email: sblack@csu.edu.au
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Author:Purnomo, Erry; Black, A. S.; Smith, C. J.; Conyers, M. K.
Publication:Australian Journal of Soil Research
Article Type:Statistical Data Included
Date:Jan 1, 2000
Words:5364
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