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Sulfur mineralisation in a coarse-textured soil after different sulfate fertilisation histories, and yield responses of wheat and lupin.

Introduction

The use on crops and pastures of high analysis phosphorus and nitrogenous fertilisers in place of low analysis fertilisers containing sulfate (S[O.sub.4.sup.2-]) has substantially changed the amounts of sulfur (S) added to agricultural soils in southern Australia. Current S[O.sub.4.sup.2-] fertiliser inputs (1-10 kg S/ha), chiefly at seeding, are believed to have a neutral to a moderately positive effect on S balance (National Land and Water Resources Audit 2001). However, S deficiency is widespread in some regions of the Western Australian wheatbelt where sandy soils dominate (Robson et al. 1995). It is possible that simple nutrient budgeting methods underestimate S needs, warranting a closer examination of the capacity of sandy soils to sustain S supply with reduced inputs of S[O.sub.4.sup.2-] fertiliser.

The bulk of S in soil is present in organic forms that cannot be utilised by plants. Scherer (2001) provides an overview of soil S transformations, describes the diverse forms of organic S in soil and outlines the processes linked to sulfur mineralisation that release S[O.sub.4.sup.2-] from soil organic matter (SOM). Mineralisation processes are either enzymatic or biochemical, or microbially mediated depending on the form of organic S. Rates of S mineralisation are thought to be dependent on SOM content and on the C : S ratio of this material (Tabatabai and Al-Khafaji 1980), with net S[O.sub.4.sup.2-] mineralisation apparent when the C : S ratio is <200, and immobilisation apparent for C : S ratios >400 (Scherer 2001). Temperature and soil water content are other key environmental factors affecting S mineralisation.

Most studies on S mineralisation have involved laboratory experimentation. There is limited information on the rates of S mineralisation in field soils. Eriksen et al. (1995) report net S[O.sub.4.sup.2-] mineralisation rates in sandy soils in Denmark to range between 3.3 and 6.7 [micro]g S/g soil.year. These rates of S mineralisation were unable to maintain S[O.sub.4.sup.2-] supply at levels needed to prevent S deficiency in rye grass, barley, and canola. The difficulties encountered in measuring net S[O.sub.4.sup.2-] mineralisation in soil have encouraged the testing of extractants that aim to release S[O.sub.4.sup.2-] from labile soil organic S pools considered to be mineralised during crop growth. The KCl-40 extraction technique introduced by Blair et al. (1991) gives satisfactory predictions of net S[O.sub.4.sup.2-] mineralisation in pasture systems but has not been tested for crop systems grown on sandy soils.

Leaching of S[O.sub.4.sup.2-] below the rooting depth of crops can be an important S loss mechanism. The amount of S[O.sub.4.sup.2-] leached is affected by S[O.sub.4.sup.2-] fertiliser application (Heng et al. 1991), S[O.sub.4.sup.2-] immobilisation (Goh and Gregg 1982), amount of drainage of soil water drainage (Heng et al. 1991), the S[O.sub.4.sup.2-] status of soil (Sakadevan et al. 1994), and the capacity of the soil to adsorb S[O.sub.4.sup.2-] (Bolan et al. 1986; Lefroy et al. 1995). The amounts of S[O.sub.4.sup.2-] leached from soils of the Western Australian wheatbelt are not known.

The aims of this paper are (1) to determine the effect of S application on grain yield in wheat (Triticum aestivum), narrow-leafed lupin (Lupinus angustifolius), and canola (Brassica napus L.); (2) to examine the residual value of applied S[O.sub.4.sup.2-] fertiliser in a sandy soil by determining the amounts of S[O.sub.4.sup.2-] in soil and rates of net S[O.sub.4.sup.2-] mineralisation; (3) to ascertain potential for S[O.sub.4.sup.2-] leaching; and (4) to examine the likely occurrence of S deficiency by determining the relationship between soil S[O.sub.4.sup.2-] supply and plant S[O.sub.4.sup.2-] demand.

Material and methods

The experimental site was located near Konnongorring, 20 km south of Wongan Hills (30[degrees]50'S, 116[degrees]38'E), in Western Australia. Annual rainfall for the area ranges between 264 mm (decile 1) and 521 mm (decile 9), with a long-term average of 386 mm. The soil was classified as grey shallow sandy duplex or Sodosol by Sehokneeht (1997). The site was cropped to wheat in 1989, lupin in 1990, and wheat in 1991. Fertiliser use over this period included 50 kg/ha of diammonium phosphate (DAP) and 60 kg urea/ha in 1989, 75 kg/ha of triple super phosphate (TSP) in 1990, and 50 kg DAP/ha and 50 kg/ha of(N[H.sub.4])[sub.2] S[O.sub.4] in 1991. From 1992 to 1995 the site was used by Wesfarmers CSBP to investigate crop responses to rates and forms of S fertiliser. Sulfur was applied each year as gypsum at 0 ([S.sub.0]), 5 ([S.sub.5]), 15 ([S.sub.15]), and 30 ([S.sub.30]) kg S/ha. In order for the residual value of the S[O.sub.4.sup.2-]fertiliser treatments to be studied, no additional S[O.sub.4.sup.2-] fertiliser inputs were made to these treatments when wheat was grown in 1996, or when lupin was grown in 1997. Gypsum was applied in both years at 15 kg S/ha ([SR.sub.15]) to plots that had not received S[O.sub.4.sup.2-] fertiliser in 1993 and 1994 to evaluate the grain yield response to current season applied S.

Each experimental plot was 2.1 m wide and 40 m long. Treatments were originally replicated 3 times in a randomised block design. However, in 1996 one replicate was damaged by spray drift from an adjacent paddock and the plots in the 2 unaffected replicates were divided in half to give 4 replicates.

Soil properties

Soil bulk density was measured in layers to a depth of 0.5 m using pits dug at 4 locations across the experimental site. Soil recovered in stainless steel rings, each 50 mm in diameter and 100 mm long, was weighed then oven-dried at 105[degrees]C and reweighed. Particle size analysis was conducted by the procedure of Day (1965). The methods used

for measurements of N[O.sub.3.sup.-] and N[H.sub.4.sup.+], bicarbonate-extractable P and K, KCl-40 extractable S, total organic carbon, 0.01 M Ca[Cl.sub.2] soil pH, and oxalate-extractable iron are outlined in Rayment and Higginson (1992). Total P and S were measured by the procedures outlined by Chen et al. (1999).

Sulfate adsorption by soil was determined by weighing 3 g of air-dried soil into a 50-mL centrifuge tube to which was added 15 mL of 0.02 M Ca[Cl.sub.2] plus 2 drops of chloroform (Lisle et al. 1991). Different amounts (0-15 mL) of 50 [micro]g S/mL [K.sub.2]S[O.sub.4] solution were added to give a range of added S[O.sub.4.sup.2-] between 0 and 250 [micro]g S/g soil. The concentration of K was balanced by the addition of 0-15 mL of 0.003 M KCl. The final volume of the extracting solution was 30 mL. Centrifuge tubes were then tumble-shaken for 48 h at 25[degrees]C. The pH of the soil solution was checked at 6, 24, and 30 h, and if necessary, adjusted to the original soil pH (0.01 M Ca[Cl.sub.2]). Each tube was centrifuged at 3000 r.p.m, for 5 min and the supernatant filtered using Whatman No 42 filter paper. The SS[O.sub.4.sup.2-] concentration in the supernatant was measured using a Waters high-pressure liquid chromatograph (HPLC). Sulfate adsorption was determined as the difference between S[O.sub.4.sup.2-] removed from solution following the addition of 0.01 M Ca[Cl.sub.2] containing [K.sub.2]S[O.sub.4] at 50, 100, 150, 200, and 250 [micro]g S/g soil, and S[O.sub.4.sup.2-] in solution when soil was extracted with 0.01 M Ca[Cl.sub.2] without added S[O.sub.4.sup.2-].

Climate

Rainfall data for the period 1 January 1992-28 May 1996 were from daily readings taken at the farmhouse located about 500 m from the research site. After 28 May 1996, weather data including rainfall, maximum and minimum air temperature, and soil temperature at 10, 50, and 100 mm were collected using an automatic meteorological station placed at the site.

The ASPIM model configured with the SOILWAT2 module (Probert et al. 1998) was used to predict drainage from 1992 to 1997. Soil water parameters used to initialise the model were taken from Asseng et al. (2001) for a sand over clay or duplex soil, and were started on 1 April of each year using soil water contents set at the plant-available lower limit.

Sulfur input in rainfall was estimated from data published by Hingston and Gailitis (1976) for sites in the central wheatbelt of Western Australia.

Crop agronomy

The cropping sequence was lupin in 1992, wheat in 1993, canola in 1994, wheat in 1995, wheat in 1996, and lupin in 1997. The cultivars used, seeding dates, seeding rates, and N inputs are given in Table 1. All crops were sown using a minimum tillage (narrow points) cultivation system. Weed management was achieved by the use of herbicides. The herbicides used included Roundup[R] (glyphosate) at 1 L/ha or Sprayseed[R] (paraquat and diquat) at 1.5 L/ha applied before sowing. Ryegrass control was achieved by using triasulfuron (35 g/ha) at seeding of wheat in 1993, fluazifop-P (0.4 L/ha) on 20 July 1994 on canola, and triflumlin (1 L/ha) at seeding of wheat in 1996. Wheat seed was treated with Baytan[R] (triadimenol and cypermethrin) and lupin was treated with Royal[R] (iprodione). Control of redlegged earth mites in canola was achieved by the application of omethoate (0.06 L/ha) at seeding and on 20 July 1994. Phosphate was applied at the rate of 13 kg P/ha in the form of TSP in all years. This rate of application resulted in an input of 0.6 kg S/ha across all treatments. Potassium was applied as muriate of potash at 50 kg K/ha on 25 August 1994 and on 7 June 1996.

Crop measurements

Machine harvest of grain was undertaken after maturity between 1992 and 1995. In 1996 and 1997 crop biomass (t/ha) was manually collected in July, August, September, and December in all treatments except the re-applied S[O.sub.4.sup.2-] fertiliser treatment, which was only measured at the grain harvest. At each manual sampling, plants were collected from a 0.5-m length of row at 4 randomly chosen points in each plot. Plant material was bulked to obtain a single plot weight. Lupin and wheat material was separated into stem and grain at final harvest. Wheat and lupin grain yield (t/ha) was also obtained from a machine harvest of each plot in 1996 and 1997.

Plant material was dried at 70[degrees]C, weighed, ground, digested using nitric/perchloric acid mixture (Johnson and Ulrich 1959), and analysed for S using an inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Matilainen and Tummavuori 1996).

Soil profile S[O.sub.4.sup.2-] content

Soil was collected from all plots in layers to 0.5 m in 1996 and 1997 to determine the S[O.sub.4.sup.2-] content within the rooting zone of lupin and wheat. The top 0.1 m of soil was collected in 0.05-m increments from within a 110-mm-diameter stainless steel ring. A 100-mm-diameter sand auger was then inserted through the stainless steel ring to collect soil in 0.1-m increments to 0.5 m. A further 0.2-m depth of soil sample was collected at some plots but the presence of a silcrete layer between 0.6 and 0.7 m in sections of the trial did not permit uniform sampling to 0.7 m.

Soil samples to 0.2 m were used in the assessment of net S[O.sub.4.sup.2-] mineralisation. These were placed in chilled polystyrene containers immediately after collection then transported to Perth within a day of collection where they were stored at 4[degrees]C for a maximum of 2 days. All soil samples were weighed and thoroughly mixed before a subsample was taken. Soil subsamples were then stored at -18[degrees]C until analysed. Soil samples collected below 0.2 m were air-dried in a glasshouse, then mixed thoroughly, sieved (2 mm), and subsampled.

Soil S[O.sub.4.sup.2-] in the top 0.2 m was extracted by shaking 50 g of moist soil with 100 mL of deionised [H.sub.2]O on a rotating shaker for 2 h at 25[degrees]C. Gravimetric water content was determined on separate subsamples. Water extracts were filtered through Whatman No. 1 paper and the filtrate was frozen until analysed for S[O.sub.4.sup.2-] using a Waters HPLC. Soil in layers below 0.2 m was analysed for S using the KCl-40 method of Blair et al. (1991). A comparison was made between water extraction and the KCl-40 method using selected samples from the 0.2-0.5 m soil layers to confirm that both methods extracted the same amount of S[O.sub.4.sup.2-] (data not presented). The amount of S[O.sub.4.sup.2-] in each soil layer (kg S/ha) was calculated from the measured concentration of S[O.sub.4.sup.2-] ([micro]g S/g soil) in the layer, the bulk density of soil in the layer (g soil/[cm.sub.3]), and the depth (cm) of each soil sampled with units expressed as kg/ha. Profile S[O.sub.4.sup.2-] content is the sum of S[O.sub.4.sup.2-] (kg S/ha) calculated for each soil layer sampled.

Net S[O.sub.4.sup.2-] mineralisation

Measurements of net S[O.sub.4.sup.2-] mineralisation were undertaken between 17 June 1996 and 24 November 1997 using the 5 residual S[O.sub.4.sup.2-] fertiliser treatments. Net S[O.sub.4.sup.2-] mineralisation during the crop growing season

was determined from the buildup of S[O.sub.4.sup.2-] in soil contained in PVC tubes (80 mm diameter by 200 mm long) and S[O.sub.4.sup.2-] that was retained in an anion exchange resin (Amberlite IRA) contained in a plastic insert that was sealed into the lower end of each soil core. A detailed description of the technique is given in Anderson et al. (1998b).

Four soil cores per plot were collected at monthly intervals from the same locations used to sample soil for profile S[O.sub.4.sup.2-]. These cores were then incubated at the site for 1 month. Soil within the PVC tubes was sectioned into 0-0.05, 0.05-0.1, and 0.1-0.2 m layers. Soil was handled in the field and prepared for analysis using procedures outlined earlier for the 0-0.2 m profile samples. Sulfate was extracted from anion exchange resin by shaking 5 g of wet resin with 100 mL of 1 M HCl for 1 h at 25[degrees]C on an end-over-end shaker. The extractant was filtered and then measured for S[O.sub.4.sup.2-] using an ICP-AES. Net S[O.sub.4.sup.2-] mineralisation was calculated from the change in soil S[O.sub.4.sup.2-] status (kg S/ha) in the top 0.2 m of soil over the incubation period plus the amount S[O.sub.4.sup.2-] (kg S/ha) retained in the resin trap.

Sulfate leached

The amount of S[O.sub.4.sup.2-] leached (SL) during the 1996 and 1997 growing seasons for each S[O.sub.4.sup.2-] fertiliser treatment was determined by difference:

SL = soil profile S[O.sub.4.sup.2-] content (0-0.5 m) at sowing

- soil profile content S[O.sub.4.sup.2-] (0-0.5 m) at harvest

+ net S[O.sub.4.sup.2-] mineralisation - plant S content

Plant S includes an estimate of root S (25% of S in plant tops) assuming the ratio of S in shoots and roots is similar to that found for N in wheat grown on similar soil (Atwell et al. 2002).

Statistical analysis

Treatment differences at each sampling time for DM yield (t/ha), S content (kg S/ha), S[O.sub.4.sup.2-] mineralisation (kg S/ha), and soil S[O.sub.4.sup.2-] content (kg S/ha) within the 0-0.2 m soil layer were subjected to analysis of variance using a package described by Burr (1982). Least significant differences were computed where treatment differences were significant at P = 0.05). Treatment differences in soil profile S[O.sub.4.sup.2-] content (kg S/ha) were analysed using the repeat measures routine in GENSTAT Version 6.

Results

Key chemical and physical properties for the soil are summarised in Table 2. The S[O.sub.4.sup.2-] adsorption capacity of the soil increased from -2 [micro]g S/g soil for the 0-0.1 m soil layer to 8.9 [micro]g S/g soil in the 0.3-0.4 m soil layer from a solution containing an initial concentration of 100 [micro]g S/g soil.

Environmental conditions

Monthly growing season rainfall and annual rainfall from 1992 to 1997 is shown in Table 3. Annual rainfall was well below the long-term average in 1994, close to the long-term average in 1993, 1996, and 1997, and above the long-term average in 1992 and 1995. The 1996 growing season was characterised by a late break on 16 June followed by 85 mm over June and 65 mm over July 1996. As a result there was 35 mm of drainage below 0.7 m from 20 June to 31 July 1996 (Table 3). In contrast, the 1997 growing season was characterised by a summer rainfall event of 32 mm on 20 February 1997, and an early start to the growing season on 5 April 1997. Subsequent individual rain events were typically < 10 mm and no drainage occurred below 0.7 m (Table 3).

Daily soil temperatures, measured at a depth of 50 mm, typically ranged between 4 and 19[degrees]C over June-August and 7 and 36[degrees]C during September and October. Outside these months, soil minimum temperatures ranged between 8 and 30[degrees]C and maximum soil temperatures ranged between 14 and 53[degrees]C.

Crop biomass and sulfur content

Crop grain yields obtained from 1992 to 1995 after the application of different rates of gypsum are shown in Table 4. Sulfur application did not affect grain yield in lupin in 1992 and wheat in 1993 but there were significant increases in grain yield following the application of S to canola in 1994 and to wheat in 1995. The amounts of sulfur exported in grain are reported in Table 5.

Previous S fertiliser inputs did not affect the early growth of wheat in 1996, but significantly larger biomass was produced in the [S.sub.30] and [S.sub.15] treatments than either the [S.sub.5] or the [S.sub.0] treatment when wheat was sampled in August and September 1996, and at grain harvest. Wheat grain yield in 1996 was sharply reduced in the absence of any external sulfur input (Table 4).

The total S content of wheat grown in 1996 ranged from 4.8 to 8.3 kg S/ha with 1.5-2.6 kg S/ha removed in the grain and 3.3-5.7 kg S/ha recycled via the wheat stubble. The S content of wheat was greater in the Sis and [S.sub.30] treatments than the [S.sub.0] treatment in August 1996 and in the [S.sub.0] and [S.sub.5] treatments for samples collected between September and November 1996 (Fig. 1b). The N/S ratio of wheat grain in 1996 was >17 : 1 for all S treatments, whereas in 1995 the N/S ratio was > 17 : 1 for the [S.sub.0] and [S.sub.5] treatments only. The N/S ratio in wheat grain in 1992 was 15 : 1 to 16 : 1 across all treatments.

[FIGURE 1 OMITTED]

Except for the final harvest, the S content in lupin biomass in 1997 was not significantly affected by previous S[O.sub.4.sup.2-] fertiliser history (Fig. 1d). Lupin biomass (Fig. 1c) and grain yield (Table 4) were also unaffected by S fertilisation. Unlike the response in wheat, re-application of S[O.sub.4.sup.2-] fertiliser (15 kg S/ha) did not affect grain yield compared to the residual S[O.sub.4.sup.2-] fertiliser treatments. Total S content of lupin biomass at harvest in 1997 ranged from 7.7 to 10.0 kg S/ha with 3.8-4.4 kg S/ha recycled via the lupin stubble and 2.8-2.9 kg S/ha removed in the lupin grain. A much higher S content (6.4-6.5 kg S/ha) was found in lupin grain in 1992 (Table 5).

Soil sulfur status

Water-extractable soil S content in the top 0.1 m of soil in June 1996 ranged from 1.8 [micro]g S/g soil in the [S.sub.0] treatment to 7.9 [micro]g S/g soil in the [S.sub.30] (Table 6). By 1997, water-extractable soil S in the [S.sub.5] and [S.sub.15] treatments had declined to the quantities found in the [S.sub.0] treatment. More S was extracted with the KCl-40 technique but the effects of prior S fertilisation on KCl-40 extractable S, including changes over time, were similar to those found for water-extractable S (Table 6). The quantities of labile organic S, determined by difference between water and KCl-40 extraction, increased in response to previous S fertilisation (Table 6).

Sulfate in soil

The quantities of soil S[O.sub.4.sup.2-] in the 0-0.2 m soil layer for all treatments are shown in Fig. 2. Significantly less S[O.sub.4.sup.2-] was found in the [S.sub.0], and [S.sub.5] treatments (6-8 kg S/ha) than the [S.sub.15] and [S.sub.30] treatments (14-16 kg S/ha) on 17 June 1996 when wheat was sown (Fig. 2a). By August 1996, S[O.sub.4.sup.2-] in the 0-0.2 m soil had declined to about 4 kg S/ha in the [S.sub.15] and [S.sub.30] treatments, to 3 kg S/ha in the [S.sub.5], and <2 kg S/ha in the [S.sub.0] treatment. Little S[O.sub.4.sup.2-] remained in the top 0.2 m of soil in the [S.sub.5] and [S.sub.0] treatments from September 1996, with marginally larger quantities of S[O.sub.4.sup.2-] in the [S.sub.15] and [S.sub.30] than the [S.sub.0] and [S.sub.5] treatments. Less than 1 kg S/ha of S[O.sub.4.sup.2-] was detected in the top 0.2 m of soil in all treatments late in the wheat-growing season (Fig. 2a).

[FIGURE 2 OMITTED]

The quantities of S[O.sub.4.sup.2-] in the top 0.2 m of soil at the start of the 1997 growing season (Fig. 2b) were lower than those detected at the start of the 1996 growing season, highlighting the decline in soil S[O.sub.4.sup.2-] supplying capacity without new S inputs. There was a similar quantity (4 kg S/ha) of soil S[O.sub.4.sup.2-] in the [S.sub.0], [S.sub.5], and [S.sub.15] treatments in June and July 1997; only the [S.sub.30] treatment had significantly greater capacity to supply S[O.sub.4.sup.2-]. This difference in S[O.sub.4.sup.2-] in soil between the [S.sub.30] treatment and the other S treatments was not observed after July 1997 because of a decline in S[O.sub.4.sup.2-] in the [S.sub.30] treatment. Only ~2 kg S/ha was detected in soil to 0.2 m in August 1997 for all treatments and < 1 kg S/ha in October 1997, also for all treatments (Fig. 3b). A small increase in soil S[O.sub.4.sup.2-] was detected from October to December 1997.

[FIGURE 3 OMITTED]

The contents of S[O.sub.4.sup.2-] in soil to 0.5 m for all S treatments in June 1996, February 1997, May 1997, and December 1997 are shown in Table 7. The distribution of S[O.sub.4.sup.2-] within the 0.5 m soil profile is only shown for the [S.sub.0] and [S.sub.30] treatments for June 1996, February 1997, May 1997, and December 1997 (Fig. 3). Significant increases in S[O.sub.4.sup.2-] content between [S.sub.0] and [S.sub.30] were apparent in many but not all layers. It is notable that between 17 June 1996 and 27 February 1997, the largest change in soil S[O.sub.4.sup.2-] content occurred above 0.2 m, with a decline of 3-5 kg S/ha for the [S.sub.0] treatment and a decrease of 11-12 kg S/ha for the [S.sub.30] treatment. The S[O.sub.4.sup.2-] content of the 0.3-0.5 m soil layer decreased by 2-5 kg S/ha across all treatments in the same period. In contrast, from May to December 1997 the soil S[O.sub.4.sup.2-] profile content to 0.5m stayed at 18-21 kg S/ha for the [S.sub.0] treatment and 26-33 kg S/ha for the [S.sub.30] treatment.

Net S[O.sub.4.sup.2-] mineralisation

Net S[O.sub.4.sup.2-] mineralisation measured over the 17-month period June 1996 to November 1997 was significantly affected by previous S fertilisation history, with a larger amount mineralised (19 kg S/ha) in the [S.sub.30] treatment than the control treatment (8 kg S/ha) (Table 8). These differences in net S[O.sub.4.sup.2-] mineralisation arose largely because of a significant difference in S mineralisation between treatments in the period February-May 1997. Growing season net S[O.sub.4.sup.2-] mineralisation was not significantly different between S treatments either in 1996 or 1997. However, net S[O.sub.4.sup.2-] mineralisation did exhibit seasonal variation (Table 8), with S immobilisation apparent soon after seeding. Rates of net S[O.sub.4.sup.2-] mineralisation were typically <0.5 kg S/ha/month over the period June-August when soil temperatures ranged between 4 and 19[degrees]C. In contrast, net S[O.sub.4.sup.2-] mineralisation was of the order of 1.6-3.6 kg S/ha.month during September-October when the soil temperatures were between 7 and 36[degrees]C. Net S[O.sub.4.sup.2-] mineralisation decreased when the surface soil dried in late spring 1996. The relationship between growing season net S[O.sub.4.sup.2-] mineralisation and labile organic S extracted by KCl-40 is shown in Fig. 4.

[FIGURE 4 OMITTED]

Sulfate leached

The neutral to positive values obtained for the unaccounted S[O.sub.4.sup.2-] pool indicate that wheat and lupin derived S[O.sub.4.sup.2-] from soil layers below 0.5 m (Table 7), precluding estimates of S[O.sub.4.sup.2-] leaching using the budgeting methods employed.

Discussion

Sulfur status in crops

Findings obtained here highlight the importance of maintaining S inputs on sandy soils for wheat production and wheat grain quality. Grain yield responses occurred in treatments where the KCl-40 soil test value was below the critical value (Blair et al. 1991), the S concentration in leaf tissue was below the critical value (Robson et al. 1995), and the N:S ratio in wheat grain was >17:1 (Randall et al. 1981). The low S content of the wheat grain found in this study (<0.12% S) for all residual fertiliser treatments except [S.sub.30], has important implications for grain quality (Moss et al. 1981; Zhao et al. 1999). In contrast, neither previous S[O.sub.4.sup.2-] fertilisation nor reapplication of S[O.sub.4.sup.2-] affected lupin growth. Robson et al. (1995) observed that wheat grown under glasshouse conditions gave an earlier and greater expression of S response than lupin. They also observed that the incidence of S deficiency in lupin crops in the northern agricultural region of Western Australia was low with most tissue samples close to the critical plant concentration whereas 36% of wheat samples were S-deficient or showed marginal S deficiency.

Sulfate inputs

The importance of previous S[O.sub.4.sup.2-] fertiliser inputs in sustaining net S[O.sub.4.sup.2-] mineralisation in the sandy soil when S[O.sub.4.sup.2-] fertilisers are not reapplied is evident in this study as noted previously by Sakadevan et al. (1993). Mineralisation of organic S was seasonally dependent, with very low rates of net mineralisation, and in some cases S immobilisation, apparent soon after seeding. This pattern of mineralisation of organic matter is common in dryland soils where stubble retention and minimum tillage are practiced (Thomas et al. 1992; Gupta et al. 1994). Rates of net S[O.sub.4.sup.2-] mineralisation increased in spring, commensurate with rising soil temperatures, but these rates of mineralisation were unable to meet wheat demand for S. Barrow (1969) observed similar seasonal variation in rates of mineralisation in sandy soils under pasture on the Swan coastal plain in southwestern Western Australia. The net S[O.sub.4.sup.2-] mineralisation of >5 kg S/ha following a rainfall event in February 1997 highlights the likely contribution of mineralisation in years with summer-autumn rain. Summer rainfall also promotes net N mineralisation in soils similar to the one studied here (Murphy et al. 1998).

Sulfate leaching

Accumulation of S[O.sub.4.sup.2-] in soil below 0.3 m is evidence of the historic displacement of S[O.sub.4.sup.2-] to depth. However, S budgets determined in this study suggested little S[O.sub.4.sup.2-] had leached into the clay below 0.5 m. This redistribution and accumulation of S[O.sub.4.sup.2-] at depth has been observed elsewhere (Chen et al. 1999). The higher S[O.sub.4.sup.2-] adsorption capacities in clay should have assisted the retention of S[O.sub.4.sup.2-] (Bolan et al. 1986; Lefroy et al. 1995) and minimised S[O.sub.4.sup.2-] movement from the rooting zone of the crops studied here. The substantial downward movement of S[O.sub.4.sup.2-] within the top 0.3 m of soil was not unexpected given the low S[O.sub.4.sup.2-] adsorption capacity in surface soil. This rapid movement of S[O.sub.4.sup.2-] from surface soil appeared to affect wheat growth more than lupin growth.

Residual value of S[O.sub.4.sup.2-] fertiliser and long-term S[O.sub.4.sup.2-] balance

In general, the residual value of previous S[O.sub.4.sup.2-] fertilisation was short-lived with as little as 15% of the applied S[O.sub.4.sup.2-] retained in the soil as S[O.sub.4.sup.2-] at the commencement of the next growing season after S application. Even lower residual values of S[O.sub.4.sup.2-] fertiliser have been found in sandy soils in the high rainfall zone of Western Australia (>750 mm) with the current season's S[O.sub.4.sup.2-] fertiliser application leached by August (Barrow 1966). These poor residual values for S[O.sub.4.sup.2-] fertiliser contrast with the 31% retention of applied S[O.sub.4.sup.2-] fertiliser found in clay loam soils (McCaskill and Cayley 2000).

The residual value of S[O.sub.4.sup.2-] fertiliser is likely to be affected by seasonal conditions, when S[O.sub.4.sup.2-] fertiliser was applied (seeding v. post seeding) and the rate of application. For example, wheat grown in 1993 was not responsive to current season S[O.sub.4.sup.2-] fertiliser application (Table 4), most likely due to the residual effect of the post seeding application of 12 kg S/ha the previous year. In contrast, the residue value of 15 kg S/ha applied in 1995 was short-lived with wheat grown in 1996 responding to S application (Table 4) after 96 mm of drainage over 2 growing seasons (Table 3).

Nutrient balances are widely used to determine the long-term sustainability of farming systems (Goh and Nguyen 1997). A common practice in the region is to use compound fertilisers at seeding that supply 1-10 kg S/ha, depending on whether fertiliser has a low or high S content. The National Land and Water Resources Audit (2001) has determined that this farming practice is likely to result in a neutral (-2-0 kg S/ha) to moderately positive (5-10 kg S/ha) S balance for the wheatbelt of Western Australia. Our findings suggest that simple budgeting techniques of the type used in the National Land and Water Resources Audit will give a good guide to the long-term S balance when S[O.sub.4.sup.2-] leaching does not occur.

Applicability of the KCl-40 soil S test

Blair et al. (1991) suggested that the extraction of soil with 0.25 M KCl at 40[degrees]C was a more accurate measure of plant-available S than the 0.01 M [Ca([H.sub.2]P[O.sub.4]).sub.2] method because the KCl-40 method extracts a pool of labile organic S that is likely to mineralise over the growing season, as well as S[O.sub.4.sup.2-]. In this study, the quantities of KCl-40 extractable labile organic S were not correlated with growing season net S[O.sub.4.sup.2-] mineralisation in soil. In particular, the KCl-40 method overestimated S mineralisation in soil that received S[O.sub.4.sup.2-] fertiliser the previous season, most probably because of the immobilisation of residual S[O.sub.4.sup.2-] fertiliser.

The nutrient status of soil is typically assessed using only the 0-0.1 m layer. In this study, wheat and particularly lupin, derived large amounts of S[O.sub.4.sup.2-] from the soil layers below 0.1 m. Accurate assessments of the S supplying capacity of soils therefore require either a deep soil S[O.sub.4.sup.2-] test or a combination of measures of net S[O.sub.4.sup.2-] mineralisation coupled with estimates of S[O.sub.4.sup.2-] leaching as suggested by Anderson et al. (1998a). Alternatively, another practical method is to assess the S status of susceptible crops, such as wheat and canola.

Conclusion

The trend to use high analysis N and P fertilisers in crops grown on sandy-textured soils with low S mineralisation and poor retention of S[O.sub.4.sup.2-] will reduce wheat and canola grain yield and degrade grain quality without supplementary S inputs. Development of soil S testing procedures that characterise net S mineralisation as well determine the residual amounts of S[O.sub.4.sup.2-] within root-zones are needed. Simple procedures for assessing the S[O.sub.4.sup.2-] retention in subsoil will assist the prediction of S[O.sub.4.sup.2-] movement from soil profiles and thus the likely fate of residual S[O.sub.4.sup.2-].

Acknowledgments

The Grains Research and Development Corporation (GRDC) and CSBP Limited provided funding for the project. The assistance of CSBP Ltd field staff in seeding and managing the trial site and technical assistance of Ms Megan Bytheway (CSIRO) during soil and plant sampling is gratefully acknowledged. The experiment was conducted on land generously made available by Ben and Wendy Davey, Konnongorring. Dr Senthold Asseng provided ASPIM predictions of drainage.

Manuscript received 24 June 2005, accepted 4 January 2006

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G. C. Anderson (A), I. R. P. Fillery, (A), D, F. H. Ripper (B), and B. J Leach (C)

(A) CSIRO, Private Bag, PO Wembley, WA 6014, Australia.

(B) CSBP Ltd, PO Box 345, Kwinana, WA 6966, Australia.

(C) Formerly CSBP Ltd, now at John Duff and Associates, Suite 5, 110 Robinson Avenue, Belmont, WA 6104, Australia.

(D) Corresponding author. Email: Ian.Fillery@csiro.au
Table 1. Crop cultivar, time of sowing, seeding rate, and
rate and date N fertiliser was applied in each year of
the study

Year Crop Variety Sowing Seeding rate
 date (kg/ha)

1992 Lupin Gungurru 12 May 108
1993 Wheat Spear 11 May 75
1994 Canola Narendra 06 June 8
1995 Wheat Spear 07 June 75
1996 Wheat Amery 06 June 75
1997 Lupin Merrit 24 May 80

Year N applied (kg N/ha)
 Date Rate Date Rate

1992 -- -- -- --
1993 11 May 20 17 June 20
1994 11 May 92 25 Aug. 50
1995 14 July 46 18 Aug. 46
1996 31 July 46 28 Aug. 46
1997 -- -- -- --

Table 2. Particle size analysis, chemical composition, and S sorption
from solution with various S042- concentrations for samples of the
soil profile collected on 17 June 1996

Measurement Depth (m):
 0-0.05 0.05-0.1

Bulk density (Mg/[m.sup.3]) 1.7 1.7
Gravel (%) 0 0
Sand (%) 89 92
Silt (%) 0 6
Clay (%) 3 2
Fe ([micro]g/g soil) 88 94
pH(Ca[C1.sub.2]) 4.9 4.6
C (%) 0.9 0.5
N[O.sub.3] ([micro]g/g soil) 20 9
N[H.sub.4] ([micro]g/g soil) 16 13
P ([micro]g/g soil) 21 17
K ([micro]g/g soil) 69 35
S ([micro]g/g soil) 4.7 3.9
Total P ([micro]g/g soil) 58.5 (A)
Total S ([micro]g/g soil) 59.1 (A)
S adsorption (50 (B)) ([micro]g/g soil) n.d.
S adsorption (100 (B)) ([micro]g/g soil) -2 (A)
S adsorption (150 (B)) ([micro]g/g soil) -0.3 (A)
S adsorption (200 (B)) ([micro]g/g soil) 3.6 (A)
S adsorption (250 (B)) ([micro]g/g soil) 2.9 (A)

Measurement Depth (m):
 0.1-0.2 0.2-0.3

Bulk density (Mg/[m.sup.3]) 1.8 1.7
Gravel (%) 0 0
Sand (%) 95 86
Silt (%) 3 2
Clay (%) 2 10
Fe ([micro]g/g soil) 149 209
pH(Ca[C1.sub.2]) 4.6 4.3
C (%) 0.2 0.1
N[O.sub.3] ([micro]g/g soil) 8 8
N[H.sub.4] ([micro]g/g soil) 8 5
P ([micro]g/g soil) 20 25
K ([micro]g/g soil) 38 49
S ([micro]g/g soil) 2.8 2
Total P ([micro]g/g soil) 84.1 21.1
Total S ([micro]g/g soil) 20.8 19.6
S adsorption (50 (B)) ([micro]g/g soil) -0.4 -1.3
S adsorption (100 (B)) ([micro]g/g soil) 6.6 8.2
S adsorption (150 (B)) ([micro]g/g soil) 1.8 n.d.
S adsorption (200 (B)) ([micro]g/g soil) 15.7 18.2
S adsorption (250 (B)) ([micro]g/g soil) 24.5 25

Measurement Depth (m):
 0.3-0.4 0.4-0.5

Bulk density (Mg/[m.sup.3]) 1.8 1.8
Gravel (%) 2 18
Sand (%) 81 84
Silt (%) 2 2
Clay (%) 13 12
Fe ([micro]g/g soil) 209 206
pH(Ca[C1.sub.2]) 4.3 4.4
C (%) 0.1 0.1
N[O.sub.3] ([micro]g/g soil) 6 6
N[H.sub.4] ([micro]g/g soil) 2 1
P ([micro]g/g soil) 12 5
K ([micro]g/g soil) 44 41
S ([micro]g/g soil) 3 4
Total P ([micro]g/g soil) 20.7 n.d.
Total S ([micro]g/g soil) 21.8 n.d.
S adsorption (50 (B)) ([micro]g/g soil) 2.2 n.d.
S adsorption (100 (B)) ([micro]g/g soil) 8.9 n.d.
S adsorption (150 (B)) ([micro]g/g soil) 17.6 n.d.
S adsorption (200 (B)) ([micro]g/g soil) n.d. n.d.
S adsorption (250 (B)) ([micro]g/g soil) 32.6 n.d.

n.d., Not determined.

(A) Soil analysis on 0-0.1 m samples.

(B) Concentration of added solution ([micro]g/g soil).

Table 3. Monthly growing season rainfall and annual rainfall (mm)
at the site, and computer-simulated drainage (mm) at 0.7m, from
1992 to 1997

Year April May June July

1992 101 15 84 54
1993 11 114 76 57
1994 -- 80 56 33
1995 16 60 81 145
1996 13 15 86 65
1997 42 27 28 38

Year Aug. Sept. Oct. Nov.

1992 107 53 18 17
1993 59 31 17 21
1994 41 26 3 --
1995 34 33 16 2
1996 48 51 13 18
1997 54 20 17 12

Year Annual Drainage
 rainfall below 0.7 m

1992 543 47
1993 395 45
1994 277 0
1995 406 56
1996 335 35
1997 307 0

Table 4. Grain yields (t/ha) obtained after different
rates of S fertiliser application to specified crops

Year Crop Sulfate treatment:
 [S.sub.0] [S.sub.5] [S.sub.15]

1992 Lupin 2.21 2.29 2.24
1993 Wheat 3.39 3.51 3.73
1994 Canola 0.24 0.32 0.45
1995 Wheat 0.90 1.35 1.97
1996 Wheat 1.30 1.90 2.00
1997 Lupin 1.66 1.65 1.61

Year Sulfate treatment: I.s.d.
 [S.sub.30] [S.sub.R15] (P = 0.05)

1992 2.19 2.20 n.s.
1993 3.78 3.67 n.s.
1994 0.46 0.33 0.14
1995 1.86 1.82 0.45
1996 2.30 2.90 0.39
1997 1.48 1.68 0.19

n.s., Not significant.

Table 5. Differences between S inputs (kg S/ha) and S removed in
grain (kg S/ha) for the period 1992-97

S in rainfall was derived from measurements of S in rainfall for
the central wheatbelt of Western Australia reported by Hingston and
Gailitis (1976), with input corrected for annual rainfall

Year S in S applied in fertiliser
 rainfall [S.sub.0] [S.sub.5] [S.sub.15] [S.sub.30]

1992 1.5 0.6 6 16 31
1993 1.1 0.6 6 16 31
1994 0.6 0.6 6 16 31
1995 1.4 0.6 6 16 31
1996 1 0.6 1 1 1
1997 0.9 0.6 1 1 1

Year S remove in grain
 [S.sub.0] [S.sub.5] [S.sub.15] [S.sub.30]

1992 6.4 6.5 6.6 6.5
1993 3.6 3.7 4.1 4.2
1994 1.1 1.4 1.9 2.2
1995 1.2 1.6 2.7 2.6
1996 1.5 2.1 2.4 2.6
1997 2.8 2.9 2.8 2.8

Year S input-output balance
 [S.sub.0] [S.sub.5] [S.sub.15] [S.sub.30]

1992 -4.3 0.9 11.0 25.9
1993 -1.9 3.4 13.0 27.9
1994 0.1 5.2 14.7 29.4
1995 0.8 5.8 14.7 29.8
1996 0.1 -0.1 -0.4 -0.6
1997 -1.3 -1.0 -0.9 -0.9

Table 6. Quantities of soil S extracted by [H.sub.2]O
and KCl-40 expressed as [micro]g S/g soil and kg S/ha

Extraction S fractions S treatments:
 [S.sub.0] [S.sub.5]

 1994

KCl-40 ([micro]g S/g soil) 4.3 5.5

 1996

[H.sub.2]O ([micro]g S/g soil) 1.8 3.0
KCl-40 (total) ([micro]g S/g soil) 4.3 5.8
[H.sub.2]O (kg S/ha) 2.8 4.8
KCl-40 (total) (kg S/ha) 7.1 9.4
KCl-40 (labile organic S) (kg 4.3 4.6

 1997

[H.sub.2]O ([micro]g S/g soil)
 1.5 1.7
KCl-40 (total) ([micro]g S/g soil) 3.2 3.1
[H.sub.2]O (kg S/ha) 2.4 2.7
KCl-40 (total) (kg S/ha) 5.2 5.1
KCl-40 (labile organic S) (kg 2.8 2.5

Extraction S fractions S treatments:
 [S.sub.15] [S.sub.30]

 1994

KCl-40 ([micro]g S/g soil) 11.6 13.4

 1996

[H.sub.2]O ([micro]g S/g soil) 6.1 7.9
KCl-40 (total) ([micro]g S/g soil) 10.6 11.5
[H.sub.2]O (kg S/ha) 10.0 11.6
KCl-40 (total) (kg S/ha) 17.1 18.4
KCl-40 (labile organic S) (kg 7.1 6.8

 1997

[H.sub.2]O ([micro]g S/g soil)
 1.6 2.2
KCl-40 (total) ([micro]g S/g soil) 4.1 5.7
[H.sub.2]O (kg S/ha) 2.4 3.6
KCl-40 (total) (kg S/ha) 6.7 9.3
KCl-40 (labile organic S) (kg 4.3 5.7

Table 7. Changes in amounts of soil profile S[O.sub.4.sup.2-](kg S/ha)
over the growing seasons, plant S[O.sub.4.sup.2-] uptake
(kg S/ha), and by difference either S[O.sub.4.sup.2-] uptake (kg S/ha)
from below 0.5 m or the amounts of S[O.sub.4.sup.2-] leached
(kg S/ha) estimated to move below 0.5 m in 1996 and 1997

Year Amount Amount of S[O.sub.4.sup.2-]
 fertiliser in soil to 0.5 m
 S applied

1996 17.vi.96 27.ii.97
 [S.sub.0] 21.8 21.2
 [S.sub.5] 25.3 24.4
 [S.sub.15] 32.3 28.4
 [S.sub.30] 40.1 34.3

1997 25.v.97 4.xii.97
 [S.sub.0] 18.7 18.2
 [S.sub.5] 21.0 19.7
 [S.sub.15] 26.0 25.7
 [S.sub.30] 32.9 33.5

Year Amount Growing season Plant
 fertiliser net S uptake
 S applied mineralisation of [S.sup.A]

1996
 [S.sub.0] 3.8 6.0
 [S.sub.5] 2.2 8.2
 [S.sub.15] 3.3 9.5
 [S.sub.30] 5.0 10.4

1997
 [S.sub.0] 3.5 9.6
 [S.sub.5] 5.1 11.0
 [S.sub.15] 8.0 10.0
 [S.sub.30] 6.7 12.5

Year Amount Plant uptake
 fertiliser S[O.sub.4.sup.2-] from
 S applied below 0.5 m or
 S[O.sub.4.sup.2-]
 leached below
 0.5 [m.sup.B]

1996
 [S.sub.0] 1.6
 [S.sub.5] 5.1
 [S.sub.15] 2.3
 [S.sub.30] -0.4

1997
 [S.sub.0] 5.6
 [S.sub.5] 4.6
 [S.sub.15] 1.7
 [S.sub.30] 6.4

(A) Content of S in root was assumed to 25% of top S based on
similar ratio for N in wheat on a similar soil (Atwell et al. 2002).

(B) Negative value indicates leaching of S[O.sub.4.sup.2-] below the
soil depth of 0.5 m.

Table 8. Net S mineralisation (kg S/ha) measured for the fertiliser
treatments in 1996 and 1997

Treatment
 17.vi.-30.vii 30.vii.-28.viii

[S.sub.0] 0.3 -1.0
[S.sub.5] -0.9 -1.0
[S.sub.5] -1.5 0.2
[S.sub.30] 1.0 -0.4
l.s.d. (P=0.05) n.s. n.s.

Treatment Growing season 1996
 28.viii.-24.ix 24.ix.-22.x

[S.sub.0] 2.8 2.3
[S.sub.5] 2.7 2.4
[S.sub.5] 2.4 2.4
[S.sub.30] 2.7 2.0
l.s.d. (P=0.05) n.s. n.s.

Treatment Fallow
 22.x.-26.xi Total 27.ii.-27.v.1997

[S.sub.0] -0.6 3.8 -0.2
[S.sub.5] -1 2.2 0.2
[S.sub.5] -0.2 3.3 5.8
[S.sub.30] -0.3 5.0 5.5
l.s.d. (P=0.05) 0.60 n.s. 1.4

Treatment
 27.v.-27.vi 27.vi.-23.vii

[S.sub.0] 0 0.4
[S.sub.5] -0.2 0.8
[S.sub.5] 0.6 0.6
[S.sub.30] -0.3 0.7
l.s.d. (P=0.05) n.s. n.s.

 Growing season 1997
 23.vii.-20.viii 20.viii.-18.ix

[S.sub.0] -0.3 1.6
[S.sub.5] 0.0 2.2
[S.sub.5] 1.6 2.4
[S.sub.30] 0.9 2.1
l.s.d. (P=0.05) n.s. n.s.

 Total
 18.ix.-24.xi Total 17.vi.96-24.xi.97

[S.sub.0] 1.8 3.5 7.1
[S.sub.5] 2.3 5.1 7.5
[S.sub.5] 2.8 8.0 17.1
[S.sub.30] 3.3 6.7 17.2
l.s.d. (P=0.05) 1.3 n.s. 3.2

n.s., Not significant
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Author:Anderson, G.C.; Fillery, I.R.P.; Ripper, F.H.; Leach, B.J.
Publication:Australian Journal of Soil Research
Geographic Code:8AUST
Date:Mar 15, 2006
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