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Tillage system affects phosphorus form and depth distribution in three contrasting Victorian soils.

Introduction

Tillage and crop rotations can significantly influence the productivity and sustainability of modern farming systems. Tillage has been shown to affect the physical and biological processes in soils such as water conservation, microbial activity, and earthworm population (Haines and Uren 1990; Li et al. 2007). Research on conservation tillage (zero- and minimum tillage) in farming systems has illustrated the greater opportunity to increase soil organic carbon (SOC), microbial biomass carbon, total nitrogen (N), and extractable phosphorus (P) due to accumulation of crop residues at the soil surface compared with conventional tillage (Carter and Steed 1992; Salinas-Garcia et al. 2001; Heenan et al. 2004; Mathers et al. 2007). The accumulation of SOC under conservation tillage has led to changes in crop productivity and nutrient cycling, especially the stratification of SOC, extractable P, and other nutrients in the soil (Diaz-Zorita and Grove 2002). Similarly, crop rotation plays an important role in SOC dynamics; continuous cropping increased biomass production, which in turn increased SOC and total N compared with rotations containing a period of fallow (Sherrod et al. 2003).

Soil P dynamics within the profile is affected by crop rotation, stubble management, and tillage practices, in part reflecting changes in the organic P (Po) pools (Bunemann et al. 2006). Zero-till practices increased the labile P fraction concomitantly with the increase in SOC in the surface layer of a non-cracking clay soil. In contrast, this fraction tended to accumulate in deeper layers in a cracking soil due to the downward movement of P through the open cracks (Muukkonen et al. 2007). The accumulation of labile P in the surface of silt loam soils was probably due to the greater mineralisation of Po compounds under a chisel plough system which incorporated crop residues into the soil (Andraski et al. 2003).

Robson and Taylor (1987) identified that tillage practice can alter nutrients via 3 processes: (i) mixing nutrients through the soil and altering their 'availability' to the crop; (ii) changing the soil physical environment, e.g. bulk density, which in turn influences root growth and function; and (iii) affecting soil biological activity. For example, long-term zero-tillage produced significantly higher concentrations of soil P in the soil surface (0-0.05 m), whereas P levels were decreased at the 0.05-0.15 m depth compared with chisel ploughing (Andraski et al. 2003). The accumulation of P in the surface soil under zero-tillage was attributed to the lack of physical disturbance that mixes fertiliser P thoroughly through the plough layer (Selles et al. 1997). Plant-available P preferentially accumulated in the soil surface of an acid soil under direct drilling compared with conventional (mechanical) cultivation. However, the crop under direct drilling had low P uptake from soil and required more P fertiliser to obtain maximum grain yields compared with conventional tillage (Cornish 1987; Wang et al. 2007). The chemical nature of soil P is also affected by tillage practice, with P solubility being increased under conservation tillage, the increased P solubility being attributed to greater microbial activity during the decomposition of soil organic matter (Zibilske and Bradford 2003). Further, tillage practices which mix the topsoil would also mix the previously applied P, and thus improve the effectiveness of P fertiliser for subsequent crops (Bolland and Brennan 2006).

Despite major changes to the management of cropping systems of southern Australia in recent decades, such as the trend away from a pasture phase to continuous cropping and the adoption of reduced tillage practices, there has been little concurrent change in nutrient management strategies, especially involving P. Furthermore, little account is made of the effect of differences in agro-ecological zones when nutrient/ fertiliser recommendations are made, despite major differences in soil types, rainfall, and the range of crops grown. Potential changes in the vertical

distribution of P and the availability of this P to crops under different tillage practices and crop rotations, particularly in relation to the relatively dry seasonal conditions that characterise cropping in south-eastern Australia, may have significant practical implications for fertiliser management (White 1990; Wang et al. 2007).

This study investigated the effects of tillage practice and crop rotations on the vertical stratification of nutrients in the soil profile and the distribution of soil P fractions in different soil types collected from 3 key agro-ecological grain production zones of Victoria. We hypothesised that the stratification of P fractions in soils would differ among soil types and tillage practices. We suggest that the effect of agronomic management practices on soil P dynamics may have significant implications for how P is managed in modem cropping systems of Victoria.

Materials and methods

Soil samples were collected from 3 long-term agronomic rotation trials that represent different soil types and agro-ecological zones in the grain-producing regions of Victoria. The samples were taken at various depths up to 0.4 m. Selected tillage treatments and crop systems from the 3 trials were used in this study and are summarised in Table 1. The depth of tillage was 0.1 m for conventional tillage and 0.02-0.07 m for other treatments, depending on the crop being sown.

Calcarosol

The field experiment was located at the Malice Research Station, Walpeup (35[degrees]08'S, 142[degrees]01'E, elevation 107m; long-term average annual rainfall 325mm) in the South Australia/ Victoria mallee agro-ecological zone (GRDC 1998). The site is flat (<2% slope). The soil type is a Calcarosol (Isbell 1996). The experiment was established in 1982 to compare long-term effects of rotation and fallow management practices on wheat production. The experiment was designed as a split-plot with 3 rotations as main plots and 2 fallow management treatments as subplots. The rotation treatments were wheat/pasture/fallow (WPF) over 3 years and wheat/pasture (WP) and wheat/fallow (WF) over 2 years. The 2 fallow management treatments were conventionally tilled and drilled fallow (CTCD) and a no-tilled fallow with direct drilling of the following crop (NTDD). Each treatment was replicated 3 times in a randomised block design with all phases of rotation represented each year. The WPF and WF rotations have a long fallow period (10-18 months) which commenced after the harvest of the previous crop in WF and in late winter or early spring (August-September) in WPF. The WP had a short fallow period (1-4 months) which commenced in midsummer and lasted to mid-autumn (January-April). In this study, soil samples were collected in November 2005 from WPF and WP rotations with direct drilling and conventional fallow. In CTCD, weeds were controlled by repeated tillage with a wide-tined implement (scarifier or chisel plough). In NTDD, weeds were controlled exclusively with herbicides.

A seeder fitted with a standard combine (0.175-m row spacing) with conventional wide points and harrow was used to sow the conventionally tilled treatments, while narrow pasture points and light harrows were used to sow the direct-drill treatments. Wheat (Triticicum aestivum cv. Millewa during 1982-1985, cv. Meering 1986-1996, cv. Ouyen 1997-2006) was sown at 60-70 kg/ha and superphosphate applied with the seed at 12kg/ha per crop. No nitrogenous fertilisers were applied. A medic pasture (Medicago truncatula) consisting of a 3:1 mixture of cultivars Jemalong and Paraggio was sown at a rate of 10 kg/ha on the pasture phase in the first cycle of the WP and WPF rotations. Medic pasture was maintained free of grass weeds with selective grass herbicides sprayed at the 3-leaf stage of medic pasture development. Pastures were slashed 2-3 times during the growing season to simulate the reduction in water use through the reduction of shoot biomass that would occur from grazing.

Vertosol

The field experiment was located in the Wimmera-Bordertown agro-ecological zone (GRDC 1998), at Longerenong in the Victorian Wimmera (36[degrees]40'S, 142[degrees]18'E, elevation 155m; average annual rainfall 420mm), and was established in 1998. The site is flat (<2% slope). The soil type is a Vertosol (Isbell 1996). The trial comprised 3 phases and was replicated 3 times. This design allowed comparison between 3- and 6-year phased treatments. Data collection focused on 3 tillage treatments: zero-tillage (ZT), reduced tillage (RT), and conventional tillage (CT) systems. The ZT treatment had been imposed on a continuous wheat/high fertiliser/reduced tillage treatment before harvest of the 2003 crop. CT comprised mechanical cultivations using a disc plough and scarifier/harrows as required after grain maturity. Stubble was burnt in the RT treatment if required (e.g. no burning occurred after a drought due to low stubble loads), whereas the stubble was physically incorporated in the CT treatment and retained on the surface in the ZT treatment. Soil samples were collected in July 2006 from the wheat phase of 4 treatments: ZT, RT, and CT canola/wheat/field pea rotation, as well as from the lucerne/ lucerne/lucerne/canola/wheat/field pea rotation to assess the effect of rotation on soil P dynamics. Phosphorus fertilisers were applied as mono-ammonium phosphate (MAP) to wheat and canola at 11.3 kg P/ha, as single superphosphate to lucerne at 9.9 kg P/ha in the first year only, and as 'Grain legume P' to field pea at 16 kg P/ha. The P fertilisers are banded with the seed (~0.05 m depth) at sowing to each of the cropping phases. Nitrogen fertilisers as MAP (banded) and urea (direct drilled) were applied to wheat and canola at 35 kgN/ha.

Chromosol

The field experiment was located in the NSW/Victorian Slopes agro-ecological zone (GRDC 1998) at Rutherglen in the Riverine Plains (36[degrees]06'S, 146[degrees]30'E, elevation 169m; average annual rainfall 650 mm). The slope at the site is approximately 1%. A gravel-filled subsurface interception drain 1 m deep surrounded the top and sides of the experimental site to help prevent interflow and surface runoff from upslope. The soil type is an eutrophic brown Chromosol (Isbell 1996). The experiment was established in 1981 to examine the long-term changes in soil properties (e.g. soil structure, biology, and chemistry) under direct drilling, stubble management (retention and burning) and CT. The 2 main treatments were direct drilled with stubble retention, and direct drill with stubble burn. Stubble from the preceding crop was left standing, mulched, or incorporated into the soil before sowing wheat. Stubble was burnt when fire restrictions were lifted in April each year. Soil samples were collected in May 2006 from a 2-year cropping rotation (faba bean/wheat) and a 3-year cropping rotation (wheat/faba bean/canola) under different tillage systems (direct drill, with either stubble retained or stubble burnt and CT with stubble burnt). The rate of P applied to each crop from 1980 to 1995 was 10 kg P/ha.year as single superphosphate. From 1996 to the present, the rate increased to 20 kg P/ha.year as either 'grain legume super' or triple superphosphate.

Soil sampling and analysis

Soil samples were collected from the 0-0.05, 0.05-0.1, 0.1-0.2, 0.2-0.3, and 0.3-0.4 m depths at all 3 sites but were not adjusted for possible differences in bulk density between the treatments. Following air drying, subsamples were ground to pass through a 0.5-mm screen for analysis. Soil P fractions were sequentially extracted using a modified version of Hedley et al. (1982), which is described in Table 2. Briefly, P was extracted by sequentially shaking 0.5 g of air-dried soil for 17 h in 30 mL of each of the following solutions: (i) deionised water with 2 strips of anion exchange resin (9 by 62 mm, converted to bicarbonate form); (ii) 0.1M NaOH; (iii) 0.5M [H.sub.2]S[O.sub.4]; and (iv) digestion in [H.sub.2]S[O.sub.4]+[H.sub.2][O.sub.2] for 10h at 360[degrees]C (Tiessen and Moir 1993). Inorganic P (Pi) in extracts was determined colourimetrically by the phospho-molybdate method (Murphy and Riley 1962). The total P (Pt) in extracts was determined by using ICP-AES (inductively coupled plasma-atomic emission spectrometry). The Po fraction was then calculated as the difference between Pt and Pi. To determine resin-Pi, the resin strips were placed in the water--soil suspension, shaken for 17 h, and removed and then placed into a clean 50-mL vial. Twenty mL of 0.5 M HCl was added to each vial prior to being shaken for 1 h. The suspension (after removing resin strips) was analysed for resin-Pi and resin Pt (using ICP-AES).

Soils were also analysed for pH (1:5 0.01 M Ca[Cl.sub.2]); total [Al.sub.2][O.sub.3], [Fe.sub.2][O.sub.3], and Si[O.sub.2] by X-ray fluorescence (XRF) analysis (Jones 1991); oxalate-extractable AI and Fe; and P buffering capacity (PBC) (Rayment and Higginson 1992). SOC and total nitrogen (N) were analysed using an Elementar Vario EL III (Elementar Analysensysteme GmbH, Germany), except that SOC of the Calcarosol samples was determined using the method of Walkley and Black (1934). PBC of the soil was determined after shaking with a range of equilibrating P solution (0-50mg P/L) for 17h. For each soil, P sorbed (mg/kg) was plotted on the y-axis against [log.sub.10]C (C is supernatant P concentration expressed as [micro]g P/L). PBC was determined as the slope of the straight line (Rayment and Higginson 1992).

Statistical analysis

Statistical analyses of data were performed by 1-way ANOVA for the effect of tillage/crop rotation at individual depths (see figures), and 2-way ANOVA for the effect of tillage/crop rotation and soil depths on P fractionations (see Table 4) using GENSTAT for Windows (8th edn) (GENSTAT Committee 2005). For figures, data were analysed to compare means using least significant difference (l.s.d.). Correlation analyses were also performed between P fractions and SOC and total N in soil.

Results

Soil characteristics

Soil from the 3 trial sites varied markedly in texture, with clay content in the surface 0.1 m ranging from 8% for the Calcarosol to 37% for the Vertosol, with the Chromosol intermediate at 20%. The Mallee Calcarosol had the lowest PBC, followed by the Vertosol and Chromosol. The PBC was not directly related to either [Al.sub.2][O.sub.3] or [Fe.sub.2][O.sub.3] content. The Vertosol had the highest contents of [Al.sub.2][O.sub.3] and [Fe.sub.2][O.sub.3] (12 and 5%, respectively), followed by the Chromosol (6 and 3%, respectively), and the Calcarosol had the lowest (3 and 1%, respectively) (Table 3).

Changes in oxalate-extractable Fe and Al content and pH throughout the profile of the 3 soils are shown in Fig. 1. The Vertosol had the highest concentration of oxalate-extractable Al (1.5-1.8g/kg). The Chromosol had higher oxalate-extractable Al in the soil surface (0-0.1 m) but lower oxalate-extractable Al than the Calcarosol below 0.1 m. In contrast, the Chromosol had the highest oxalate-extractable Fe (1.5-2.0 g/kg), followed by the Vertosol (1.0-1.3 g/kg), and the Calcarosol had the lowest (0.4-0.6g/kg). The oxalate-extractable Fe content in the Chromosol decreased with soil depth. The Vertosol (pH 7.0-7.1) and the Calcarosol (pH 6.8-7.6) were alkaline, whereas the Chromosol was more acidic with soil pH ranging from 5.3 to 5.7 throughout the profile (Fig. 1).

[FIGURE 1 OMITTED]

Effects of tillage and crop rotation on soil C and N

The distribution of SOC and N throughout the topsoil also varied markedly among soil types. Both SOC and total N of all 3 soils were highest in surface soil (0-0.05 m) and decreased with depth (Fig. 2). In general, SOC and N concentrations were lowest in the Calcarosol (2-4 mg C/g and 600 [micro]g N/g, respectively), and remained relatively constant with soil depth below 0.05 m. The Vertosol and the Chromosol had similar concentrations of SOC and N in the top 0.05 m (12 mg C/g and 1200 [micro]g N/g), whereas the Vertosol had greater SOC and N in the subsurface layers (Fig. 2).

In the Calcarosol, crop rotation significantly affected SOC and total N (P < 0.01) (Fig. 2). SOC was higher in the wheat/ pasture rotation than the wheat/pasture/fallow rotation in the upper layers (0-0.2 m), whereas tillage did not affect SOC. Total soil N was significantly greater (P < 0.05) in the 0-0.05 and 0.1-0.2 m layers of the wheat/pasture treatment than the wheat/ pasture/fallow treatment. Surface soil in the direct drilling treatments of the wheat/pasture/fallow treatment exhibited higher N content than conventional tillage.

In the Vertosol, SOC and N in the surface soil (0-0.1 m) were lower under conventional tillage than zero-tillage and reduced tillage in the wheat/canola/pasture rotation (P < 0.05, Fig. 2). Tillage practice generally did not have a significant effect on SOC or N below 0.1 m (Fig. 2).

In the Chromosol, SOC and N contents were not affected by tillage or burning except in the 0.05-0.1 m soil depth where wheat/faba bean rotation with conventional tillage and stubble burning had a greater SOC and the wheat/faba bean/canola rotation with direct drill and stubble retention had a lower N content than the other treatments (Fig. 2).

[FIGURE 2 OMITTED]

Effects of soil type and tillage on the distribution of P fractions

Tillage practice/crop rotation and depth significantly affected the labile and moderately labile P pools, and the recalcitrant residual P pool within the profile of the 3 soils. While the effects of tillage on the 3 Pi fractions, except NaOH-Pi in the Vertosol, mainly occurred in the topsoil, there was no interaction between tillage and depth on the residual P (Table 4). Most of the Po fractions were also affected by tillage and depth (Table 4).

In the Calcarosol (Fig. 3), Pi fractions were greatest in the upper soil layer (0-0.05 m) and decreased with depth. Resin-Pi was greatest in the top 0.05 m where it averaged 40 mg/kg and sharply decreased to very low concentrations (average 2-10 mg/kg) below 0.1 m. Likewise, the Pi extracted by NaOH (average 14 mg/kg) and [H.sub.2]S[O.sub.4] (average 38 mg/kg) was strongly stratified in the topsoil and decreased markedly with depth. In contrast, the Po fractions were generally greatest in the 0.05-0.1 m layer. The residual P fraction was distributed uniformly through the soil profile.

Inorganic P fractions were generally greater in the topsoil (0-0.1 m) of the direct drill treatment than the conventional tillage treatment (P < 0.05), irrespective of crop rotations. Tillage and crop rotation had little effect on resin-Pi and NaOH-Pi fractions in soil below 0.1 m. Crop rotations appeared to have a greater effect on Po fractions than did tillage, but these were not significant. The wheat/pasture rotation led to higher resin-Po and [H.sub.2]S[O.sub.4]-Po but lower NaOH-Po than wheat/pasture/fallow. The residual P fraction was significantly higher in wheat/pasture than wheat/pasture/fallow (Fig. 3) (P<0.05), reflecting the more regular application of P fertiliser (every second v. every third year).

In the Vertosol (Fig. 4), resin-Pi and [H.sub.2]S[O.sub.4]-Pi fractions were concentrated in the upper soil layers (0-0.1 m) and decreased substantially (P<0.05) with soil depth below this layer. The NaOH-Pi fraction was much lower than other Pi fractions but tended to increase with depth. The largest component of soil P extracted was [H.sub.2]S[O.sub.4]-Pi (average 128mg/kg at 0-0.05m). Organic P fractions also tended to accumulate in the plough layer. Resin-Po was highest in the 0.05-0.1 m layer with an average of 27 mg/kg. NaOH-Po was distributed uniformly in the upper 0.3 m layers and was higher than NaOH-Pi. [H.sub.2]S[O.sub.4]-Po was highest in the top layer and was much lower than the [H.sub.2]S[O.sub.4]-Pi fraction.

Under reduced- and zero-tillage, resin-Pi and [H.sub.2]S[O.sub.4]-Pi in the Vertosol were concentrated in the topsoil but there was no effect on NaOH-Pi compared with conventional tillage. Reduced tillage resulted in significantly higher concentrations of resin-Po than did zero-tillage and conventional tillage in surface soil (0-0.1 m), whereas tillage did not affect NaOH-Po content in the 0-0.3 m layers. In contrast, [H.sub.2]S[O.sub.4]-Po was significantly higher in zero-tillage than other tillage treatments (P<0.05). The inclusion of 3 seasons of lucerne prior to canola/wheat/ pasture (6-year rotation) with reduced tillage led to a marked decrease in [H.sub.2]S[O.sub.4]-Po in the top 0.2 m. The residual P fraction was significantly higher in zero-tillage than in reduced tillage and conventional tillage treatments (P < 0.05) in the 0.05-0.1 m soil depth (Fig. 4).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

In the Chromosol (Fig. 5), all P fractions tended to accumulate in the top 0.1m except [H.sub.2]S[O.sub.4]-Po which increased in the 0.1-0.3m layer. For example, resin-Pi averaged 48mg/kg in the top 0.1 m and was 77-84% higher than in depths greater than 0.1 m. Among the Pi fractions, NaOH-Pi was the largest P fraction (average 97mg/kg), followed by [H.sub.2]S[O.sub.4]-Pi (82 mg/kg) in the topsoil. Among the Po fractions, NaOH-Po was the largest Po fraction with an average of 129mg/kg at 0-0.05m. In comparison, the residual P fraction tended to be distributed uniformly throughout the profile.

[FIGURE 5 OMITTED]

Tillage practice had the greatest effect on the relative distribution of soil P in the different fractions in the topsoil (0-0.1 m). Conventional tillage associated with stubble burning produced increases of 16-34% in resin-Pi compared with other cultivation methods in the topsoil (Fig. 5). Similarly, NaOH-Pi and [H.sub.2]S[O.sub.4]-Pi were significantly higher in conventional tillage than other treatments at 0-0.05 m. Stubble retention decreased resin-Pi compared with stubble burning in the top 0.05 m only. Stubble burning resulted in a sharp increase of [H.sub.2]S[O.sub.4]-Pi compared with stubble retention also in the top 0.05 m. In contrast, the resin-Po fraction was unaffected by tillage method. Stubble retention increased NaOH-Po and decreased [H.sub.2]S[O.sub.4]-Po in comparison with stubble burning. Tillage and crop rotation did not affect the residual P fraction in the top 0.2 m, but direct drilling with stubble retention tended to have a higher residual P fraction in the soil depth 0.2-0.3m (P<0.05) (Fig. 5).

Total P

The concentration of Pt varied considerably between the 3 soil types but was always greatest in the top 0.1 m and decreased with depth (Figs 3-5, Table 4). In the topsoil (0-0.05 m), Pt was greatest in the Chromosol (Fig. 5, average 535mg/kg), followed by the Vertosol (Fig. 4, average 377mg/kg), with the Calcarosol having the lowest Pt concentration (Fig. 3, average of 262 mg/kg). Tillage practice significantly affected Pt in the topsoil of both the Vertosol and Calcarosol, where it was higher under reduced/zero-tillage than conventionally tilled treatments (Fig. 3). In the Chromosol, however, conventional tillage associated with stubble burning had a higher Pt than other cultivation methods. At soil depths greater than 0.2 m, neither tillage method nor crop rotation affected soil Pt in any soil type examined (Figs 3-5).

In the Calcarosol, SOC was positively correlated with resin-Pi, residual P, and Pt but negatively correlated with NaOH-Po. In contrast, SOC was negatively correlated with resin-Pi, NaOH-Pi, [H.sub.2]S[O.sub.4]-Pi, NaOH-Po, and Pt. The N content of the Calcarosol and the Vertosol did not have significant correlations with the forms of P in these soil types (Table 5).

Proportions of P fractions in surface soils

The P percentage of different fractions followed the same pattern as P concentrations of these fractions in the topsoil across all 3 soil types (Fig. 6). In the Calcarosol, Pi fractions were concentrated in the topsoil, especially the top 0.05 m (on average 35% of the total P). Resin-Pi was the largest Pi fraction, followed by [H.sub.2]S[O.sub.4]-Pi and NaOH-Pi. In the 0.05-0.1 m increment, the Pi fractions decreased compared with the Pi content in the top 0.05 m. In contrast, Po fractions were higher in the 0.05-0.1 m layer than in the top 0.05 m (Fig. 6).

In the Vertosol, Pi and Po fractions did not vary between 0-0.05 and 0.05-0.1 m depths (Fig. 6). The [H.sub.2]S[O.sub.4]-Pi (34%) was the largest fraction, followed by resin-Pi (13%), with NaOH-Pi accounting for only 1% of total P. Organic P fractions accounted only for 17% of the total soil P. Resin-Po was slightly higher than [H.sub.2]S[O.sub.4]-Po and NaOH-Po.

[FIGURE 6 OMITTED]

In the Chromosol, the Pi fractions at 0-0.05 and 0.05-0.1 m accounted for 42% and 36% of total P, respectively (Fig. 6). NaOH-Pi was the largest fraction (18%), followed by [H.sub.2]S[O.sub.4]-Pi (15%) and resin-Pi (9%). Total Po fractions did not change between 2 soil depths and accounted for approximately 32% of the total soil P. NaOH-Po was the largest fraction while [H.sub.2]S[O.sub.4]-Po was the smallest fraction of total P.

Discussion

Distribution of C and N in soil profiles

The amount and distribution of nutrients such as N, C, and P within the soil profile varied markedly between soil types, regardless of agronomic management, reflecting previous land use, climate, and physical processes such as shrinking and swelling that may occur in soils. In addition to this natural variation, agronomic management such as tillage method, stubble retention, and crop rotation also influenced the distribution and chemical form of P. Although the effect of agronomic management was generally small compared with the inherent underlying soil properties, small changes in both chemical form and relative distribution of P were potentially sufficient to influence crop nutrition. Soil type, tillage, and crop rotation also had significant impacts on the distribution of SOC and N in different Victorian agro-ecological zones, although the experimental design used did not permit soil type effects to be separated from environmental effects.

Soil organic C levels across the trials reflected a balance between inputs resulting from biomass production and losses such as decomposition. In the cropping systems examined in this study, biomass production was strongly linked to water (rainfall) and N supply (from both fertiliser and legume [N.sub.2] fixation). For example, in the Calcarosol, the greater SOC and N concentrations found in upper soil layers of the wheat/pasture rotation than in the wheat/pasture/fallow system reflected differences in dry matter production (Latta and O'Leary 2003). Although a fallow period can increase soil mineral N supply in the short-term (O'Leary and Connor 1997), longer term N supply would have relied on [N.sub.2]-fixation during the pasture phase as no fertiliser N was applied in this particular trial. Studies in other countries have also shown that soil C and N tend to be greater in more intensive cropping systems due to decreased the length of fallow, greater residue production, and different crop residues (Sherrod et al. 2003; Dou et al. 2007).

Differences in the frequency and intensity of tillage can strongly influence the distribution and availability of the soil nutrients studied. Conventional (mechanical) tillage incorporates crop residues throughout the upper soil layers (0-0.1 m). In contrast, conservational tillage systems (zero- and reduced-tillage) do not incorporate the crop residues, resulting in a higher concentration of soil nutrients and SOC in the soil surface, before decreasing sharply with depth (House et al. 1984; Dou et al. 2007). In the current study, conventional tillage produced lower concentrations of SOC and N than direct-drilling and zero-tillage in the Vertosol (Wimmera), but tillage practice had no significant effect on SOC or N in the Chromosol (high rainfall zone). This effect was surprising, particularly as the Mallee and high rainfall zone trials had run for >25 years, whereas the Wimmera trial had been in progress only 8 years at the time of sampling and the zero-tillage treatment in place <3 years. It is speculated that the much higher average rainfall experienced at the Chromosol site (>650 mm) than the Wimmera (<420 mm) might have resulted in increased rates of residue decomposition that overrode potential management effects. Conservation tillage, however, has been found to decrease the residue decomposition rate compared with conventional tillage (Lorenz and Lal 2005; Dou et al. 2007).

The pattern of vertical stratification of SOC and N within the profile varied markedly with soil types/region. Although SOC and N tended to concentrate in the soil surface (0-0.05 m) for all the soils, the Vertosol and the Chromosol had greater concentrations of C and N than the Calcarosol. This probably resulted from the greater biomass production, higher average rainfall, and finer texture in these 2 soils that resulted in increased organic matter in the topsoil. Furthermore, while SOC and N decreased sharply with depth in the Chromosol only, they decreased gradually in the Vertosol (Fig. 2). This is probably related to the vertic nature of the Vertosol, where large cracks develop following drying that can also produce a natural 'tillage' effect to depths >1 m under very dry conditions.

Nature of P across soil types

The pattern of P distribution in soil profiles, and the form of this P, varied significantly between the 3 different soil types. In this study, we assessed the chemical nature of soil P using a scheme originally based on a widely used method (Hedley et al. 1982) that sequentially extracts soil P using extractants of differing composition and increasing strength. As such, many of the fractions do not correspond to a precise chemical structure but the scheme does provide a broad indication of the nature of the soil P: organic v. inorganic and highly labile v. recalcitrant to plants. Very few studies have examined the nature of soil P in the major soils used for cropping in south-eastern Australia, and none to our knowledge have simultaneously compared alkaline (Calcarosol and Vertosol) and acid soils (Chromosol), which together produce >80% of Victoria's grain harvest. The Vertosol and highly alkaline Calcarosol soils of south-eastern Australia tend to have very high P concentrations and moderate P buffering capacities (Bertrand et al. 2003), although this was not the case in the current study where the Calcarosol had the lowest PBC and the acidic Chromosol the highest, which followed the same trend as total P. The decrease in PBC in a Calcarosol with an increase of P application rates in a previous study was probably due to the increased saturation of P sorption sites on soil after long-term P fertilisation (Vu et al. 2008).

The Calcarosol had a greater concentration of resin-Pi and [H.sub.2]S[O.sub.4]-Pi than NaOH-Pi in the top 0.1 m. The low concentration of NaOH-Pi in this soil is related to the lower concentrations of oxalate-Al and -Fe and the oxides [Al.sub.2][O.sub.3] and [Fe.sub.2][O.sub.3]. In general, NaOH-Pi is considered to be Pi strongly associated with Fe and Al compounds in soils (Perrott et al. 1989). Resin-Pi is regarded as readily available to plants, and concentrations between 11 and 28 mg/kg are generally sufficient for annual crops (Cantarella et al. 1998). The levels of resin-Pi in the top 0.05 m of all 3 soil types were >30mg/kg, reflecting a long history of fertiliser P application in all trials and indicating that none of the soils were likely to produce P deficiencies for cereal production.

Although the Vertosol had much greater oxalate-extractable Al and Fe and the oxides [Al.sub.2][O.sub.3] and [Fe.sub.2][O.sub.3] than the Calcarosol in this study (similar to that reported by Bertrand et al. 2003), only a small amount of the Pi was extracted with NaOH. In contrast, more Pi was extracted by [H.sub.2]S[O.sub.4]. The predominance of the [H.sub.2]S[O.sub.4]-Pi fraction in the Vertosol suggests that the applied P was eventually transformed to this fraction rather than the NaOH-Pi pool. Similarly, in a black Vertosol of south-eastern Queensland, Wang et al. (2007) also found that acid-extractable P (HCl-Pi) was the largest Pi pool. This suggests that in the sequential extraction method, acid extractants (i.e. [H.sub.2]S[O.sub.4], HCl) may be more effective in extracting Pi bound with sesquioxides than the NaOH extract. This also indicates that the transformation of applied P into the sparingly soluble pools might be dependent upon the soil type and rainfall, because in the 2 other soils, which were both high in oxalate-extractable Al and Fe (amorphous forms), the applied P should be transformed into both Al-P and Fe-P (Vu 2004). Alternatively, the predominance of acid-soluble Pi compared with alkali-soluble P may reflect the strong association of P with Ca in the Calcarosol.

In the Chromosol, the reverse trend was observed as more NaOH-Pi was extracted than resin-Pi, possibly due to the greater annual rainfall and/or the acid pH of this soil. In this soil, fertiliser P appeared to be preferably transformed into an inorganic P fraction which was more readily solubilised by NaOH. In a clay soil, Zhang et al. (2004) found that continuous P fertilisation increased the NaOH-Pi fraction at a slower rate in the first 6 years than in the following years, suggesting that in this clay soil the fertiliser P was first transformed into other Pi fractions at a higher rate than into NaOH-Pi. Like the NaOH-Pi fraction, NaOH-Po was the predominant Po fraction in this soil (approximately 30% of total P). In a Chromosol of South Australia, Dougherty et al. (2006) found a high organic P content, which accounted for 65% of the total soil P. The positive correlation between SOC (and N) with soil P fractions except resin-Po and [H.sub.2]S[O.sub.4]-Po in the Chromosol indicates the potential influence of biological processes on P dynamics in this soil.

Stratification of soil P fractions in 3 contrasting soils

Compared with soil N or SOC, relatively few Australian studies have examined the effect of tillage and rotation on soil P (with the exception of Bunemann et al. 2006), especially across different soil types and agro-ecological zones (e.g. Kirkegaard 1995). Although there was a pronounced effect of soil type on both the vertical distribution of P and its chemical availability to plants, agronomic management also significantly affected this distribution. The vertical distribution of P in the soil profile is potentially of great importance to plants, particularly in the dryland grain-producing regions of Australia, as most roots are located in the upper soil layers. As the soil surface is likely to undergo periods of drying during the growing season, especially in environments such as the Wimmera and Mallee, most of the P located in this dry soil may be positionally unavailable to plants (Cornish 1987; Wang et al. 2007; Dunbabin et al. 2008). However, some crop species may be able to offset this effect by using hydraulic lift of water in the subsoil into surface layers. Several studies in southern Australia (Gates et al. 1981; Cornish 1987; Fischer et al. 1988) have noted poorer P nutrition under reduced tillage practices, although some of these studies were on acid soils in southern New South Wales, which was probably related to the P-fixing capacity of the soils.

In the current study, there was a pronounced concentration of readily plant-available P fractions in the top 0.1 m soil depth, which declined rapidly with depth below the plough layer (0.1m). This effect was greatest on the Calcarosol at 0.1-0.2m soil depth (Fig. 3), the site with the lowest average rainfall (325 mm). However, tillage practice/rotation still produced differences in residual P as deep as 0.3-0.4m across all soil types, providing evidence of deep soil P use by crops (Wang et al. 2007). Previous research found a significant contribution of subsoil P to plant P uptake when the topsoil P was continuously depleted and/or naturally low (Pothuluri et al. 1986; Richards et al. 1995; Schwab et al. 2006; Wang et al. 2007). Leaching of P is highly unlikely on these moderately buffered clay subsoils in low to medium rainfall environments (Wang et al. 2007). Although reduction in tillage significantly increased the vertical stratification, available P levels (resin extractable) were more than adequate for crop growth at all sites.

In the Calcarosol, the higher residual P fraction in the 2-year (wheat/pasture) than the 3-year (wheat/pasture/fallow) rotation indicates that a significant proportion of the P applied as fertiliser tended to transform into plant-unavailable P forms. Interestingly, the effects of crop rotation on this residual P fraction were seen throughout the whole soil profile studied (0-0.4 m). The movement of applied P from topsoil to subsoil is unlikely due to the P-fixing nature of the soil and low rainfall. However, increased water storage in the fallow period may have enhanced root proliferation which, in turn, stimulated mobilisation of the residual P through rhizosphere acidification (Vu et al. 2008). In the Vertosol, the inclusion of lucerne (Medicago sativa L.) in the canola/wheat/pasture rotation resulted in decreased resin-Pi in the topsoil (Fig. 4), probably reflecting the less frequent application of P fertiliser (4 years out of 6) in this rotation compared with annual applications in the continuous cropping treatments. In the lucerne plots, biomass was slashed at least once annually (or greater depending on growth during the season) and left on the plot, creating a source of low C : N residues that may have facilitated mineralisation/immobilisation of soil P in the surface layer. In the Chromosol, where the faba bean pulse crop provided a potential source of N through [N.sub.2]-fixation, there was no evidence of a change in the P fractions (Fig. 5). Conventional tillage with stubble burning in the Chromosol significantly increased the amount of P in the inorganic fraction compared with stubble retention. In contrast, it was found that crop rotation and stubble management did not have significant effects on the dynamics of inorganic P fractions in a long-term field experiment with a similar acid soil type and environment (Chromic Luvisol) in southern New South Wales (NSW) (Bunemann et al. 2006). This discrepancy might be due to the P fertiliser management in the NSW study where P was applied to every crop at a rate of 20 kg/ha, resulting in the steady accumulation of inorganic P in all treatments. In addition, the significant cultivation method (tillage and crop rotation) x depth interaction for Pi fractions in the current study suggests that crop rotations and tillage practices affected the distribution of inorganic P pools throughout the soil profiles. The build-up of both Pi and Po fractions in the surface 0.1 m of soil indicates that this accumulation was, in part, attributable to the P fertiliser application on the surface soil regardless of the tillage method used.

Effects of tillage systems on P dynamics

The accumulation of crop residues in the surface soil induced by tillage methods was reported to be closely related to the plant-available pools of both Pi and Po (Zibilske et al. 2002; Bunemann et al. 2006), and the high accumulation of SOC in the surface soil by conservation tillage systems was closely related to the Po dynamics (Zibilske and Bradford 2003; Bunemann et al. 2006). Soil Po pools play an important role in P cycling and crop P nutrition upon mineralisation (Dalai 1977; McLaughlin et al. 1988; Frossard et al. 1995). Given the general increase in SOC under zero/reduced tillage, it was not the case in the Vertosol where SOC level was higher under conventional than other tillage practices in this present study.

Our study showed an increase in Pi fractions as well as Pt in the topsoil following direct drilling of the Calcarosol, zero- and reduced-tillage in the Vertosol, and conventional tillage in the Chromosol. These results are consistent with a previous study which showed that reduced- and zero-tillage practices increased total P in the topsoil by 15% compared with conventional tillage in an Oxisol (Selles et al. 1997). The small effects of tillage on P fractions below the plough layer are consistent with previous findings in a Calciortidic Haploxeralf (Martin-Rueda et al. 2007) and Udic Mollisol (Song et al. 2007).

The increase of Pi fractions but decrease of NaOH-Po fractions under conventional tillage in the 0.05-0.1 m increment of the Chromosol suggests that the effects of tillage or crop rotation varied with soil type and agro-ecosystem. For example, Bunemann et al. (2006) found that during 24 years of experimentation on a Chromic Luvisol, crop rotations and tillage methods did not significantly affect the dynamics of Pi, and the transfer of P to deeper soil layers was limited.

The stratification of soil Pi and Po fractions in soil profiles induced by conservation tillage may result in lower P-use efficiency, especially in semi-arid regions where rapid drying of soil surfaces due to high temperature and evaporation rates can occur. Greater rates of P fertilisers may be needed to maximise yields. Deep-banded P application into the subsoil is considered a solution to this problem. Such a practice has been shown to increase P-use efficiency, although this practice involves greater cost and time to the grower (Singh et al. 2006). However, the availability of soil P is closely related to soil water content, which is generally higher under conservation tillage practices and the use of long fallows (Latta and O'Leary 2003). We suggest that different management practices may be required for different soil types/agro-ecological zones. For example, in low rainfall regions such as the Mallee, deep placement of P rather than surface broadcasting would increase the P-use efficiency. Nevertheless, the crop response to deep-banded P application can differ with root architecture and ability of the root to extract P from the surface soil (Schwab et al. 2006). Therefore, P management will also need to account for the crop system and the extent of P stratification within the soil profile.

Conclusions

The proportions of Pi and Po fractions varied markedly among the 3 soils studied. In the Calcarosol, a greater proportion of total P was present as labile (resin-P) forms, whereas in the finer textured soils, P was preferentially transformed into sparingly soluble pools such as NaOH-Pi (Chromosol) and [H.sub.2]S[O.sub.4]-Pi (Vertosol). Tillage practice and crop rotation significantly affected the distribution of the labile and moderately labile Pi and Po pools. The accumulation of all soil P fractions in the topsoil under conservation tillage may limit plant access to P during dry seasonal conditions. Rapid drying of the soil surface may also result in immobilisation of soil P, thus effectively decreasing P-use efficiency and necessitating higher rates of P fertiliser to obtain full yield potentials in the dry year (Latta et al. 2008). This creates a quandary, in that incorporating crop residue and fertiliser into the soil through mechanical tillage in low rainfall areas may improve nutrient supply, but may also result in environmental degradation such as increased rates of erosion. There are also potential environmental consequences of increased P stratification in the soil surface which may result in greater P runoff.

Acknowledgements

We thank Mr Phil Newton, Mr Ron Sly, and Mr Roy Latta (DPI Victoria) for sampling the soils and Mr Joe Edward (La Trobe University) for technical support in P analysis. We acknowledge the Victorian Department of Primary Industries for providing access to the long-term sites and the Grains Research and Development Corporation for financial support through the Nutrient Management Initiative project UM00023.

Manuscript received 1 May 2008, accepted 18 August 2008

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D. T. Vu (A,C), C. Tang (A,D), and R. D. Armstrong (B)

(A) Department of Agricultural Sciences, La Trobe University, Bundoora, Vic. 3086, Australia.

(B) Department of Primary Industries, PMB 260 Horsham, Vic. 3401, Australia.

(C) Institute for Soils and Fertilisers, Hanoi, Vietnam.

(D) Corresponding author: Email: C.Tang@latrobe.edu.au
Table 1. Soil types and treatments chosen for the study

Soil types Crop rotation Tillage

Calcarosol Wheat/pasture/fallow Direct drill
 Wheat/pasture/fallow Conventional tillage
 Wheat/pasture Direct drill
 Wheat/pasture Conventional tillage

Vertosol Canola/wheat/peas Zero tillage
 Canola/wheat/peas Reduced tillage
 Canola/wheat/peas Conventional tillage
 Three lucerne/canola/ Reduced tillage
 wheat/peas (^)
Chromosol Wheat/faba bean Direct drill, stubble retention
 Wheat/faba bean/canola Direct drill, stubble retention
 Wheat/faba bean Direct drill, stubble burning
 Wheat/faba bean Conventional tillage,
 stubble burning

Soil types Crop rotation Abbreviation

Calcarosol Wheat/pasture/fallow WPF-DD
 Wheat/pasture/fallow WPF-CT
 Wheat/pasture WP-DD
 Wheat/pasture WP-CT

Vertosol Canola/wheat/peas CWP-ZT
 Canola/wheat/peas CWP-RT
 Canola/wheat/peas CWP-CT
 Three luceme/canola/ LCWP-RT
 wheat/peas (^)
Chromosol Wheat/faba bean WFa-DD-Ret
 Wheat/faba bean/canola WFaC-DD-Ret
 Wheat/faba bean WFa-DD-Brn
 Wheat/faba bean WFa-CT-Brn

(^) Lucerne/luceme/luceme/canola/wheat/peas.

Table 2. Phosphorus fractionation scheme and type of
extracted P

Extractant P fraction extracted

Resin strips in Labile P. Soluble and
 distilled water ion-exchangeable
 forms. Readily
 available P for plant
 roots (Kuo 1996)

0.1 M NaOH Pi more strongly
 associated with Fe
 and Al at mineral
 surfaces. Po more
 strongly associated
 with soil organic
 surfaces and humic
 and fulvic acids
 (Perrott et al. 1989)

0.5 M [H.sub.2]S[O.sub.4] Ca-associated Pi
 (apatite-P). Some
 occluded Pi and Po
 released on dissolution
 of sesquioxides
 (Tiessen and Moir 1993)

Digest with Residual P fraction. Most
 [H.sub.2]S[O.sub.4] + chemically stable
 [H.sub.2][O.sub.2] and insoluble forms
 of Pi and Po

Table 3. Selected properties of 3 contrasting soils (0-0.1 m)

 Si[O.sub.2] (A)
Soil Sand Silt Clay (%)

Calcarosol 91 1 8 91
Vertosol 39 24 37 63
Chromosol 53 27 20 84

 [Al.sub.2] [Fe.sub.2] PBC (B)
Soil [O.sub.3] (A) [O.sub.3] (A) (mg/kg)

Calcarosol 3 1 19
Vertosol 12 5 72
Chromosol 6 3 78

(A) Analysed by XRF (Jones 1991).

(B) P buffering capcacity (Rayment and Higginson 1992).

Table 4. Significance levels of the effects of tillage (and crop
rotation) and depth and their interactions on P fractions in a
Calcarosol, a Vertosol, and a Chromosol

 [H.sub.2]
 S[O.sub.4]-
 Resin-Pi NaOH-Pi Pi Residual P

 Calcarosol

Depth 0.001 0.001 0.001 0.001
Tillage 0.001 0.001 0.026 0.001
Depth x tillage 0.001 0.001 0.001 0.927

 Vertosol

Depth 0.001 0.001 0.001 0.001
Tillage 0.001 0.004 0.001 0.010
Depth x tillage 0.001 0.613 0.001 0.083

 Chromosol

Depth 0.001 0.001 0.001 0.001
Tillage 0.001 0.001 0.001 0.135
Depth x tillage 0.001 0.001 0.001 0.965

 [H.sub.2]
 S[O.sub.4]-
 Resin-Po NaOH-Po Po Total P

 Calcarosol

Depth 0.001 0.001 0.001 0.001
Tillage 0.086 0.029 0.008 0.001
Depth x tillage 0.810 0.252 0.002 0.001

 Vertosol

Depth 0.001 0.001 0.001 0.001
Tillage 0.001 0.202 0.001 0.001
Depth x tillage 0.001 0.395 0.001 0.001

 Chromosol

Depth 0.001 0.001 0.001 0.001
Tillage 0.098 0.001 0.002 0.117
Depth x tillage 0.079 0.001 0.001 0.008

Table 5. Correlation coefficients (r, P<0.05) between
P fractions and contents of organic C and total N in
the surface soil (0-0.1m) of the 3 soils

--, Not significant at P=0.05

 Calcarosol Vertosol

Variable Organic C N Organic C N

Resin-Pi 0.77 0.83 -- --
NaOH-Pi -- -- -- --
[H.sub.2]S
 [O.sub.4]-Pi -- -- -- --
Residual P 0.74 -- -- --
Resin-Po -- -- --
NaOH-Po -0.90 -0.75 -- --
[H.sub.2]S
 [O.sub.4]-Po -- -- -- --
Total P 0.68 0.62 -- --

 Chromosol

Variable Organic C N

Resin-Pi 0.68 0.50
NaOH-Pi 0.86 0.73
[H.sub.2]S
 [O.sub.4]-Pi 0.79 0.68
Residual P -- --
Resin-Po -- --
NaOH-Po 0.78 0.81
[H.sub.2]S
 [O.sub.4]-Po -0.50 -0.63
Total P 0.86 0.76
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Article Details
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Author:Vu, D.T.; Tang, C.; Armstrong, R.D.
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:8AUST
Date:Feb 1, 2009
Words:9873
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