The Effect of Soil Water Status on Fertilizer, Topsoil and Subsoil Phosphorus

Transcription

1 1 2 The Effect of Soil Water Status on Fertilizer, Topsoil and Subsoil Phosphorus Utilisation by Wheat 3 4 T.M. McBeath *, M.J. McLaughlin, J.K. Kirby and R.D. Armstrong T.M. McBeath, M.J. McLaughlin, School of Agriculture, Food and Wine, The University of Adelaide, PMB1, Waite Research Institute, Glen Osmond, SA 5064, Australia M.J. McLaughlin and J.K. Kirby, CSIRO Sustainable Agriculture Flagship, CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia R.D. Armstrong, Grains Innovation Park, Department of Primary Industries, PMB 260, Horsham, VIC, 3400, Australia. T.M. McBeath current address: CSIRO Ecosystem Sciences, PMB 2, Glen Osmond, SA 5064, Australia * Corresponding Author; therese.mcbeath@csiro.au Phone: ACKNOWLEDGEMENTS Funding for this work was from South Australian Grains Industry Trust and Australian Research Council (LP ). Thanks to Minnipa Agriculture Centre and the Hunt family for providing land and watering for EP sites and to the Loller, Obst and Schober families for providing land for Mallee sites. Thanks to Bill Davoren, Anthony Whitbread, Rick Llewllyn, Glenn McDonald and Bill Bovill for collaboration in the Mallee region. We gratefully acknowledge Colin Rivers and Caroline Johnston for technical support, Sean Mason for DGT analyses and Erik 1

2 25 26 Smolders and Yibing Ma for discussions about the topsoil-subsoil experimental design. 27 2

3 ABSTRACT Crop phosphorus (P) comprises uptake from both fertilizer and from P throughout the soil profile, and is controlled by both soil and climatic factors. An accurate knowledge of how season and soil type can govern the source (fertiliser or soil) of crop P is important for understanding how to best manage this nutrient. The effect of above and below average rainfall on crop fertilizer P uptake was measured using radioisotopes of P at seven sites in Southern Australia. At three of these sites a dual isotopic technique (using 32 P and 33 P) was used to distinguish between uptake of P by wheat from the fertilizer, topsoil (0-15 cm) and subsoil (below 15 cm). The amount of P fertilizer used by wheat increased with increasing rainfall but was not directly related to whether the soil was initially deficient or sufficient in P. When sufficient P was present in the subsoil, the use of subsoil P increased with the addition of P fertilizer to topsoil, suggesting that the P fertilizer stimulated root growth into the subsoil. 42 3

4 43 44 INTRODUCTION A recent decadal drought in southern Australia, coinciding with the fertilizer price spike of 2008 and the likely increase in P fertilizer prices over time due to the expense of extraction (Cornish, 2010), has ignited debate about best practice use of the fertilizer resources (McLaughlin et al., 2011). There has also been an increasing trend towards less tillage in southern Australian cropping systems with > 60% of paddocks recently described as no-tillage (Llewellyn and D'Emden, 2010). This move to no tillage systems has reduced soil mixing and therefore increased stratification of both the fresh and residual P fertilizer in a zone banded at approximately 5 cm below the soil surface (Wright et al., 2007). The banded fertilizer P in the topsoil is prone to surface drying, reducing the availability of P to crops under conditions of drought (Cornish, 1987; Moody et al., 2010). Consequently, the efficiency of P fertilizer use (Singh et al., 1977; Strong and Barry, 1980) and the relationship between the relative proportion of crop P derived from the fertilizer, topsoil (here defined as the top 10-15cm) and subsoil (Cornish and Myers, 1977; Dunbabin et al., 2008; Kuhlmann and Baumgartel, 1991; Mannikar and Subbiah, 1972; Pinkerton and Simpson, 1986; Shierlaw and Alston, 1984) is influenced by both soil fertility and seasonal soil moisture conditions Soil P uptake by crops comprises P derived from freshly applied fertilizer, P from previously applied fertilizer and native soil P reserves of mineral and biological origin (McLaughlin et al., 2011). In larger scale field trials, the efficiency of freshly applied P can be measured using indirect P difference methods where a control of no added 4

5 P is compared with added P treatments, and a range of trials measuring P fertilizer efficiency using the indirect method are reviewed by Syers et al. (2008). This indirect measurement is susceptible to interference from other plant and soil factors (disease, soil type change etc.) and a lack of response does not mean that the fertilizer did not contribute P to the crop. The P difference methods do not provide a direct measure of the contribution of P to plants from background soil or fertilizer sources. The very few experiments that have measured P fertilizer use from alkaline soils directly using isotopes suggest that it is in the order of less than 12% of fertilizer P freshly applied, and most of the crop P uptake was from residual soil P (Dorahy et al., 2008; McLaughlin et al., 1988). However, this freshly applied fertilizer is available to subsequent crops with decreasing availability over time (Alessi and Power, 1980; Barrow, 1973; Strong et al., 1997) In this study, we used radioisotopes of P to directly measure freshly applied P fertilizer efficiency at seven field sites across the Mallee and Eyre Peninsula cropping regions of southern Australia. We compared the effect of decile two (lowest 20% rainfall of all seasons) and decile seven (highest 30% rainfall of all seasons) simulated rainfall on single-year fertilizer efficiency by growing plants in the field under rainout shelters and irrigating to the required level. This technique allowed for the determination of total P uptake in plants (growth stage head in the boot), and a fertilizer efficiency value (or amount of fertilizer used in year added). In addition, at three of the seven field sites, we further examined the relative contribution of topsoil and subsoil to crop P uptake and P fertilizer efficiency in response to wet and dry conditions using a double labelling ( 33 P and 32 P) methodology. This experiment allowed us to test the hypothesis that in dry conditions a plant might push more roots 5

6 93 94 into the subsoil and access nutrients from deeper in the profile, due to the inaccessibility of nutrients in the dry topsoil MATERIALS AND METHODS Soil Collection and Chemical Properties There were seven field sites selected for this study (Table 1). The seven field sites were Wanbi (Site 1), Langhorne Creek (Site 2), Halidon (Site 3), Minnipa (Site 4), Wharminda (Site 5) and Karoonda (two soil types, sites 6 and 7), in the low rainfall cropping zone of South Australia. These soil types ranged from neutral to alkaline ph and P deficient (Sites 1 and 2) to sufficient (remaining sites) using the diffusive gradient in thin film phosphorus soil test (C DGT-P ) critical value (Table 1). The soils were collected at 0-10 cm depth, air-dried and sieved at <2 mm prior to use. Selected soil physical and chemical properties are shown in Table Soil ph (H 2 O) and electrical conductivity (EC) were measured in a 1:5 soil: solution suspension (Rayment and Higginson, 1992). Calcium carbonate content was determined according to the procedure of Martin and Reeve (1955), particle size analysis according to the method of the US Department of Agriculture (1982) and total organic carbon (TOC) according to the method of Matejovic (1997). Cation exchange capacity (CEC) was measured using method 15E1 of Rayment and Higginson (1992). The field capacity (-10 kpa) and wilting point (-1500 kpa) were measured at each site and down the soil profile according to the methodology of Klute (1986)

7 The DGT technique was used to measure C DGT-P with a mixed binding layer containing 0.8 mm diffusive layer and a protective filter membrane as outlined by Mason et al. (2010). Bicarbonate-extractable P was determined according to the method of Colwell (1963) using 0.5 M sodium bicarbonate (ph 8.5) as the extractant and P-concentration was determined colorimetrically (Murphy and Riley 1962). The phosphorus buffering index (PBI) was measured according to Burkitt et al. (2002) and the approach of Moody (2007) was used to calculate the critical Colwell concentration to identify whether the soil Colwell P status was deficient or sufficient. Both Colwell and DGT soil test P values are presented as only the Colwell test is currently commercially available in Australia Fertilizer Efficiency Experiment The uptake of P fertilizer from soils was measured directly in wheat (Triticum aestivum cv. Axe) using a radioisotope tracer method, under wet (decile seven, the top 30% of all growing season rainfall records) and dry (decile two, the bottom 20% of all records) growing season conditions at all seven sites (decile figures for each site are given in Table 2). The wheat cultivar Axe was selected for this study because it has a short growing season. A short growing season variety was required due to the rapid decay of the 32 P radioisotope (t 1/ days) that limited the length of the experiment to three months. It was expected that Axe would be near completion of the P uptake phase of the growth cycle within three months (root uptake of P tends to be limited from flowering onwards) (Nayakekorala and Taylor, 1990; Romer and Schilling, 1986)

8 Soils were sampled (0 10 cm depth) from each field trial site and returned to the laboratory where they were dried at 40 C and then sieved <2 mm. The sieved soils were then packed into 10-cm diameter polyvinyl chloride (PVC) cores at a density of 1.36 g cm -3 to a total dry soil weight of 1.6 kg Two phosphoric acid fertilizer spikes containing a radioactive tracer were prepared by adding 1.1 MBq ml P for cores where only fertilizer efficiency was measured (Sites 1, 2, 5 and 7), and 3 MBq ml P for cores in the topsoil-subsoil experiment (Sites 3, 4 and 6). A 1mL aliquot of radioactive P fertilizer was added 3 cm below the soil surface in a band, at a rate equivalent to 15 kg P ha -1 (surface area basis). A no added P fertilizer control was included and all treatments received 20 kg N ha -1 as urea and 2.5 kg Zn ha -1 as Zn sulfate in a separate band at the time of P fertilizer addition. Soils were wet to 50% of field capacity (ideal sowing moisture) during the packing process. The field capacity of each soil is listed in Table 1. Three pregerminated Axe wheat seeds were then sown in each core ready for transportation to the field site the following day At each field site, plots (3 m by 15 m) was sown conventionally with wheat (Triticum aestivum cv. Axe) to allow establishment of a buffer of plants. Following sowing of the buffer zone, a 3 m by 2 m rain-out shelter was erected following the specifications outlined in Burk and Dalgliesh (2008). The rain-out shelter was split in two with a plastic lined channel placed across the middle to prevent water moving between the irrigation treatments. One end of the shelter was irrigated to decile 2, and the other to decile 7 growing conditions (Table 2). The decile 7 end of the shelter was selected based on the direction of prevailing weather (most rainfall events in the Southern 8

9 Australian winter come from the south to south west direction) to prevent rainfall wetting up the decile 2 treatment. The amount of equivalent rainfall irrigated at each site is outlined in Table 2. Cores were watered individually and the bulk area under the rain-out shelter was watered according to the same level of equivalent rainfall. In the first week of June the prepared cores with a root permeable membrane at the base were planted in the soil under the rain-out shelter in a completely randomised design containing 4 replicates within each irrigation area. One week after planting the emerged wheat seedlings were thinned to one plant per core. Sites 1, 3, 6 and 7 received a further 50 kg N ha -1 at Zadoks 30 (late tillering (Zadoks et al., 1974)) due to some yellowing of leaves The wheat plants were harvested at Zadoks 47 (head in the boot (Zadoks et al., 1974)) by cutting the stems at the soil surface. The plants were dried at 70 C for 72 h and dry weights were recorded. The plants were ground using a coffee grinder, digested in concentrated nitric acid (Aristar) at 40 C for 12 h and 140 C for 3 h and analysed for total P concentrations using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) according to the procedure of Zarcinas et al. (1987) The 32 P or 33 P activity in digest solutions were determined by pipetting a subsample of 1 ml of each digest solution, 1 ml of deionised water and 10 ml of National Diagnostics Ecoscint A (Georgia, USA) scintillant into 15mL glass scintillation vials. The 32 P activities in digest filtrates were measured using a Rackbeta II Wallac Liquid Scintillation Counter. All counts were corrected for decay and dilution. The specific activity (SA) of P in the added P fertilizer was calculated as; SA (kbq mgp -1 ) = Equation 1 9

10 Where, fertilizer activity is the amount of 32 P radioactivity in the added fertilizer (kbq core -1 ). The percentage (%) of P derived from the fertilizer in plants was calculated according to Equation 2, P in plants from fertilizer (mg P core -1 ) = And according to Equation 3, Fertilizer recovery (% of P added) = This methodology to determine fertilizer efficiency was similar to the approach developed by (Bertrand et al., 2006). Topsoil-Subsoil Experiment A double spike P radioactive procedure using radioactive 32 P and 33 P was used to measure the contribution of fertilizer P, topsoil and subsoil P to plant nutrition at three of the field sites (Sites 3, 4 and 6). The phosphoric acid fluid fertilizer was labelled with 32 P to measure fertilizer efficiency (3 MBq core -1 ) as described above, and topsoil was uniformly labelled with 33 P as outlined below to determine wheat uptake of background P from topsoil and subsoil. The experimental design comprised 3 sites, 2 watering treatments, 2 fertilizer treatments and a treatment of confined and unconfined cores as explained below to determine P in wheat plants from subsoil, in a balanced factorial design with 4 replicates In order to measure the individual contribution of topsoil and subsoil P to plant nutrition an isotopic dilution technique was developed where the topsoil of cores was labelled with carrier free 33 P at 1.3 MBq kg -1 soil To label the soil, the soil was tumbled in a plastic lined cement mixer while the spray nozzle of a battery operated garden sprayer was inserted in a small opening and sprayed continuously to ensure 10

11 even homogenous labelling of the soil. The soils were wet to 50% of field capacity (Table 1) using deionised water. After the mixing procedure, the soils were allowed to equilibrate at room temperature for 10 days prior to initiation of the experiment. In order to measure the contribution of subsoil P to plant nutrition an extra treatment was established where a 2 µm mesh fabric was used to provide a physical barrier to roots growing into subsoil allowing the plant roots to only access topsoil P (confined). The subsoil P uptake was determined using a similar procedure developed by Shierlaw and Alston (1984) using the difference between the specific activity of the confined and unconfined cores. The radioactivity of 32 P and 33 P in digested wheat plants was determined using a dual scintillation counting procedure (McBeath et al. 2008). The specific activity (SA) of P for each radioactive P isotope in the plants was calculated as; SA plant (kbq mg -1 ) = Equation 4 Where plant activity is the amount of 32 P or 33 P radioactivity in plants (kbq core -1 ). The P in plants from subsoil was calculated according to the following equation: P in plant from subsoil (mg P core -1 ) = ) Equation 5 where, SA plant is the specific activity in plants using 33 P activity in unconfined and confined cores. The P in plants from topsoil (mg P core -1 ) can then be calculated using the following equation: P in plants from topsoil (mg P core -1 ) = total P in plant (mg P core -1 ) - P in plant from subsoil (mg P core -1 ) - P in plant derived from the fertilizer (mg P core -1 ) Equation 6 11

12 P in the plant derived from the 32 P fertilizer was calculated using Equation 2. An assumption of this technique is that the plant roots do not have the capacity to solubilise significant amounts of sparingly soluble forms of P when grown in the confined cores Following sampling, each core was tested for soil P concentrations using the DGT procedure outlined above. Furthermore, when each core was removed, a core subsample and samples from below the core were removed at 10 cm intervals for testing of P status using a resin procedure (McLaughlin et al., 1994) Statistical Analysis Analysis of variance (ANOVA) was undertaken using the Genstat V.13 statistical package. The design used was a treatment structure of fertilizer x rainfall x site with a block structure of replicates/site. Least significant difference (LSD) between treatments was determined at the 5% level of significance using Fisher s Protected LSD RESULTS The topsoils of the seven selected sites were all neutral to alkaline ph with no salinity (EC) constraints on wheat growth (Table 1). All sites had low TOC (<1.1%) and CEC (< 14.2 cmol + kg -1 ) and only sites 1 and 4 contained detectable carbonate (6.1 % and 1.6%, respectively) and clay (8.1% and 14.4%, respectively) contents. Sites 1 and 2 had P deficiency according to the critical value for the DGT test (Mason et al., 2010) 12

13 (Table 1). None of the sites were P deficient according to the critical values derived from the measurement of Colwell P and the PBI (Moody, 2007). Insert Table 1 here Due to the high in-season rainfall causing lateral flow and subsoil wetting under the rain-out shelter in 2010 (decile 9-10 growing season), the amount of water applied as irrigation at all sites was reduced below the calculated amount for Decile 2 and Decile 7 rainfall. This adjustment was made using weekly soil moisture measurements (data not shown) to guide the amount of water to apply, and the amount applied is shown in Table 2. There was less soil water (a greater deficit) at harvest than there was at sowing at all sites with this deficit greater in most instances for the low rainfall (decile 2) treatment (Table 2). Insert Table 2 here While extra rainfall increased plant dry weight at each site (3.23 g core -1 for decile 2 and 3.78 g core -1 for decile 7, P<0.05, LSD 0.36 g), the interaction of site x fertilizer x rainfall was not significant, and therefore plant dry weight is presented as the average of the two rainfall treatments (Figure 1). The addition of P increased dry weight at sites 1, 2, and 5. Phosphorus application reduced shoot dry weight at site 7; the reason for which is unknown. Insert Figure 1 here At sites 2, 5 and 6, adding P fertilizer increased crop P uptake for both rainfall treatments, but shoot P uptake was greatest in the decile 7 treatment (Figure 2). At sites 1, 3 and 4, P addition only increased P uptake in the higher rainfall (decile 7) treatment (Figure 2). Insert Figure 2 here 13

14 Fertilizer P efficiency ranged from 3-33% and was greater with increased rainfall at sites 1, 2, 3, 4 and 6. Of these sites, sites 1 and 2 also produced significant (P < 0.05) dry matter responses to P addition while sites 3, 4 and 6 did not (Figure 1, Figure 3). Insert Figure 3 here More soil P was utilized by plants in the higher rainfall treatment, especially when P fertilizer was applied (Figure 4). The interaction between rainfall x fertilizer x site was not significant (P > 0.05) while the interaction of site x fertilizer was significant (P< 0.001, LSD 62, Figure 4). As shown in Figure 5, the cores treated with P fertilizer and lower (decile 2) rainfall had significantly lower post-harvest C DGT values, presumably due to lower soil P removal by the plants. Insert Figures 4and 5 here At site 6 both fertilizer and subsoil P uptake was greater for the higher rainfall treatment suggesting that fertilizer addition has stimulated root growth and utilisation of subsoil P (Figure 4). In the absence of fertilizer, there was greater P uptake at all sites in the higher rainfall treatment but the majority of P uptake was derived from the topsoil. Subsoil P was a very significant contributor to crop P uptake at Site 6, the only site with significant concentrations of available P at depth (see below), with P uptake further stimulated by addition of P fertilizer to the topsoil. Insert Figure 6 here The post-harvest soil profile resin P values suggest that more soil P was utilised in the high rainfall fertilized treatments as the post-harvest resin P value was lower for decile 7 treatments at sites 3 and 6 in the top layer (Figure 6). It was especially the case for site 6 where subsoil P was of greater importance to crop P nutrition and the differences in profile resin P were also present at 25 cm depth. The convergence of 14

15 resin P values below 25 cm depth may reflect reduced differences in root growth and/or soil P uptake, or a lack of P supply at depth (Figure 6) DISCUSSION The efficiency by which wheat seedlings utilised freshly applied fertiliser ranged from 3 to 33% depending on the site; P fertiliser use efficiency was not necessarily related to the soil P status but was affected strongly by soil moisture status. Few studies have directly measured (via radioactive tracers) P fertiliser use efficiency under field conditions. In three different studies (including soils in the study presented here) using alkaline soils (Calcixerollic xerochrepts) with similar Colwell P status (20-28 mg P kg soil -1 ), Dorahy et al. (2008) found a very low efficiency of P fertilizer use of % in cotton crops, McLaughlin et al. (1988) found a P fertilizer efficiency for wheat of 12%, while in this study the P fertilizer efficiency of wheat was % (site 6 and 7). Direct measurement of fertilizer P use efficiency using isotope tracing or dilution generally gives % fertilizer recovery values (in crop) similar to those determined by the difference method (measuring P uptake in control and fertilized treatments (Syers et al., 2008), usually in the range 1-35% (Mattingly and Widdowson, 1958a; Mattingly and Widdowson, 1958b; Mitchell, 1957). However, it is evident here that crop P uptake from P sources in both the topsoil and subsoil are strongly affected by fertilization, so that the difference method does not provide the most accurate or true measure of fertilizer P use efficiency Rainfall (and therefore soil moisture) is the biggest determinant of crop growth and grain yield response of dryland cropping systems in southern Australia (Kirkegaard and Hunt, 2010). Crop nutrition, especially that of P in turn also has a significant influence on early vegetative growth (Batten et al., 1986). In this study in-season 15

16 (simulated) rainfall treatments had a major influence on the amount of P fertilizer used in the year applied as well as the use of P derived from both the topsoil and subsoil P. Other studies, conducted on alkaline cracking clay soils found that the residual value of previously applied fertilizer was linked to soil moisture in the growing season, with lower availability in seasons with low rainfall and with a greater effect on residual fertilizer than freshly applied fertilizer (Bolland, 1999; Strong and Barry, 1980; Strong et al., 1997). In the current study the Decile 2 treatment exhibited lower P uptake. In the presence of freshly applied fertilizer, the amount of P uptake from the topsoil (where residual fertilizer P is likely to be located, Figure 5) was reduced compared to P derived directly from the fertilizer, suggesting that this is the favoured source of P. Dunbabin et al. (2008) suggested that fertilizer placed in the topsoil stimulated root uptake of P in seasons of lower rainfall. This response was attributed to this part of the profile being kept moist from relatively small (< 5 mm) rainfall events that occur in such seasons but insufficient rainfall occurring to wet up soil deeper in the profile (> 50 mm depth). As fertilizer P in soil becomes less available with time (Alessi and Power, 1980; Barrow, 1973; Strong et al., 1997) and higher in-season rainfall increases the removal of freshly applied fertilizer P from the system, it is important that agronomic strategies for managing fertilizer P are altered accordingly. In seasons with higher rainfall, yield potential is similarly increased and with it, overall P demand by the crop. Phosphorus export from the cropping system (as grain) is also greatest during these seasons, so that in the long term sufficient P (as fertiliser) needs to be applied if overall soil fertility is to be maintained. However, in the short term (one season), P derived from fertiliser contributed only a small proportion of total P uptake by the crop, but this P had a major influence on the ability of the crop to access soil P (in both the topsoil and subsoil). 16

17 The most interesting outcome of this study was the stimulation of subsoil P uptake when P fertilizer was added in combination with the high rainfall treatment (Figure 4). We propose that the fertilizer P, combined with adequate soil water, stimulated the development of roots early in the growing season. These roots grew into the subsoil and accessed P reserves where these were available (Site 6). The implications of this finding require careful management in situations where subsoil P reserves are limited (Moody et al., 2010; Pinkerton and Simpson, 1986) and supplementary P fertilisation of the subsoil needs to be considered if continued use of these reserves is desired. There have been several studies showing the benefits of subsoil fertilisation (Adcock et al., 2006; Adcock et al., 2007; Cornish and Myers, 1977; Graham and Ascher, 1993; McBeath et al., 2010) but a more economic way to deliver these nutrients to the subsoil is desirable (Adcock et al., 2007; McLaughlin et al., 2011). Further to this, many Australian subsoils, especially alkaline soils such as used in the current study, contain a number of subsoil constraints including high boron, salinity and sodicity which will limit root growth and the supply of water (and nutrients) to the crop (Adcock et al., 2007; Dang et al., 2006). An assessment and understanding of subsoil constraints in the system is essential before it can be considered a reliable source of P nutrition Banding P fertilizer with seed is an important fertilizer management strategy in both dry seasons, where it is co-located with moisture arising from small rainfall events, and in wet seasons where it stimulates early root growth to enable the crop plant to better exploit fertility throughout the soil profile. Extraction of nutrients from the subsoil provides enormous potential for increased yield in wet seasons but requires 17

18 careful monitoring and maintenance of fertility to be a sustainable management practice REFERENCES Adcock D., Wilhelm N., McNeill A., Armstrong R. (2006) Subsoil amelioration on a sand-over-clay: Crop performance and residual yield benefits., Australian Agronomy Conference, Australian Society of Agronomy, Perth. Adcock D., McNeill A.M., McDonald G.K., Armstrong R.D. (2007) Subsoil constraints to crop production on neutral and alkaline soils in Southern Australia: a review of current knowledge and management strategies. Australian Journal of Experimental Agriculture 47: Alessi J., Power J.F. (1980) Effects of Banded and Residual Fertilizer Phosphorus on Dryland Spring Wheat Yield in the Northern Plains. Soil Science Society of America Journal 44: Barrow N.J. (1973) Relationship between a soil's ability to adsorb phosphate and the residual effectiveness of superphosphate. Australian Journal of Soil Research 11: Batten G.D., Wardlaw I.F., Aston M.J. (1986) Growth and distribution of phosphorus in wheat developed under various phosphorus and temperature regimes. Australian Journal of Agricultural Research 37: Bertrand I., McLaughlin M.J., Holloway R.E., Armstrong R.D., McBeath T.M. (2006) Changes in P bioavailability induced by the application of liquid and powder sources of P, N and Zn in alkaline soils. Nutrient Cycling in Agroecosystems 74: Bolland M.D.A. (1999) Decreases in Colwell bicarbonate soil test P in the years after addition of superphosphate, and the residual value of superphosphate 18

19 measured using plant yield and soil test P. Nutrient Cycling in Agroecosystems 54: Burk L., Dalgliesh N.P. (2008) Estimating plant available water capacity: A methodology, CSIRO, Canberra. Burkitt L.L., Moody P.W., Gourley C.J.P., Hannah M.C. (2002) A simple phosphorus buffering index for Australian soils. Australian Journal of Soil Research 40: Colwell J.D. (1963) The estimation of phosphorus fertilizer requirements of wheat in Southern New South Wales by soil analysis. Australian Journal of Experimental Agriculture 3: Cornish P.S. (1987) Effects of direct drilling on the phosphorus uptake and fertilizer requirements of wheat. Australian Journal of Agricultural Research 38: Cornish P.S. (2010) A postscript to "Peak P"- an agronomist's response to diminishing P reserves, in: H. Dove (Ed.), "Food Security from Sustainable Agriculture"Proceedings of 15th Agronomy Conference 2010, Australian Society of Agronomy, Lincoln, New Zealand. Cornish P.S., Myers L.F. (1977) Low pasture productivity of a sedimentary soil in relation to phosphate and water supply. Australian Journal of Experimental Agriculture 17: Dang Y.P., Dalal R.C., Routley R., Schwenke G.D., Daniells I. (2006) Subsoil constraints to grain production in the cropping soils of the north-eastern region of Australia: an overview. Australian Journal of Experimental Agriculture 46:

20 Dorahy C.G., Rochester I.J., Blair G.J., Till A.R. (2008) Phosphorus use-efficiency by cotton grown in an alkaline soil as determined using 32phosphorus and 33phosphorus radio-isotopes. Journal of Plant Nutrition 31: Dunbabin V.M., Armstrong R.D., Officer S.J. (2008) Identifying fertiliser management strategies to maximise nitrogen and phosphorus acquisition by wheat in two contrasting soils from Victoria. Australian Journal of Soil Research in press. Graham R.D., Ascher J.S. (1993) Nutritional limitations of subsoils, in: N. J. Barrow (Ed.), Plant Nutrition- From Genetic Engineering to Field Practice, Kluwer Academic Publishers. pp Kirkegaard J.A., Hunt J.R. (2010) Increasing productivity by matching farming system management and genotype in water-limited environments. Journal of Experimental Botany 61: Kuhlmann H., Baumgartel G. (1991) Potential importance of the subsoil for the P and Mg nutrition of wheat. Plant and Soil 137: Llewellyn R.S., D'Emden F.H. (2010) How much is enough: the steep final steps to extensive no-tillage cropping in Australia, in: H. Dove (Ed.), "Food Security from Sustainable Agriculture"Proceedings of 15th Agronomy Conference 2010, Australian Society of Agronomy, Lincoln, New Zealand. Mannikar N.D., Subbiah B.V. (1972) Root activity and uptake of subsoil phsophorus by maize (Zea mays L.) and bersem (Trifolium alexandrinum Juslen.) as influenced by phosphorus levels in surface layer using P 32 as a tracer. Indian Journal of Agricultural Science 42: Martin A.E., Reeve R. (1955) A rapid manometric method for determination of soil carbonate. Soil Science 79:

21 Mason S., McNeill A., McLaughlin M., Zhang H. (2010) Prediction of wheat response to an application of phosphorus under field conditions using diffusive gradients in thin-films (DGT) and extraction methods.. Plant and Soil 337: Matejovic I. (1997) Determination of carbon and nitrogen in samples of various soils by the dry combustion method. Communications in Soil Science and Plant Analysis 28: Mattingly G.E.G., Widdowson F.V. (1958a) Uptake of phosphorus from P32-labelled superphosphate by field crops. Part 1. Effects of simultaneous application of non-radioactive phosphorus fertilizers. Plant and Soil 9: Mattingly G.E.G., Widdowson F.V. (1958b) Uptake of phosphorus from P32-labelled superphosphate by field crops. Part II. Comparison of placed and broadcast applications to barley. Plant and Soil 10: McBeath T.M., Grant C.D., Murray R.S., Chittleborough D.J. (2010) Effects of subsoil amendments on soil physical properties, crop response, and soil water quality in a dry year. Australian Journal of Soil Research 48: McLaughlin M.J., Alston A.M., Martin J.K. (1988) Phosphorus Cycling in Wheat- Pasture Rotations. I. The Source of Phosphorus Taken up by Wheat. Australian Journal of Soil Research 26: McLaughlin M.J., Lancaster P.A., Sale P.G., Uren N.C., Peverill K.I. (1994) Comparison of cation/ anion exchange resin methods for multi-element testing of acidic soils. Plant and Soil 32: McLaughlin M.J., McBeath T.M., Smernik R.J., Stacey S.P., Ajiboye S., Guppy C. (2011) The chemical and biological nature of P-accumulation in agricultural soils: implications for fertiliser design. Plant and Soil in press. 21

22 Mitchell J. (1957) A review of tracer studies in Saskatchwan on the utilization of phosphates by grain crops. Journal of Soil Science 8: Moody P., Bell M., Klepper K., Lawrence D., Pu G. (2010) Implications of minimum till dryland cropping systems for diagnostic P and K soil tests, World Congress of Soil Science, Brisbane, Australia. Moody P.W. (2007) Interpretation of a Single-Point P Buffering Index for Adjusting Critical Levels of the Colwell Soil P Test. Australian Journal of Soil Research 45: Nayakekorala H., Taylor H.M. (1990) Phosphorus uptake rates of cotton roots at different growth stages from different soil layers. Plant and Soil 122: Peverill K.I., Sparrow L.A., Reuter D.J. (1999) (Ed.)^(Eds.) Methods of Soil Analysis-An Interpretation Manual, CSIRO Publishing, Collingwood, VIC. pp. Pages. Pinkerton A., Simpson J.R. (1986) Interactions of surface drying and subsurface nutrients affecting plant growth on acidic soil profiles from an old pasture. Australian Journal of Experimental Agriculture 26: Rayment G.E., Higginson F.R. (1992) Australian laboratory handbook of soil and water chemical methods Inkata Press, Melbourne. Romer R., Schilling G. (1986) Phosphorus requirements of the wheat plant in various stages of its life cycle. Plant and Soil 91: Shierlaw J., Alston A.M. (1984) Effect of soil compaction on root growth and uptake of phosphorus. Plant and Soil 77: Singh R., Chadha R.K., Verma H.N., Singh Y. (1977) Response of dryland wheat to phosphorus fertilizer as influenced by profile water storage and rainfall. Journal of Agricultural Science, Cambridge 88:

23 Strong W.M., Barry G. (1980) The availability fo soil and fertilizer phosphorus to wheat and rape at different water regimes. Australian Journal of Soil Research 18: Strong W.M., Best E.K., Cooper J.E. (1997) Phosphate fertiliser residues in wheatgrowing soils of the Western Downs, Queensland. Australian Journal of Soil Research 35: Syers J.K., Johnston A.E., Curtin D. (2008) Efficiency of soil and fertilizer phosphorus use. Reconciling changing concepts of soil phosphorus behaviour with agronomic information IPNI. USDA. (1982) Particle size analyse, in: S. C. Service (Ed.), Procedures for Collecting Soil Samples and Methods of Analysis for Soil Survey, Washington, D.C. Wright A.L., Hons F.M., Lemon R.G., McFarland M.L., Nichols R.L. (2007) Stratification of nutrients in soil for different tillage regimes and cotton rotations. Soil and Tillage Research 96: Zadoks J.C., Chang T.T., Konzak C.F. (1974) A decimal code for the growth stages of cereals. Weed Research 14: Zarcinas B.A., Cartwright B., Spouncer L.R. (1987) Nitric acid digestion and multielement analysis of plant material by inductively-coupled plasma spectrometry. Communications in Soil Science and Plant Analysis 18:

24 FIGURE CAPTIONS Figure 1. Dry weight response of Axe wheat plants to addition of P fertilizer (g core - 1 ). A column appended by a different letter is significantly different (site x P addition, P<0.001, LSD 0.99) Figure 2. Phosphorus uptake of Axe wheat plants (mg core -1 ) in response to addition of P fertilizer and rainfall simulating decile 2 and decile 7. Within a site columns appended by a different letter are significantly different (P addition x Rainfall, P<0.05, LSD 1.11) Figure 3. Plant recovery of P fertilizer added under simulated decile 2 and decile 7 expressed as fertilizer efficiency (% of P added in plant). Columns appended by a different letter are significantly different (site x rainfall, P <0.05, LSD 6) Figure 4. Plant P uptake (mg core -1 ) from fertilizer, subsoil and topsoil in response to P addition and simulated rainfall at A. Site 3, B. Site 4, C. Site 6. A component of a column appended to the left with a different letter is significantly different (P source x watering P<0.001, LSD 0.82 mg) Figure 5. Post-harvest topsoil P soil test values (CDGT-P (µg L -1 ). The interaction of site x rainfall x P addition was not significant while site x P addition was (P<0.001, 24

25 LSD 62) but the effect of decile 2 with P addition is illustrated by not averaging the effect of P addition across the two rainfall treatments Figure 6. Post-harvest soil resin P (mg kg -1 ) to 50 cm depth in response to soil profiles treated with P addition and simulated decile 2 and 7 rainfall at A. Site 3, B. Site 4 and C. Site

26 Table 1. Soil (0-15 cm) physical and chemical properties for each field site Site Wanbi Langhorne Halidon Minnipa Wharmind Karoonda Creek a Site No Soil Type Sandy Sand over Sand Sandy Coarse Deep Sand Loam over calcrete over Loam sand over Sand over calcrete clay calcrete clay ph 1: EC 1: TOC% CEC (cmol + kg -1 ) Carbonate 6.1 <0.2 < <0.2 <0.2 <0.2 (%) Clay (%) Field capacity (% w w -1 ) Wilting Point (% w w -1 ) Colwell P (mg kg -1 ) PBI Critical Colwell P (mg kg -1 ) Resin P (mg kg -1 ) C DGT-P * (μg L -1 ) OC, organic carbon, CEC, cation exchange capacity, CDGT-P Effective diffusive gradient in thin film phosphorus concentration. *Critical value for C DGT-P (diffusive gradient in thin films phosphorus soil test) is 60 μg L -1 (Mason et al., 2010).Critical value for EC affecting wheat growth is 0.34 ds m -1 (Peverill et al., 1999) 26

27 Table 2. Median growing season rainfall and decile 2 and 7 June to August rainfall from 100 year rainfall records, are compared with the experimental plant available water at sowing and irrigation levels used to create decile 2 and 7 conditions. This required significantly less irrigation than rainfall would apply due to decile 9-10 conditions in the surrounding field. The water deficit at the end of the experiment for the decile 2 and 7 treatment indicates that decile 2 was significantly drier than decile 7 in most cases Site No Median GSR (April- October) (mm) Decile Jun-Aug R (mm) Decile Jun-Aug R (mm) PAW at sowing (mm) Decile Jun-Aug I (mm) Decile Jun-Aug I (mm) Deficit (mm) Decile 2 Deficit (mm) Decile # Rainfall data is based on analysis of 100 yr rainfall records from the nearest Bureau of Meteorology weather station. GSR, growing season rainfall, R, rainfall, PAW, plant available water, I, irrigation, and the deficit is the difference between plant available water at sowing + irrigation and plant available water at harvest. 27

28 Plant Wt (g core -1 ) Minus P 7 a a Plus P 6 5 b 4 cd cd c 3 e ef e e de 2 1 g fg g g Figure Site 28

29 Plant P uptake (mg core -1 ) Minus P Decile 2 Plus P Decile c c b a d b c a c c b a c c b a c b b a c b b a Minus P Decile 7 Plus P Decile 7 a a a a Figure Site 29

30 Fertilizer Efficiency (% added P in plant) Decile 2 35 a Decile b bcd bc bcd bcd de cde ef 10 5 gh gh h fg gh Site Figure 3. 30

31 Plant P (mg core -1 ) Plant P (mg core -1 ) Plant P (mg core -1 ) Site 3 c a c d b de e de de de Decile 2 Minus P Decile 2 Plus P Decile 7 Minus P Decile 7 Plus P A a 16 Site 4 B c 10 a 8 6 b d a 4 b 2 0 d d cd cd Decile 2 Minus P Decile 2 Plus P Decile 7 Minus P Decile 7 Plus P 16 Site 6 C f c g a b ef de g d g Decile 2 Minus P Decile 2 Plus P Decile 7 Minus P Decile 7 Plus P fertilizer topsoil subsoil Figure 4. 31

32 Post-harvest topsoil CDGT-P (ug L -1 ) Minus P Decile 2 Plus P Decile 2 Minus P Decile 7 Plus P Decile LSD site x fert Figure Site 32

33 Soil Depth (cm) Soil Depth (cm) Soil Depth (cm) Site 3 Resin P (mg kg -1 ) A Site 4 Resin P (mg kg -1 ) B Site 6 Resin P (mg kg -1 ) C Figure 6. Decile 2 Minus P Decile 2 Plus P Decile 7 Minus P Decile 7 Plus P 33