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Australian Journal of Soil Research, 2004, 42, 89–98
Phosphorus auditing cannot account for all of the phosphorus applied to different pasture soils L. L. BurkittA,D, C. J. P. GourleyB, and P. W. G. SaleC A
B
Tasmanian Institute of Agricultural Research, PO Box 3523, Burnie, Tas. 7320, Australia. Department of Primary Industries, RMB 2460, Hazeldean Rd, Ellinbank, Vic. 3821, Australia. C Department of Agricultural Sciences, La Trobe University, Bundoora, Vic. 3086, Australia. D Corresponding author; email:
[email protected]
Abstract. Five field sites established in the high rainfall zone of southern Victoria were used to examine the downwards vertical movement of phosphorus (P) fertiliser on soils which ranged in P sorption capacity. Fertiliser was applied either as a single application of 280 kg P/ha at the beginning of the experiment (April 1998), or as 35 kg P/ha reapplied every 6 months (totalling 210 kg P/ha by the end of the third year). Soil cores were sampled in June 2001 to a depth of 40 cm, and soil at depths of 0–5, 5–10, 10–20, 20–30, and 30–40 cm was analysed for a range of soil properties and total P concentration. Total P concentration changed very little down the profile, indicating that there was minimal vertical movement of P fertiliser below the 10 cm layer of 5 pasture soils following the single application of 280 kg P/ha or 35 kg P/ha reapplied every 6 months. Soils with low to moderate surface P sorption capacity showed a trend for higher total P concentrations at depth. However, quantitative relationships between vertical P movement and soil properties at depth were poor. A P audit resulted in variable recovery of the applied P (45–128%) in the surface 40 cm at each of the 5 sites. Consistently low P recoveries were achieved at one site, where the surface soil had a high P sorption capacity. Some applied P may have bypassed the high P sorbing surface layers at this site through macropore flow and moved beyond the 40 cm sampling zone, or have been lost to surface runoff. These results question the usefulness of P audit or mass-balance methods for accounting for P movement in a pasture-based system, as spatial heterogeneity of soil properties, both horizontally and vertically, was high in the current study. SeLPRta. 0lLu.Bu3di2t5rnkgtoindifernetsoiltypes
Additional keywords: budget, mass-balance, reapplication, nutrient loss, vertical movement, sorption, buffering, fertiliser.
Introduction The loss of phosphorus (P) from agricultural land to water is a major concern, as it can lead to the eutrophication of surface water by stimulating toxic algal growth (Sharpley et al. 1994). Although P losses from soil to water in the form of surface runoff and erosion are well recognised, P movement down through the soil profile is likely to be limited by the soil’s capacity to remove P from soil solution through P sorption processes (Wild 1950; Sharpley et al. 2000). Phosphorus movement down the soil profile has been demonstrated in a range of soil types. This movement has been shown to occur in sandy soils, which have low P sorption capacity (Russell 1960; Lewis et al. 1981), in soils which are high in organic matter or poorly drained (Sims et al. 1998), in soils which have underground drainage (Heckrath et al. 1995) or significant macropore flow (Cox et al. 2000), and in soils that are saturated with P © CSIRO 2004
(Breeuwsma and Silva 1992). Phosphorus losses through these soils are of increasing concern. This is particularly so, given that recent studies have found P movement down the soil profile increases with increasing bicarbonate-extractable P concentration in the surface layers (Heckrath et al. 1995; Hesketh and Brookes 2000; McDowell and Sharpley 2001). It is essential that the potential for vertical movement of P be identified and managed accordingly, in order to improve the efficiency of P fertiliser use and reduce off-farm impacts from P losses. Methods of measuring P leaching are not always suitable for determining the vertical movement of P in the field. For example, the use of intact soil cores, isolated from the surrounding soil, allows the study of P leaching and transport mechanisms through relatively undisturbed soil. However, the surface area of soil that can be examined using these approaches is limited and may be difficult to replicate on a 10.1071/SR03025
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field scale. Field studies using plots where tile drains have been installed are limited by the range of different soil types that can be investigated. An alternative approach is to measure the movement of P fertiliser through the profile using a mass balance (Haynes and Williams 1993) or P audit approach. Such an approach involves measuring the total P content down the soil profile on fertilised and control plots. The measurement of total P is preferred in a P audit approach as this measure accounts for all P forms in soil including plant-available, sorbed, and organic P. Losses of P to animal product and redistribution are accounted for, enabling the amount of P recovered in the soil to be expressed as the percentage of the total amount of P applied as fertiliser. A P audit approach has been used in previous studies to show that considerable P movement occurs down the profile of lightly textured, sandy soils in Australia (Hingston 1959; Russell 1960; Ozanne et al. 1961; Lewis et al. 1987). These soils are problematic, as regular applications of P fertiliser are required to maintain the productivity of pasture species, but this leads to further P loss through the soil profile. Lewis et al. (1981), who studied 15 sandy soils, suggested that the soil’s capacity to sorb P in the surface 0–10 cm was highly related to P movement through the soil profile. Although P movement on these soils was measured under commercial rates of P fertiliser, few studies have compared the movement of P following standardised P fertiliser treatments, in soils which range in P sorption capacity. This paper describes the changes in total P concentration down the soil profile for 5 pasture soils, following a large single application of 280 kg P/ha or the reapplication of 35 kg P/ha every 6 months. The objective was to determine whether downward vertical movement of fertiliser P occurred in these field soils and whether such movement could be related to chemical and physical properties and P sorption characteristics of soil at different depths.
Table 1.
Soil texture Clay (%) pH(H2O) Organic C (g/kg) Olsen P (mg/kg) Colwell P (mg/kg) Oxalate-extr. Fe (mg/kg) Oxalate-extr. Al (mg/kg) PBCO&S (mg P/kg)A PBI+ColPB B
Soils and site management Five soil types (Sites 1, 2, 4, 5, and 6) were selected from a major field study established in April 1998 on 9 rainfed commercial grazing properties across the major grazing areas of southern Victoria (Burkitt et al. 2001). All sites were situated in zones where mean annual rainfall was >800 mm/year (40-year average). All sites had permanent pasture, comprising predominantly perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.), with sites both set-stocked or rotationally grazed by dairy or beef, with stocking rates varying between sites. Field sites were initially selected for their low to moderate P fertility in the 0–10 cm layer, with Olsen P (Olsen et al. 1954) concentrations 60 g/kg and >0.17 dS/m, respectively) in the surface soil layer of all sites and declined consistently with depth; organic C contents measured at the 30–40 cm depth were
