Direct recovery of 33 P-labelled fertiliser phosphorus in subterranean clover ( Trifolium subterraneum ) pastures under field conditions: The role of agronomic management

Abstract

Grazing systems are a major producer of food and fibre across the world. These systems often require the addition of fertiliser phosphorus (P) for maximum pasture growth, and it is now estimated that a four-fold increase in the use of P fertiliser in grasslands is needed to meet increased food demand by the year 2050. However, the recovery of P from fertiliser is often inefficient and global issues associated with P scarcity will continue to worsen. Knowledge on the uptake of fertiliser P by grasslands, including the effect of agronomic management, remains incomplete under field conditions. The aim of this study was to quantify the effects of soil P fertility (across three levels of soil P fertility), time of fertiliser application (at one level of soil P fertility), and placement of fertiliser (at one level of soil P fertility) on the growth and uptake of fertiliser P by clover pastures during a growing season. Subterranean clover (Trifolium subterraneum L.) monocultures established at two field sites in Australia were used to test the growth response to, and recovery of: (i) early-season (autumn) additions of fertiliser P to the soil surface at three levels of soil P fertility; (ii) mid-season (late winter) additions of fertiliser P to the soil surface; and (iii) early-season additions of fertiliser P placed 6 cm below the soil surface. Fertiliser P was applied to the pastures as single superphosphate that was labelled with a ³³P radiotracer to supply ~20 kg P ha-¹. Total herbage yield and recovery of fertiliser P by the clover pastures was generally highest when fertiliser P was applied to the soil surface early in the growing season and to soils maintained at the optimum level of soil P fertility for maximum pasture growth. An audit of the ³³P recovery of fertiliser P in the clover pasture revealed that up to 50% of the fertiliser P was recovered by the clover plant (shoots and roots), 5-15% remained in the fertiliser granule, and 20-25% was recovered in the 0-4 cm soil layer (largely as inorganic P) by the end of the growing season. We demonstrate that clover pastures are able to recover a relatively large proportion of surface applied fertiliser P during a growing season. Surface application is the simplest and most cost-effective strategy for management of fertiliser P in pastures.