Accumulation and phosphatase-lability of organic phosphorus in fertilised pasture soils

The accumulation and phosphatase-lability of organic P was investigated in soil taken from 3 pastures that had received contrasting fertiliser management over 8 years. The soils were either unfertilised or had received superphosphate either from 1994 to 1997 or applied annually to 2002. P-fertilisation led to an increase in both the inorganic and organic P content of the soils, but with differences in the distribution of organic P in various extractable pools. Fertilisation also affected the amount of organic P that was amenable to hydrolysis by a non-specific phosphatase. In particular, the amount of water-extractable organic P that was phosphatase-labile was greatest in soil that had received continuous fertiliser application. Despite improved phosphatase-lability of different organic P pools in the fertilised soils, transgenic Trifolium subterraneum L., which releases extracellular phytase, showed no consistent advantage in growth and P nutrition compared with either wild-type or azygous controls when grown in intact cores of soil. This indicates that organic P that accumulates with P-fertilisation is either not an effective substrate for transgenic plants that exude phytase or is equally available to transgenic and control plants.