Dung decomposition in temperate dairy pastures - II. Contribution to plant-available soil phosphorus

Changes in dung and soil inorganic and organic P pools were measured in an experiment investigating the effects of dung on soil properties in 2 grazed dairy pasture systems ( low and high input). Total dung P and bicarbonate-extractable P pools were measured in decomposing pads and compared with the changes in inorganic and organic bicarbonate-extractable soil P, P sorption, and microbial P flush of soils beneath dung pads and control 'pads' where nothing was applied. Bicarbonate-extractable total P in the dung pads declined to the same level in both systems by the last sample date. Organic P comprised a similar proportion (36%) of the bicarbonate- extractable total P pool in dung from both low and high input systems. However, the transformations of dung P were different in each system, with the bicarbonate- extractable total P in the low input dung increasing by 25% before declining, and a greater decrease in percentage bicarbonate- extractable organic P in this system. Bicarbonate- extractable soil organic P did not vary under dung pads despite the large increases in Olsen and Colwell P observed. Soil Olsen P trends observed in the low and high input systems indicate a potential for leaching losses of P in high input systems compared with low. This increase in soil Olsen P was not due to decreased P sorption as dung did not reduce the sorption capacity of the soil. Microbial soil P increased under dung pads only after 60 days in this experiment. Neither the organic P nor microbial P pools contributed significantly to soil labile P pools in these temperate dairy pasture systems. The transformations of P thought to occur during the decomposition of dung are discussed.