Dissolved reactive phosphorus in a Calcaric Fluvisol as affected by the addition of agricultural wastes

Land application of organic wastes can result in the accumulation of available soil phosphorus (P) and in an increase risk of eutrophication of surface and shallow groundwaters. We conducted laboratory experiments to examine the effect of waste application on the concentration of dissolved reactive phosphorus (DRP) in 1:5 soil to 0.005 M CaCl(2) suspensions. Ten organic wastes, of which eight were mature composts, were applied to a P-rich Calcaric Fluvisol at rates ranging from 0 to 10% of soil by weight, and the difference in DRP concentration between suspensions containing waste and the untreated control (ADRP) was measured over a period of 300 days. In half of the suspensions of each waste-soil combination, 80 mg P (as KH(2)PO(4)) kg(-1) soil was applied at day 14. Values of ADRP were generally positive, but a significant number of negative values were also recorded for some wastes and application rates, particularly at later sampling times if inorganic P had been added. Regression analyses revealed that ADRP (i) increased with increasing soluble reactive P at all times and (ii) increased in the short term, but decreased in the long term with increasing water soluble organic carbon in the waste. The fact that ADRP was sometimes negative for some waste types suggests that application of organic waste to soil does not necessarily increase eutrophication risks caused by soil P losses.