Effects of long-term phosphorus fertilization and winter cover cropping on soil phosphorus transformations in less weathered soil

Information concerning sources and sinks of available P in soil is needed to improve soil P management and protect water quality. This study, conducted from 1989 to 1998 on a Sultan silt loam soil (Aquantic Xerochrept), determined the annual P removal rate by corn (Zea mays L.) and P transformation as affected by P rate and winter cover cropping. Treatments included two P rates ( 0 and 44 kg P ha(-1)) applied to corn at planting each year. All cover crops received 19.6 kg P ha(-1) at seeding each fall. Also included was a control without any cover crop and with no P addition. Corn yield and P uptake were affected by P fertilizer additions, but not by cover crops. A fairly constant amount of P was supplied from indigenous soil P when no external P was added. When the amount of P added exceeded that removed by corn, the excess P was converted mainly to NaOH-extractable inorganic P (NaOH-P-i). When the amount of P applied was below that removed by corn, indigenous soil NaOH-P-i acted as a source of available P for the plant. With no reduction of organic P (P-o) extractable by NaOH or NaHCO3, the contribution from P-o to the available P pool appeared limited. The role of NaOH-P-i in P availability in the soil was substantiated by its significant correlation with labile NH4Cl-extractable P ( NH4Cl-P; r(2) = 0.60, P <0.001) or NaHCO3-P-i ( r(2) = 0.81, P <0.001) pools. The NaOH-P-i for the soil reflected the changes in soil P resulting from past fertilizer P input and P removal by the crops.