CHANGES OF SOIL-PHOSPHORUS FRACTIONS UNDER LONG-TERM CORN MONOCULTURE

Corn (Zea mays L.) monoculture is a common management practice due to high production levels with economic and agronomic advantages. Phospho rus reactions in soil affect sustainable, economic corn management and environmental quality. Our objectives were to quantify changes in soi l P fractions as related to additions of manure and inorganic P with t ime using a modified Hedley fractionation scheme. A field experiment w as conducted on a Chicot sandy clay loam soil (fine-loamy, mixed, frig id Typic Hapludalf) from 1984 to 1993. With an annual rate of 60 kg P ha(-1) manure P plus 44 kg P ha(-1) inorganic P, labile inorganic P (P i; Bicarb-Pi) increased and moderately labile Pi (NaOH-Pi) decreased. A higher annual P rate of 132 kg P ha(-1) plus manure P increased soil Pi through Bicarb-Pi, NaOH-Pi, and stable Ca-bound Pi (HCl-R). Inorga nic P at either rate combined with manure P decreased labile Bicarb-Po fractions and increased moderately labile NaOH-Po, with the result th at total extractable soil organic P (Po) increased. Without manure, 44 kg P ha(-1), approximately equal to P removal by the crop, resulted i n no change in soil P fractions except for residual P (Res-P), which d ecreased. Soil Po and Pi levels increased continuously in soil fertili zed with 132 kg P ha(-1) yr(-1) inorganic P only. With the manure-inor ganic system, 86% of the Bicarb-Pi was supplied from added inorganic P , indicating reduced P fixation compared with the inorganic P system. The NaOH-Pi and Res-P were soil P sinks, with Res-P being supplied fro m both the NaOH-Po and HCl-Pi fractions. In contrast, NaOH-Pi in the i norganic P system was the major source for Bicarb-Pi.