Effects of repeated manure and fertilizer phosphorus additions on soil phosphorus dynamics under a soybean-wheat rotation

Soil P availability and efficiency of applied P may be improved through an understanding of soil P dynamics in relation to management practices in a c ropping system. Our objectives in this study were to evaluate changes in pl ant-available (Olsen)P and in different inorganic P (P-i) and organic P (P- o) fractions in soil as related to repeated additions of manure and fertili zer P under a soybean-wheat rotation. A field experiment on a Typic Haplust ert was conducted from 1992 to 1995 wherein the annual treatments included four rates of fertilizer P (0, 11, 22 and 44 kg ha(-1) applied to both soyb ean and wheat) in the absence and presence of 16 t ha(-1) of manure (applie d to soybean only). With regular application of fertilizer P to each crop t he level of Olsen P increased significantly and linearly through the years in both manured and unmanured plots. The mean P balance required to raise O lsen P by 1 mg kg(-1) was 17.9 kg ha(-1) of fertilizer P in un manured plot s and 5.6 kg ha(-1) of manure plus fertilizer P in manured plots. The relat ive sizes of labile [NaHCO3-extractable P-i (NaHCO3-P-i) and NaHCO3-extract able P-o (NaHCO3-P-o)], moderately labile [NaOH-extractable P-i (NaOH-P-i) and NaOH-extractable P-o (NaOH-P-o)] and stable [HCl-extractable P (HCl-P) and H2SO4/H2O2-extractable P (resisual-P)] P pools were in a 1:2.9:7.6 rati o. Application of fertilizer P and manure significantly increased NaHCO3-P- i and -P-o and NaOH-P-i, and -P-o fractions and also total P. However, HCl- P and residual-P were not affected. The changes in NaHCO3-P-i, NaOH-P-i and NaOH-P-o fractions were significantly correlated with the apparent P balan ce and were thought to represent biologically dynamic soil P and act as maj or sources and sinks of plant-available P.