LAUSR

ABSTRACT A 32-year field experiment was performed to examine the impact of applications of pig manure and mineral fertilizers alone or in combination on phosphorus (P) forms, transformations and availability in a Mollisol. The concentrations of total P, Olsen P, inorganic P (Pi) and organic P (Po) and their fractions were generally higher in treatments with than without mineral fertilizers and manure, and with high than with low manure rate. The proportions of Olsen P to total P, dicalcium bound P (Ca2-P) and octacalcium bound P (Ca8-P) to Pi and labile Po (LPo) to Po were significantly higher in manured than in unmanured treatments. The direct path coefficient of Ca2-P on Olsen P (1.63) was larger than that of other P fractions on Olsen P (–0.87 to 0.24), and the indirect path coefficients of all P fractions on Olsen P via Ca2-P (1.50 to 1.62) were greater than those through other P fractions (–0.86 to 0.24). The results suggested that manure promotes accumulation of Ca2-P and LPo, and mineral fertilizers combined with high-rate manure are especially effective for improving P availability. Contributions of other P fractions on Olsen P are mainly via their indirect effects for Ca2-P. Abbreviations: CK: control; NPK: nitrogen, phosphorus and potassium fertilizers; M1: pig manure at low rate; M2: pig manure at high rate; TP: total phosphorus; Pi: inorganic phosphorus; Po: organic phosphorus; Ca2-P: dicalcium bound phosphorus; Ca8-P: octacalcium bound phosphorus; Ca10-P: phosphorite bound phosphorus; Al-P: aluminum bound phosphorus; Fe-P: iron bound phosphorus; O-P: occluded iron/aluminum bound phosphorus; LPo: labile organic phosphorus; MLPo: moderately labile organic phosphorus; MRPo: moderately resistant organic phosphorus; HRPo: highly resistant organic phosphorus; PAC: phosphorus activation coefficient; SOC: soil organic carbon; TN: total nitrogen; PCA: principal component analysis.