Effects of repeated swine manure applications on legacy phosphorus and phosphomonoesterase activities in a paddy soil

Swine manure application could affect soil legacy P and related transformation. Soil samples were taken for two rice seasons to uncover the responses of soil P fractions and phosphomonoesterase activities in a middle-term swine manure-applied rice field. The field experiment treatments included three swine manure organic amendment rates (26 [low], 39 [middle], and 52 kg P ha(-1) [high]) and two controls (no fertilizer and 26 kg P ha(-1) superphosphate). On seasonal average, the relative sizes of labile (H2O-P, NaHCO3-P-i, and NaHCO3-P-o), moderately labile (NaOH-P-i and NaOH-P-o), and stable (HCl-P and residual-P) P pools were in 1: 3.1: 3.6 and 1: 2.3: 3.7 ratios in 2011 and 2012, respectively. According to P fractions (except for residual-P) and phosphomonoesterase activities, treatments could be ranked as M-52 > M-39 > M-26 > P-26 > CK over the whole rice growth stages, but the difference between the 39 and 52 kg P ha(-1) manure rates was not significant. Most of P fractions showed the highest content at rice seedling stage for all treatments while phosphomonoesterase activities reached their maximums at rice tillering stage for all treatments. Moreover, repeated swine manure did not affect most of P fractions (except for NaOH-P-o) and phosphomonoesterase activities under the plow layer due to the mature plow pans. Our results suggest that the swine manure application should be kept at about 39 kg P ha(-1) to decrease potential environmental risks and to maintain soil legacy P in the paddy soil.