Impact of land use and nutrient addition on phosphatase activities and their relationships with organic phosphorus turnover in semi-arid grassland soils

Information on the relationships between phosphatase activities and organic phosphorus (P) turnover is fundamental to understanding soil P dynamics but remains poorly understood. An 8-year field study was conducted in a steppe and an abandoned cropland under semi-arid grasslands to explore the effects of nitrogen (N) and P additions on P composition in soil as determined by P-31 nuclear magnetic resonance (NMR) and associated phosphatase activities. Results showed that the phosphate monoester content, soil acid phosphomonoesterase, alkaline phosphomonoesterase, and phosphodiesterase activities were higher in the steppe than in the abandoned cropland soil. Nitrogen addition significantly suppressed phosphatase activities. Phosphorus addition significantly increased acid phosphomonoesterase, alkaline phosphomonoesterase, and phosphodiesterase activities in the steppe but significantly decreased them in the abandoned cropland. Structural equation modeling revealed that both phosphodiesterase and alkaline phosphomonoesterase activities showed significant negative effects on diesters and monoesters in the steppe, but there were no significant effects of phosphatase activities on organic P composition in the abandoned cropland. Our findings highlight the variation of dominant mechanisms involved in organic P turnover with land use change. Phosphorus deficiency in the steppe appeared to promote the production of phosphatases and the subsequent biochemical mineralization of organic P. While in the abandoned cropland, previous cultivation resulted in a proportionally greater loss of soil organic carbon than that of organic P, indicating that organic P was mineralized as a result of biological mineralization of organic matter.