Long-term changes in land use influence phosphorus concentrations, speciation, and cycling within subtropical soils

While land conversion from native vegetation to agriculture influences the concentration, speciation, and cycling of soil phosphorus (P), the nature of these changes remain poorly understood. We collected surface soils (0-10 cm) from paired sites at three locations, comparing soil from native vegetation with adjacent soils converted for cropping, pasture, and plantation for up to 115 y. The extent of organic P loss upon land use change differed between the three soils, with conversion to cropping causing the largest decreased in soil organic P as well as phosphatase activity. Using solution P-31 nuclear magnetic resonance (NMR), it was found that cropping caused a pronounced decrease in organic P - both orthophosphate monoesters and orthophosphate diesters. Furthermore, cropping soils had a substantial reduction in the diester-P/monoester-P ratio, indicating the preferential degradation of the more labile orthophosphate diester-P upon conversion to cropping. Importantly, by subsequently converting cropping land to pasture or plantation, these adverse P-related changes could be reversed or at least halted. Synchrotron-based X-ray absorption near-edge structure (XANES) demonstrated that land use change had no pronounced impact on inorganic P, with sorbed P dominating in all treatments. Changes in land use also influenced bacterial diversity, with land use change effects being specific to the three soils. This study provides information on how land use changes alters P behaviour and cycling in soils, with this being important for ensuring the sustainable use of soils for food production.