Differences in Soil Nitrogen Availability and Transformation in Relation to Land Use in the Napahai Wetland, Southwest China

Changes in land use can affect soil nitrogen availability and transformation as well as atmospheric N2O concentration and global warming. The total nitrogen (N), NH4+-N, NO3−-N, net N mineralization, and denitrification of natural wetland, grazing meadow, and cropland soils in Napahai Wetland were measured to determine whether the conversion of natural wetland to grazing meadow or cropland significantly impacts soil N availability and transformation. The results showed that grazing meadow and cropland soils exhibited significantly lower total N and inorganic N concentrations than those of natural wetland soils. The net N mineralization rate was higher in the natural wetland soils than that in the grazing meadow soils. The net N mineralization rate of cropland soils was highest in spring and summer but was at its lowest in autumn. Soils of natural wetlands had higher denitrification rates than those of grazing meadows, while cropland soils had higher denitrification rates than those of natural wetlands. These data suggest that the conversion of natural wetland to grazing meadow substantially decreases total N and inorganic N concentrations, net N mineralization, net nitrification, and denitrification. Conversions of natural wetland to cropland could also decrease the total N and inorganic N concentrations throughout the growing seasons but increase both N mineralization, in spring and summer, and denitrification rates, in May, September, and November. Natural wetland soils have higher N availability and N transformation rates than those of grazing meadow soils and cropland soils, indicating that natural wetland would have the greater potential effect on climate change.