Effects of nitrogen addition on above-and belowground litter decomposition and nutrient dynamics in the litter-soil continuum in the temperate steppe of Inner Mongolia, China

Anthropogenic activities have increased nitrogen (N) inputs to grassland ecosystems, greatly affecting ecosystem carbon cycling. To investigate the litter decomposition and nutrient dynamics for different litter types under N addition, we conducted a 2-year litter bag experiment at N addition rates ranging from 0 to 25 g N m(-2) yr(-1). The decomposition rates (k) of Leymus chinensis and Stipa grandis were both highest in the 25 g N m(-2)yr(-1) treatment, whereas decomposition of mixed litter (aboveground) was highest in the 10 g N m(-2)yr(-1) (N10) treatment. The decomposition rate of all the aboveground litter was significantly positively correlated with the NO3-N:NH4+-N ratio. Mixed litter and mixed roots exhibited net C, N, and P release in all treatments. Nitrogen addition promoted N immobilization and slowed P release for S. grandis. For L. chinensis, net P immobilization occurred in most N-addition treatments except for N10 in the first year, followed by a net P release. Soil C:N and C:P ratios were significantly negatively correlated with the litter C:N and C:P ratios, respectively, of L. chinensis. This result may indicate that the N release and P immobilization during decomposition of L. chinensis litter were the main factors influencing the soil stoichiometric ratios.