Consistent responses of litter stoichiometry to N addition across different biological organization levels in a semi-arid grassland

Changes in litter chemistry and stoichiometry following N enrichment have important consequences on litter decomposition and plant-mediated nutrient cycling. Our knowledge about the responses of litter stoichiometry at different biological organization levels to N enrichment remains poorly understood. Moreover, whether the impacts of N enrichment on stoichiometric ratios in litter would depend on ecosystem management strategies remains unknown. We examined the effects of N addition and mowing on C:N:P stoichiometry in standing litter at plant organ, species, functional group, and community levels in a semi-arid grassland of northern China, taking advantage of a field experiment that has been running for seven years. Nitrogen addition altered biomass allocation between different organs and among different plant functional groups, with consequences on litter stoichiometry at community level. N addition had no impacts on litter C concentration, increased litter N, and N:P ratio, and decreased litter C:N ratio from plant organ to community. The impacts of N addition on litter P and C:P depended on the identity of plant species and functional group. Mowing did not affect litter nutrient characters across all organization levels. Furthermore, no interactive effects between N addition and mowing on litter nutrient characters were observed from plant organ to community levels. We conclude that N deposition will enhance litter quality even in the heavily-used grasslands with shifts in biomass allocation and species composition, which may contribute to the enhancement of plant-mediating nutrient cycling.