Responses of carbon cycling and soil organic carbon content to nitrogen addition in grasslands globally

Understanding how N addition status (i.e., duration, rate, and form of N addition) impacts carbon (C) cycling has great implications for C storage prediction and grassland management. We examined 257 studies related to C cycling in grasslands and obtained a dataset of 1073 observations for meta-analysis. N addition significantly increased plant C input (plant above-ground biomass +49.1%, below-ground biomass +16.6%; litterfall biomass +17.4%; dissolved organic carbon +16.4%) across natural grasslands globally. For C loss processes, N addition enhanced litter decomposition by 9.2% and decreased soil respiration by 1.1%. In topsoil, microbial biomass C was reduced by 8.7% and soil organic C (SOC) content was increased by 3.1%. For C fluxes, N addition increased gross primary productivity and ecosystem respiration by 24.7% and 11.7% respectively, leading to negative net ecosystem exchange. These results indicated that grasslands were C sinks under N addition. Besides climate, the duration of N addition was the most important factor affecting C cycling. The response of SOC to N addition increased with time but weakened after a decade, associated with accumulative effects of N-induced soil acidification. N addition at any rate or form increased SOC content in temperate grassland/meadow while the effects in other grasslands depended on N addition status. There were higher SOC contents and the lowest net ecosystem exchange at low rates of N addition. Therefore, N addition should be limited to under 60 kg N ha−1 yr−1 to increase plant production and SOC content in grasslands globally and to maintain soil function as a C sink. More attention should be paid to the effects of the duration of N addition and soil acidification on C cycling to improve theoretical models or to help with grassland management practices and policies.