Global pattern and drivers of nitrogen saturation threshold of grassland productivity

Ecosystem productivity usually exhibits first increase and then saturated response to increasing nitrogen (N) additions, yet the broad-scale pattern and potential drivers of the N saturation threshold are little investigated.

By synthesizing N addition experiments with at least four N-input levels from the global grasslands, we applied the quadratic-plus-plateau model to fit the aboveground net primary productivity (ANPP)－N rate relationship, and estimated the saturation threshold for N rate (critical N rate, NCR) and maximum ANPP (ANPPmax) from the inflection point where ANPP no longer statistically increased with N rate for individual experiments. Based on these estimations, we investigated the spatial pattern and driving factors of NCR and ANPPmax.

The mean NCR and ANPPmax were 15.0 and 477.0 g m-2 year-1, respectively, but varied greatly across single-site experiments. Management strategies (e.g., biomass harvest, different N forms and addition frequency) minimally influen...