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 influenced both parameters. Structural equation models demonstrated that the spatial difference in NCR and ANPPmax were mainly explained by aridity index, and soil carbon (C)/N ratio availability also predicted the variation in NCR.

Given that grasslands are important not only for the trend and variability of the land C sink but also for the maintenance of pasture yield, the pattern and controls of NCR and ANPPmax, as revealed by the current study, are crucial for constructing robust predictions of C sink capacity and improving N fertilizer management in grasslands.