Data from: 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.