Nitrogen deposition increases global grassland N2O emission rates steeply: A meta-analysis

Nitrous oxide (N2O) emissions in grasslands contribute considerably to global warming and are influenced substantially by nitrogen deposition. However, the impacts of N deposition impact on soil N2O emission rates across global grassland remain unclear due to spatial heterogeneity. Here, we synthesized data from 34 published studies to clarify the response of N2O emission rates to N deposition across global grassland. The priming effect size of N deposition on global grassland N2O emission rate was 1.64 +/- 0.14 (95% confidence interval 1.36-1.92, P < 0.0001). Categorical variable analysis revealed that various N types influenced effect size significantly, in the orders of urine > slurry > dung > NH4SO4 > NH4NO3 > urea. Furthermore, explained heterogeneity analysis revealed that air temperature, nitrogen dose, precipitation and soil bulk density was positively associated with effect size. In addition, air temperature, altitude, pH and nitrogen dose were the major factors influencing effect size. Path analysis also indicated that N dose took a significant role on direct effect (P < 0.01), the indirect effects were relative higher of soil pH, air temperature and altitude through soil bulk. We provide a key insight that continuous increase in N deposition would significantly stimulate grasslands N2O emission. Controlling greenhouse effect and soil acidification would mitigate the priming effect availably.