Grass and forbs respond differently to nitrogen addition: a meta-analysis of global grassland ecosystems

Nitrogen (N) deposition has increased globally and has profoundly influenced the structure and function of grasslands. Previous studies have discussed how N addition affects aboveground biomass (AGB), but the effects of N addition on the AGB of different functional groups in grasslands remain unclear. We conducted a meta-analysis to identify the responses of AGB and the AGB of grasses (AGB_grass) and forbs (AGB_forb) to N addition across global grasslands. Our results showed that N addition significantly increased AGB and AGB_grass by 31 and 79%, respectively, but had no significant effect on AGB_forb. The effects of N addition on AGB and AGB_grass increased with increasing N addition rates, but which on AGB_forb decreased. Although study durations did not regulate the response ratio of N addition for AGB, which for AGB_grass increased and for AGB_forb decreased with increasing study durations. Furthermore, the N addition response ratios for AGB and AGB_grass increased more strongly when the mean annual precipitation (MAP) was 300-600 mm but decreased with an increase in the mean annual temperature (MAT). AGB_forb was only slightly affected by MAP and MAT. Our findings suggest that an acceleration of N deposition will increase grassland AGB by altering species composition.