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.