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.