Direct and indirect effects of nitrogen enrichment and grazing on grassland productivity through intraspecific trait variability

In the context of global change, the effects of livestock grazing and nitrogen (N) deposition on ecosystem structure and function of grasslands are not isolated, but simultaneous or even interactive. However, most studies on variations in plant functional traits and linkages to ecosystem function have focused on grazing or N enrichment alone. Few studies have combined these two factors to explore the role of intraspecific trait variability in community assembly and primary productivity, and their pathways on ANPP remain unclear. Here, we examined the effects of N addition and grazing on intra- and interspecific variations in plant functional traits in Inner Mongolia steppe based on two manipulative experiments with six N addition rates or grazing intensities over 6 years. The direct and indirect effects of N addition and grazing on ANPP were further analysed. Our results showed that N enrichment greatly enhanced intraspecific trait variability compared to grazing, which played an important role in maintaining species diversity and primary productivity. There was a negative linear relationship between species richness and intraspecific trait variance within community (T-IP/IC), indicating that N enrichment promoted intraspecific trait variability and increased interspecific niche width, leading to a decrease in species richness. ANPP was positively correlated with T-IP/IC values under N addition or grazing, suggesting that higher intraspecific trait variability was beneficial to maintaining grassland productivity. We also found that N addition had a direct positive effect on ANPP, and intraspecific trait variation was the secondary cause of ANPP variation; in contrast, grazing had an indirect negative effect on ANPP, mainly through species richness and interspecific trait variation. Synthesis and applications. Our findings have important implications for the restoration and management of semi-arid grasslands. Considering the offsetting effects of N addition and grazing on primary productivity, fertilization measures can be integrated into pasture management to quickly restore productivity. In the restoration of degraded grassland by artificial reseeding, the optimal seedling density related to intraspecific variation should be determined, and the forge species should be selected according to plant functional traits, taking into account the strategies of both conservative resource-use and grazing tolerance.