Grazing exclusion enhances plant and topsoil carbon stocks in arid and semiarid grasslands

Abstract

Grazing exclusion (GE) is widely considered to be an important strategy for restoring overgrazed grasslands and promoting carbon (C) storage. However, the changes in the components of ecosystem C with GE and their related drivers remain largely unexplored. Here, we investigated the effects of GE on the ecosystem C components (plant and soil C stocks) and their key driving factors through sampling inside and outside 15 grazing exclosures across the Inner Mongolia arid and semiarid grasslands in northern China. Our results showed that, except for dead root C, GE significantly promoted plant C stocks. The increase of C stocks in AGB and litter resulted from the increases of both biomass and C concentration, while the increase in live root C stock was mainly attributed to biomass accumulation. GE also promoted the topsoil (0–10 cm) C stocks but had little effect on the subsoil (10–30 cm) C stocks. Overall, GE had no significant effect on total ecosystem C stocks. Our results further indicated that across the grasslands, GE likely enhanced C accumulation in AGB, litter and live root in more humid and/or fertile sites. We also found that the increases of C stocks in AGB and litter were driven by the direct effects of GE and its indirect effects mediated by soil water content. The increases of C stocks in live root and topsoil with GE were linked to only high soil water contents. The generalizability of the soil water effect was confirmed by a meta-analysis which further revealed that across the grasslands of China, the responses of ecosystem C components to GE were associated with changes in soil water conditions. Therefore, our study highlighted the important role of soil water in regulating ecosystem C dynamics with GE at the national scale. Overall, our results showed that across arid and semiarid grasslands, GE is generally beneficial for plant and topsoil C accumulation. However, GE effects mediated by soil water on ecosystem C components may change to some extent due to altered soil water conditions under climate change.