Legumes’ positive effects on soil carbon magnify via microbial pathways in warm-arid grasslands

Legumes play a crucial role in nitrogen cycling; however, it is unclear how this effect is related to soil organic carbon dynamics in grassland across climatic gradients. Here, we investigate how legumes affect soil nitrogen levels and thus microbial necromass accumulation across a 2500 km climatic gradient spanning warm-arid to cold-humid grasslands, using nitrogen-fixing Caragana shrubs as indictors for legumes. We show that microbial necromass and its contribution to organic carbon were markedly lower in warm-arid than in cold-humid grasslands, associated with lower soil nitrogen availability in arid environments. The impacts of legumes on increasing soil nitrogen availability in warm-arid grasslands were greater than in cold-humid regions. This led to a 64–112% increase in microbial necromass under legumes relative to non-legumes in warm-arid regions, whereas a 6–29% increases in cold-humid regions, contributing to more pronounced soil carbon gains under legumes in arid areas. Our findings indicated microbial necromass as a key mediator linking legume-driven nitrogen enrichment to soil carbon storage, highlighting legumes’ potential to stabilize grassland carbon pools under climate-induced aridification in global drylands.