Legumes reduce the effects of salt stress on co-existing grass.

Nitrogen (N) fixing legumes typically enhance the ability of coexisting non-N-fixing species to resist disease and drought, but whether legumes enhance their ability to resist salt stress remains unknown, restricting our ability to explore the potential of legumes to rehabilitate salt-affected ecosystems. We conducted a simulation experiment to examine whether and how legumes influence the response of coexisting grass to salt stress. We compared the effects of salt stress on the plant biomass, root cell viability, antioxidant enzyme activities, soil extracellular enzyme activities and microbial functional gene abundances associated with N and phosphorus (P) cycling between pure grass communities and legume-grass mixtures. We found that salt stress decreased grass biomass and the abundance of most N and P cycling genes in rhizosphere soils. However, these negative effects were smaller in legume-grass mixtures than in pure grass community. Additionally, salt stress increased the activities of soil N and P cycling enzymes, with greater positive effects observed in legume-grass mixtures than in the pure grass community. The structural equation modelling results showed that the most direct and indirect path coefficients of salt stress effects on biomass were smaller in legume-grass mixtures than in the pure grass community. Our findings provide direct evidence that legumes can reduce the negative impact of salt stress on co-existing grass community, highlighting the potential of including legumes with high N fixation abilities to restore salt-affected ecosystems.