Soil microbiome mediates plant community productivity in grass–legume mixtures

Background and aims: Arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria (NFB) can form symbiotic relationships with host plants, and this enhances nutrient uptake and plant performance of host plants. However, how AMF and NFB influence the complementarity and facilitation dynamics between grasses and legumes in grasslands is still not well understood. Methods: We conducted a greenhouse experiment to assess effects of different grass: legume ratios (GLRs) and microbial symbionts on plant production comprising two native C3 species from a semiarid steppe in northern China (grass (G): Leymus chinensis; legume (L): Medicago ruthenica). Different GLRs (G4L0: 4:0, G3L1: 3:1, G2L2: 2:2, G1L3: 1:3, G0L4: 0:4) were compared with or without AMF or NFB inoculation. Results: Grass–legume mixtures, especially G1L3, produced significantly higher biomass than monocultures via altering the community structure of soil microorganisms. AMF inoculation increased the availability of nutrients (mainly soil available P (SAP) and/or NO3−–N), which increased plant community productivity due to increased biomass in both grass and legume. Co-occurrence networks revealed that keystone genera (e.g., Arenimonas, Dominikia, Claroideoglomus and Scutellospora) were the primary factors influencing plant community productivity among grass–legume mixtures. Structural equation model showed that GLRs and inoculation types consistently produced overyielding directly and indirectly via affecting SAP and NO3−–N concentrations. Conclusion: Our findings highlight that suitable grass: legume mixture ratios (e.g., 1:3) positively affect productivity, and soil microorganisms that improve soil resource acquisition (SAP and NO3−–N), especially AMF, play an important role in grass: legume mixtures in this process.