Hairy Vetch Intercropping Attenuates Mycorrhizal Benefits to Walnut Growth and Soil Organic Carbon Sequestration via Glomalin

Intercropping is a prevalent soil management strategy within walnut orchards, while its impacts on the functionality of arbuscular mycorrhizal fungi (AMF) in walnuts (Juglans regia) remain unclear, especially concerning soil carbon (C) sequestration via glomalin-related soil protein (GRSP). This study aimed to explore the effects of inoculation with the AMF species Diversispora spurca and intercropping with hairy vetch (Vicia villosa) on walnut biomass accumulation, soil water-stable aggregate (WSA) stability, leaf and root C (C_leaf and C_root) content, soil organic carbon (SOC), GRSP, and GRSP-contained C (C_GRSP), in addition to the contribution rate of C_GRSP to SOC. The intercropping treatment significantly inhibited root mycorrhizal colonization rate, soil hyphal length, and spore density in AMF-inoculated walnut plants. Individual AMF inoculation, rather than individual intercropping, significantly promoted shoot and root biomass accumulation, WSA stability, SOC, C_leaf and C_root, the levels of purified easily extractable (EEG), difficultly extractable (DEG), and total GRSP (TG), as well as their C contents. The combination treatment (AMF inoculation + intercropping) displayed limited benefits, improving just WSA stability without yielding synergistic advantages over individual treatments. Arbuscular mycorrhizal fungal inoculation significantly increased C_GRSP, especially C_DEG, while individual intercropping resulted in a reduction of C_DEG. The combination treatment elevated both C_DEG and C_TG, albeit to a lesser extent than AMF alone. The contribution rates of C_EEG, C_DEG, and C_TG to SOC were 0.33% - 0.53%, 1.16% - 1.78%, and 1.49% - 2.31%, respectively. Although AMF inoculation significantly increased the contribution rates of C_DEG and C_TG to SOC, this effect was diminished when combined with intercropping. Notably, C_DEG, rather than C_EEG, exhibited a significantly positive correlation with SOC and WSA stability. The findings provide new insights into the mechanisms of SOC sequestration in walnuts grown in controlled environments and offer a theoretical basis for the application of AMF in walnut cultivation.