Arbuscular mycorrhizal fungi hyphal density rather than diversity stimulates microbial necromass accumulation after long-term Robinia pseudoacacia plantations

Robinia pseudoacacia plantations are an effective strategy for preventing soil erosion, enhancing soil fertility, and stimulating carbon (C) sequestration in barren systems, supported by symbioses with arbuscular mycorrhizal (AM) fungi and rhizobia. However, the effects of AM fungal diversity and hyphal density on microbial necromass and SOC accumulation after long-term Robinia pseudoacacia plantations remain unclear. We hypothesize that increased AM fungal diversity and hyphal density after afforestation stimulate SOC formation by facilitating fungal and bacterial necromass C (FNC and BNC), glomalin (GRSP), and extracellular polymeric substances (EPS), with the contributions increasing as forest age. To test this hypothesis, microbial necromass, SOC, AM fungal diversity, and hyphal density were measured in surface soil (0–20 cm) and subsurface soil (20–40 cm) of Robinia pseudoacacia plantations aged 10, 15, 20, 35, and >50 years. Results showed that SOC accumulation was largely confined to surface soil, predominantly as mineral-associated organic C (MAOC). The content of FNC, GRSP, and EPS-polysaccharide in surface soil also increased with stand age, which is closely associated with MAOC. This emphasizes that long-term Robinia pseudoacacia plantations primarily stimulate C accumulation in surface soil, likely due to GRSP and EPS-polysaccharide aiding in the aggregation and protection of microbial necromass C. Although the AM fungal diversity in surface soil decreased with stand age, the hyphal density increased alongside root biomass. The increase in hyphal density could facilitate FNC and EPS formation, thereby contributing to the MAOC and SOC accumulation. In contrast, the content of SOC and microbial necromass C in subsurface soil showed an absence of differences or was reduced. Overall, this study reveals that microbial necromass and SOC accumulation after Robinia pseudoacacia plantations occur in surface soil, with AM fungal hyphal density and associated exudates, rather than AM fungal diversity, serving as key predictors.