Impacts of Spartina alterniflora invasion on fractions and fungal communities of mineral-associated organic carbon in subtropical coastal wetlands of China

Understanding the stabilization pathways of mineral-associated organic carbon (MAOC) under plant invasion is critical for predicting soil carbon dynamics in coastal wetlands. Spartina alterniflora , a widespread invasive species, exerts substantial ecological impacts, yet its influence on MAOC stabilization remains poorly understood. This study investigated the shifts in MAOC fractions，specifically iron and aluminum oxide-associated (MAOC Fe(Al)-OC ), calcium-associated MAOC (MAOC Ca-OC ), and residual MAOC (MAOC Residual-OC ), along with soil fungal communities before and after invasion at a subtropical estuarine wetland. Results show that invasion significantly increased MAOC Fe(Al)-OC and MAOC Ca-OC by 31% and 21%, respectively, while the chemically stable MAOC Residul-OC remained quantitatively dominant. FTIR analysis revealed an enrichment in MAOC aliphatic-OC functional groups, suggesting a shift toward more chemically diverse yet labile carbon inputs. Although fungal α-diversity remained unchanged, the invasion drove a distinct functional reconfiguration of the community, characterized by a transition from r- to K-strategies and the proliferation of Saprotroph. Random Forest and redundancy analyses (RDA) identified saprotrophs as pivotal biological regulators (explaining 55% of the variation), exhibiting an apparent ecological coupling with the responsive MAOC Ca-OC . These findings suggest that Spartina alterniflora invasion enhances carbon sequestration not only by increasing organic inputs but also by orchestrating a Saprotroph heavy community that strengthens specific mineral-organic associations. This study highlights the synergy between fungal functional shifts and geochemical pathways as a fundamental mechanism governing blue carbon resilience under biological invasions.