Moss biocrusts enhance soil carbon stabilization via microbial necromass accumulation in the karst rocky desertification region of eastern Yunnan, China

Karst ecosystems are globally important carbon sinks, yet the escalating severity of rocky desertification is causing continuous losses of soil carbon stocks. As a key biological component of karst rocky desertification environment, moss biocrusts play crucial ecological roles, but their impacts on the accumulation of soil microbial necromass and organic carbon remain largely unexplored. In the present study, soils covered by moss biocrusts were collected from rocky desertification sites with different degrees of degradation in Zhanyi District, Qujing City, Yunnan Province, China, at both the biocrust layer (0–1 cm) and the underlying sub-layer (1–5 cm). Soil physicochemical properties, microbial biomass, enzyme activities, and amino sugar content were quantified, and the contribution of microbial necromass carbon to soil organic carbon formation was assessed. The results showed that: (1) with increasing rocky desertification intensity, habitats covered by moss biocrusts exhibited pronounced resource island effects, thereby improving soil physicochemical properties and further promoting the accumulation of microbial necromass carbon and soil organic carbon. (2) fungal necromass carbon accounted for a larger proportion of total microbial necromass carbon than bacterial necromass carbon. Meanwhile, the formation of microbial necromass carbon in the biocrust layer was primarily driven by nutrient supply capacity, microbial activities, and nitrogen cycling processes, whereas its accumulation in the sub-layer was strongly regulated by soil moisture conditions, nutrient availability, and substrate decomposition processes. (3) Microbial necromass carbon was positively associated with both particulate organic carbon and mineral-associated organic carbon, and the relationship was stronger for mineral-associated organic carbon, suggesting that microbial necromass carbon in soils covered by moss biocrusts may contribute to organic carbon stabilization through the mineral-associated organic carbon pool. Meanwhile, along the degradation gradient, microbial necromass carbon and soil organic carbon generally co-increased, and changes in soil microbial carbon pump efficacy indicated a stronger relative contribution of microbial necromass carbon to the stabilization of soil organic carbon in biocrust layers. Overall, moss biocrusts could enhance the contribution of microbial necromass carbon to organic carbon accumulation and stabilization potential under rocky desertification conditions. These findings provide theoretical insights into carbon sequestration and restoration mechanisms in karst ecosystems, and have important implications for ecological restoration and carbon management in rocky desertification regions.