Multitrophic interactions support belowground carbon sequestration through microbial necromass accumulation in dryland biocrusts

Belowground organisms play essential roles in biogeochemical cycling of carbon. However, it remains unknown how species interactions across multiple trophic levels influence soil carbon sequestration. Biological soil crusts (biocrusts) comprise multiple trophic groups, forming an ideal model system to study species interactions in natural communities. This study explored the critical role of multitrophic interactions in shaping the accumulation of microbial necromass carbon (MNC), comparing biocrust-covered and bare soils in the dryland ecosystem of the Loess Plateau. Amino sugars were used as indicators of soil microbial necromass, and environmental DNA sequencing was used to characterize multitrophic communities in soil samples. Biocrust-associated soils exhibited 2.5 times higher MNC than bare soils, with bacterial necromass carbon (BNC) constituting a larger proportion of soil organic carbon than fungal necromass carbon (FNC). Greater network complexity and more frequent within-trophic associations (WTAs) were observed for bare soils. The proportion of negative WTAs was negatively correlated with MNC, whereas the proportion of cross-trophic associations (CTAs) was positively correlated with MNC. Community composition, hierarchical interactions, and network complexity all shaped microbial necromass carbon accumulation. This study illustrates a novel mechanism contributing to carbon sequestration in dryland ecosystems, wherein multitrophic interactions within the soil micro-food web regulate microbial necromass accumulation, and sheds light on the dynamics and stabilization of soil microbial necromass.