Grassland Degradation-induced Soil Organic Carbon Loss Associated with Micro-food Web Simplification

Soil micro-food webs play a vital role in sustaining soil carbon cycling and stocks through the activities and interactions of individual organisms. However, grassland degradation disrupts these micro-food webs and is expected to reduce soil carbon stocks. This hypothesis was tested along degradation transects that were established in alpine meadows and steppes in arid regions, examining how multitrophic organisms and microbial metabolic efficiency respond to grassland degradation and how these responses relate to soil organic carbon (SOC). Grassland degradation reduced microbial necromass accumulation coefficient (the ratio of microbial necromass carbon to microbial biomass carbon) and increased microbial metabolic quotient (the ratio of soil respiration rate to microbial biomass carbon), indicating that microbes may prioritize SOC decomposition for resource acquisition over growth and necromass accumulation. Degradation led to increased bacterial and fungal diversity, reduced protist and nematode diversity, and simplified the structure of micro-food web (network complexity). Overall, grassland degradation reduced microbial metabolic efficiency, linked to reduced plant biomass, lower soil clay content, and a simplified micro-food web—particularly weakening interactions among microbes, microbivores, and predators—which is associated with SOC loss in degraded grasslands. These findings indicate the necessity of maintaining micro-food web structures to promote soil carbon sequestration in degraded grasslands.