Nitrogen addition decouples the microbial necro-mass from soil organic carbon formation in a temperate grassland

Abstract

Increasing anthropogenic nitrogen (N) inputs has profoundly altered soil microbial necro-mass carbon (MNC), which serves as a key source of soil organic carbon (SOC). Yet, the response pattern of MNC and its contribution to SOC across a wide range of N addition rates, remain elusive. In a temperate grassland with six years’ consecutive N addition spanning seven rates (0–50 g N/(m²·year)) in Inner Mongolia, China, we explored the responses of soil MNC and its contribution to SOC. The soil MNC showed a hump-shaped pattern to increasing N addition rates, with the N saturation threshold at 18.07 g N/(m²·year). The soil MNC was driven by nematode abundance and the ratio of bacterial to fungal biomass below the N threshold, and by plant biomass allocation pattern and diversity above the N threshold. The contribution of soil MNC to SOC declined with increasing N addition rates, and was mainly regulated by the ratio of MNC to mineral-associated organic carbon and plant diversity and the ratio of bacterial to fungal biomass. In addition, the soil MNC and SOC differentially responded to N addition and were mediated by disparate biological and geochemical mechanisms, leading to the decoupled MNC production from SOC formation. Together, in this N-enriched temperate grassland, the soil microbial necro-mass production tends to be insufficient as a general explanation linking SOC formation. This study expands the mechanistic comprehension of the connections between external N input and soil carbon sequestration.