Nitrogen availability and mineral particles contributed fungal necromass to the newly formed stable carbon pool in the alpine areas of Southwest China

Nitrogen (N) addition can essentially enhance soil carbon (C) storage in terrestrial ecosystems. However, there is little information concerning how N availability regulates new C sequestration and potential microbial mechanisms in different forest soils. Here, we investigated the effects of N addition on the distribution of newly formed C in particulate organic C (POC) and mineral-associated organic C (MAOC) fractions, as well as the newly formed amino sugars C (i.e. biomarkers for fungal and bacterial-derived microbial necromass) in MAOC under coniferous and broad-leaved forest soils in alpine areas of Southwest China, based on 60-day incubation experiments by adding 13C labeled glucose and NH4Cl. The newly formed C derived from glucose was heavily distributed in MAOC fractions, with 90.8% in coniferous forest soil and 83.2% in broad-leaved forest soil, reflecting that new C was predominantly stabilized by soil mineral particles under glucose addition. Despite the divergent soil organic C (SOC) content and quality (C:N ratio) in both forest soils, we detected that N addition decreased the newly formed C by lowering the new POC and MAOC. Conversely, N addition significantly enhanced the new microbial necromass, with more fungal necromass associated with mineral particles, resulting in a high ratio of new fungal/bacterial necromass in the MAOC fraction in both forest soils. However, the lower ratio of new fungal/bacterial necromass was observed in the broad-leaved forest soil with higher mineral particles, which was primarily attributed to the preferential accumulation of bacterial necromass on mineral particles. These results further underpinned the importance of soil mineral particles in regulating the contribution of fungal necromass to a new stable C pool. Collectively, this study revealed that N availability and soil mineral particles determined the contribution of fungal necromass to the new stable C pool, thereby providing novel insights for accurately assessing the soil C stability of various forest types under climate change.