[Response of Soil Microbial Necromass Carbon and Community Structure to Warming and Nitrogen Addition in an Alpine...

Climate change， especially global warming and increasing atmospheric nitrogen （N） deposition， has become a major driver of carbon cycle in terrestrial ecosystems. To the role of microbial residues in soil organic carbon （SOC） accumulation and identify their controlling factors under climate change scenarios， a field experiment was conducted in an alpine swamp meadow on the Qinghai-Tibet Plateau. The experiment included four treatments： control group， warming， N addition， and combined warming with N addition. Soil microbial necromass carbon （MNC） was quantified by separately assessing fungal necromass carbon （FNC） and bacterial necromass carbon （BNC） using amino sugar biomarkers. Warming significantly increased FNC and FNC/SOC by 23.20% and 26.76%， respectively （P<0.05）. In contrast， N addition and the combined treatment significantly decreased FNC by 61.86% and 50.86% （P<0.05） and decreased FNC/SOC by 15.81% and 19.05% （P<0.05）. N addition and the combined warming with N addition significantly promoted the BNC/SOC by 92.24% and 73.58%， respectively （P<0.05）. Mantel tests indicated significant correlations between FNC and dominant fungal phyla such as Ascomycota， Mucoromycota， Basidiomycota， and Zoopagomycota， and bacterial phyla including Acidobacteriota and Rokubacteria. Correlation analysis and PLS-PM revealed that soil nutrient properties were the primary factors influencing MNC. Specifically， SOC， total phosphorus （TP）， and organic phosphorus （O-P） were significantly and positively correlated with MNC， while total nitrogen （TN） and nitrate nitrogen （NO₃^--N） were negatively correlated with it. Regarding soil enzymatic activities and microbial diversity indices， N-acetyl-β-D-glucosaminidase （NAG） activity， peroxidase （POD） activity， the fungal Simpson diversity index， and the bacterial Chao1 richness index positively correlated with MNC. Conversely， urease （URE） activity showed a significant negative correlation with MNC. These findings highlight divergent responses of MNC to climate warming and N deposition， driven primarily by soil nutrient properties. This study provides evidence for understanding the carbon stabilization mechanisms in cold region ecosystems under future climate change scenarios.