Microbial Necromass Nitrogen Accumulation and Its Response to Soil Multifunctionality Following Plantation Restoration in a Subtropical Karst Region

ABSTRACT Microbial necromass nitrogen (MNN) is increasingly recognized as a major source of soil N, playing a crucial role in N sequestration and sustaining N balance. However, quantitative data on the contribution of MNN to total N and its relationship with soil multifunctionality (SMF) in karst plantation ecosystems remain lacking. In this study, MNN (measured via amino sugar analysis) and SMF (calculated using the mean value method) were determined across five plantations and unafforested land (control) in the Wuling Mountains of subtropical China. The soil MNN content ranged from 1.02 to 1.67 g kg−1, contributing 53.8% to 75.4% of the total N, with Cinnamomum camphora pure forest exhibiting the highest values among all stand types and control. The afforestation of Cinnamomum camphora enhanced soil nutrient provision, microbial activity, and biomass, increasing SMF by 54% compared to the control. The soil functional parameters were generally higher in the organic horizon than in the topsoil and subsoil, and the contribution of MNN to total N was greater in the subsoil. Enzyme stoichiometry ratios indicated that microbial P limitation was prevalent in both plantation and control soils, with MNN content positively correlated with C/P and N/P ratios (p < 0.001). Increases in MNN were closely associated with SMF, encompassing nutrient provision, microbial activity, and biomass, with available N (AN) being a key predictor of MNN accumulation. The partial least squares path model revealed that nutrient provision contributed positively to MNN accumulation, and AN had a direct effect path coefficient of 0.72 on it (p < 0.01). This work underscored the critical role of MNN in contributing to N sequestration and its potential to support ecosystem functioning in the karst region.