Coniferous Tree Species-Induced Shifts in Soil Total Nitrogen and pH Regulated Microbial-Derived Carbon Accumulation and Thus Promoted Soil Organic Carbon Sequestration

Forest soil constitutes a critical reservoir within terrestrial carbon pools. Understanding the dynamics of soil organic carbon (SOC) in coniferous forests is crucial for enhancing ecosystem carbon sequestration capacity, yet systematic quantification of SOC characteristics and their driving factors remains limited across critical bioclimatic zones. This study examined SOC features in topsoil and driving factors across eight representative coniferous forest types in Longnan—an ecologically significant transition region of northwestern China. SOC concentrations ranged from 31.76 to 80.86 g·kg−1, where Abies fargesii var. faxoniana exhibited significantly higher concentrations than other conifers. Fungal necromass dominated SOC formation (29%–45% contribution) versus minimal bacterial necromass inputs (3%–5%). Redundancy analysis identified that soil total nitrogen, C/N ratio, and tree evenness showed significant correlations with SOC concentrations and their fractions. Partial least squares path modeling revealed that tree species exerted a direct positive impact on soil total nitrogen, while having an adverse effect on soil pH. Lower soil pH and higher total nitrogen were associated with higher microbial-derived carbon and SOC concentrations. In contrast, plant-derived carbon exerted no direct influence on SOC concentrations, operating exclusively through microbial-derived carbon pathways. These results indicated that coniferous tree species-induced shifts in soil total nitrogen and pH facilitate the accumulation of microbial necromass carbon, rather than plant residues, and thus promote SOC sequestration. A. fargesii var. faxoniana can be regarded as a key strategic tree species for SOC sequestration and sustainable forest management, and its cultivation should be prioritized due to improvements in total nitrogen and microbial-derived carbon.