Spatial distribution pattern and influence path of soil carbon and nitrogen in the Heihe River source region, northeast Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau (QTP) is critical for global carbon and nitrogen cycling and carbon sequestration because of its sensitive positive response to global warming. Although the factors affecting soil carbon and nitrogen in the sophisticated mountainous terrain of the QTP have been extensively investigated, the specific “direct” or “indirect” roles and their influence path remain unclear. In this study, the patterns of soil total carbon contents (STC) and soil total nitrogen contents (STN) were examined based on 445 soil samples collected from 89 plots in the Heihe River source region, northeastern QTP. The objective is to elucidate the interactions and driving mechanisms among climate, vegetation, geographical environmental factors, and soil properties for STC/STN. We found; (1) notable differences in STC/STN between altitudinal gradients and LULC types, exhibiting an overall increasing trend, followed by a decreasing trend with an increase in altitude; (2) the STC/STN in the 0–50 cm soil layer declined as soil depth increased, such that the distinctive “surface aggregation” characteristics were confirmed. In general, regions with higher soil total carbon densities (STCD) and soil total nitrogen densities (STND) were concentrated in the southeast of the watershed, whereas regions with lower densities were identified in the northwest. (3) Soil water content (SWC) and bulk density (BD) are critical in distributing STC/STN. Among the significant environmental factors influencing STC, SWC had the maximum direct path coefficient (0.42), suggesting a direct effect. Other factors, such as land-use and land-cover change (LULC), BD, pH, and normalized difference vegetation index (NDVI) affected the STC content via a wide variety of pathways. For STN, SWC (0.46), and pH (−0.2) showed direct path coefficients with magnitudes greater than the indirect path coefficients, suggesting direct effects. Moreover, NDVI, BD, and precipitation affected the STN through other factors. These findings provide insights into soil carbon and nitrogen dynamics in alpine soil under sophisticated mountainous terrain. They provide valuable support for studying soil carbon and nitrogen cycling, and for evaluating carbon and nitrogen stocks in cold regions.