Spatial Patterns and Controlling Factors of Soil Organic Carbon and Total Nitrogen in the Three River Headwaters Region, China

The alpine ecosystem has great potential for carbon sequestration. Soil organic carbon (SOC) and total nitrogen (TN) are highly sensitive to climate change, and their dynamics are crucial to revealing the effect of climate change on the structure, function, and services of the ecosystem. However, the spatial distribution and controlling factors of SOC and TN across various soil layers and vegetation types within this unique ecosystem remain inadequately understood. In this study, 256 soil samples in 89 sites were collected from the Three River Headwaters Region (TRHR) in China to investigate SOC and TN and to explore the primary factors affecting their distribution, including soil, vegetation, climate, and geography factors. The results show that SOC and TN contents in 0–20, 20–40, 40–60, and 60–80 cm soil layers are 24.40, 18.03, 14.04, 12.40 g/kg and 2.46, 1.90, 1.51, 1.17 g/kg, respectively; with higher concentrations observed in the southeastern region compared to the northwest of the TRHR. One-way analysis of variance reveals that SOC and TN levels are elevated in the alpine meadow and the alpine shrub relative to the alpine steppe in the 0–60 cm soil layers. The structural equation model explores that soil water content is the main controlling factor affecting the variation of SOC and TN. Moreover, the geography, climate, and vegetation factors notably indirectly affect SOC and TN through soil factors. Therefore, it can effectively improve soil water and nutrient conditions through vegetation restoration, soil improvement, and grazing management, and the change of SOC and TN can be fully understood by establishing monitoring networks to better protect soil carbon and nitrogen.