Soil organic carbon density and its driving factors in forest ecosystems across a northwestern province in China

Abstract

Forest soils are an important component of the global carbon cycle, as they sequester and store much more organic carbon than other terrestrial ecosystems. Thus, reliable information on the forest soil organic carbon (SOC), along with the factors that influence it, is crucial from the perspective of regional carbon budgets and climate modeling. Gansu Province in northwestern China is characterized by complex climatic gradients and diverse forest types, suggesting a large variability in the spatial distribution of SOC, yet the SOC stock in this region has not been comprehensively investigated because of insufficient soil profile data. Our study covered the entire forest area in the province via a regional-scale sampling campaign, and provided soil organic carbon density (SOCD) data obtained from 112 natural forests and 100 plantations. Influencing factors including forest type, forest stand age, soil type, litter carbon, climate, and topography were evaluated in both natural forests and plantations. The results indicated a SOC stock of 433.4 ± 13.2 Tg C and a SOCD value of 175.3 ± 33.0 Mg C ha−1 for the 0–100 cm layer at the regional scale. Specifically, the SOC stock and the forest area-weighted average SOCD in natural forests were estimated at 349.0 ± 16.5 Tg C and 207.3 ± 23.2 Mg C ha−1, which were about quadruple and double, respectively, of the values for the plantations. Regardless of natural forests and plantations, SOCD increased with both stand age class and elevation and decreased with mean annual temperature (MAT). SOCD also increased with mean annual precipitation (MAP) for natural forests with elevation <2500 m and for all plantations. The overall importance to SOCD of the explanatory variables increased with increasing soil depth in natural forests while decreased with increasing soil depth in plantations. Forest type, stand age, soil type, MAT, MAP, elevation, litter carbon, slope, and aspect explained 58 and 65% of the total variation in the SOCD at 100 cm depth in natural forests and plantations, respectively. In natural forests, the most important explanatory variable for the uppermost 30 cm was soil type, while it was MAT for the whole 100 cm; in plantations, the most prominent factors were forest type and elevation, regardless of the soil depth. These suggest that environmental changes will likely have different effects on SOCD in natural forests and plantations.