Changes in topsoil organic carbon from 1986 to 2010 in a mountainous plateau region in Southwest China

The carbon cycle and underlying environmental drivers are not yet fully understood, and whether climate warming is inducing soil carbon loss remains controversial, largely due to limited evidence from large-scale observations. With relatively less human disturbance, Yunnan Province, a typical mountainous plateau region in Southwest China, is a favorable place to evaluate environmental effects on soil organic carbon (SOC) dynamics. In this study, soil sampling of the upper 20-cm soil layer was conducted at sites across Yunnan Province in 1986 and 2010 to estimate topsoil SOC density (SOCD). The changes in SOCD and environmental factors, including initial SOCD value, temperature, precipitation, longitude, latitude, elevation, and slope gradient, were analyzed. The results indicated that the topsoil of Yunnan Province mainly acted as a carbon source during the study period. The topsoil SOCD in 1986 averaged 5.82 kg C m(-2) and was higher than that of 2010 (5.45 kg C m(-2)), corresponding to a 0.14 Pg loss in SOC stock over the 25 years. The northwestern subregion experienced the most drastic SOCD changes, and the SOCD decreased most notably in areas with higher initial SOCD values. Compared with cropland and grassland, more significant SOCD reductions were observed in forest. Among the studied environmental factors, initial SOCD was the main factor associated with SOCD variations. The next most important factor was temperature, followed by precipitation. With high elevation and initial SOCD, the northwestern subregion of Yunnan Province had the most sensitive SOCD changes, underscoring the need for further protection and appropriate management.