Spatiotemporal variation of soil organic carbon in the cultivated soil layer of dry land in the South-Western Yunnan Plateau,China

The dynamics of soil organic carbon (SOC) in cropland is one of the central issues related to both soil fertility and environmental safety. However, little information is available at county level regarding the spatiotemporal variability of SOC in the southwestern mountainous region of China. Thus, this study aimed to explore spatiotemporal changes of SOC in the cultivated soil layer of dry land in Mojiang County, Yunnan Province, China. Data were obtained from the second national soil survey (SNSS) of 1985 and soil tests for fertilizer application carried out by the Mojiang Agricultural Bureau in 2006. The ANOVA test was applied to determine any significant differences between the datasets, while semivariogram analysis was performed on geostatistics via an ordinary Kriging method in order to map spatial patterns of soil organic carbon density (SOCD). The results revealed that SOCD in the cultivated soil layer significantly decreased from 3.93 kg m^-2 in 1985 to 2.89 kg m^-2 in 2006, with a total soil organic carbon stock (SOCS) decrease of 41.54×10⁴ t over the same period. SOCS levels fell most markedly in yellow-brown soil at a rate of 51.52%, while an increase of 8.70% was found in the analysed latosol. Geostatistical analysis also showed that the recorded changes in SOCD between 1985 and 2006 were spatially structured. The decreasing trend might be attributed to the combined action of intense cultivation, major crop residue removal without any protective tillage measures, unreasonable fertilization and natural climatic diversity inducing a large decrease in SOC in the studied cultivated dry land region of Mojiang County. Therefore, management measures such as protective tillage should be undertaken in order to enhance soil C sequestration.