Temporal variation of soil organic matter content and potential determinants in Tibet, China

Understanding temporal variations in soil organic matter (SOM) is important because SOM improves soil, sustains soil productivity, and sequesters carbon. The Tibetan Plateau plays a critical role in climate change at both regional and global scales. Thus, the main objective of this study was to investigate temporal variations in SOM content in Tibetan soils as an important indicator of climate change. SOM content data were obtained from soil surveys conducted in the 1980s and resampled in the 2000s. In this study, SOM content data from the A horizon of the soil profile were compared over time, and a paired t-test was conducted to verify differences between the two datasets. The results reveal that SOM content in the A horizon decreased significantly. To determine the causes of SOM content decreases in the A horizon, soil samples were grouped into ten soil types, four land-use types, and seven physiogeographical units. SOM content in four soil types (felty soils, cambisols in the World Reference Base (WRB) for soil resources; cold calcic soils, cambisols in the WRB; bog soils, gleysols in the WRB; and meadow soils, umbric gleysols/haplic phaeozem in the WRB), pasture, and broad valleys and basins of the south Tibetan zone of montane shrubby steppe decreased significantly in the A horizon. Based on the analysis of some known realities, this change should be attributed to pasture degradation resulting from overgrazing and climate warming.

Research Highlights
► SOM content decreased significantly in the A horizon.
► SOM content in four soil types, pasture, and southern Tibet decreased significantly.
► Pasture degradation may be regarded as the major determinant of SOM content change.