[Spatial-temporal variation of soil organic carbon and its relations to soil physical properties in degraded alpine grasslands].

By using grid sampling method, the spatial-temporal variation of soil organic carbon (SOC) and its relations to soil physical properties in degraded alpine grasslands in North Tibet were studied. The SOC content and its density both in surface (0-10 cm) and in subsurface (10-20 cm) layers decreased in order of slightly degraded grassland > normal grassland > moderately degraded grasslands > seriously degraded grasslands, and the differences of SOC content and its density between the two layers showed the same variation trend. An opposite trend was observed in the annual variation rates of SOC content and its density in different grasslands, and the variation was larger in surface than in subsurface layer. The annual accumulation rate of SOC in 0-10 cm layer in not degraded and slightly degraded grasslands was 0.018 and 0.003 g x kg(-1), being 6.0 and 2.0 times of that in 10-20 cm layer, while the annual loss rate of SOC in 0-10 cm layer in moderately and seriously degraded grasslands was 0.150 and 0.231 g x kg(-1), being 2.3 and 2.2 times of that in 10-20 cm layer, respectively. The total annual loss of SOC in moderately and seriously degraded grasslands was 3.8 times of the total annual accumulation of SOC in not degraded and slightly degraded grasslands. The degraded alpine grasslands in North Tibet had an average annual SOC loss of 7.87 x 10(7) t C x a(-1), and a greater potential to further loss in the future. The SOC content was significantly positively correlated with the contents of 5.0-1.0 mm, 1.0-0.5 mm, and 0.5-0.25 mm soil aggregates, soil bulk density, and soil moisture content.