Effects of cultivation on soil carbon and nitrogen along an altitudinal gradient in the Southeastern Tibetan Plateau

The effects of cultivation on the Tibetan Plateau at high altitudes remain uncertain. To answer these questions, this study to compare the differences in soil total carbon (TC) and nitrogen (TN) between farmland and adjacent forests was conducted at 755-1850 m above sea level (a.s.l.) in Medog, southeast of the Tibetan Plateau, using an altitude gradient sampling strategy. 14 farmland-forest pairs were used for comparison. Cultivation caused significant soil TC and TN changes when comparing forests and farmlands. More than half of the farmland-forest pairs soil TC and TN content exhibited decreasing trends. The soil at 0-10 cm was the most susceptible to cultivation, where soil TC decreased by 1.10 kg/m2 (about 21 %), and TN decreased by 0.07 kg/m2 (about 19 %). With increasing altitude, lower soil temperatures were more favorable for soil carbon and nitrogen accumulation in both forests and farmland. Soil water content was a key factor in determining the accumulation or consumption of carbon and nitrogen in agricultural soils, with a threshold of 40 %. Beyond this value, agricultural soils tended to accumulate TC and TN rather than consume them. Furthermore, the use of organic fertilizers was beneficial for maintaining soil fertility. Soil TC and TN increased by up to 5.57 kg C/m2 and 0.33 kg N/m2 in 0-10 cm soil after using organic fertilizers. However, overutilization of the soil eventually caused a dramatic loss of soil carbon (up to 19.39 kg C/m2 TC) and nitrogen (1.21 kg N/m2 TN) in the 0-30 cm soil, resulting in abandoned land. In addition to field experiments, this study also reviewed the status of agricultural cultivation on the Tibetan Plateau since 2000 using data from 79 farmland observations. The results showed that most regions within an altitude range of 2745-4088 m (a.s.l.) experienced significant soil carbon and nitrogen losses. That led to decreased soil organic carbon (SOC) at 1.5 Tg C/year, while soil TN increased at 0.03 Tg N/year after 2000. Therefore, cultivation on the Tibetan Plateau must receive adequate attention for the sustainable development of plateau agriculture.