Effects of agricultural land use on soil nutrients and its variation along altitude gradients in the downstream of the Yarlung Zangbo River Basin, Tibetan Plateau

Agricultural development in alpine ecosystems can cause significant changes in soil nutrients. With large altitude spans, the combined effect of the two is still unclear in existing research. To answer this problem, this study took the downstream of the Yarlung Zangbo River Basin (YZRB) as the study area, and designed a comparative soil sampling scheme along the altitude gradient. We compared soil nutrient characteristics facility agricultural land (FA) and field cultivated land (FC), using grassland (GL), the main source of agriculture expansion, as a reference. A total of 44 sampling areas were designed within an altitude range of 800-3500 m to reveal the effects of agricultural land development along the altitude gradient on soil nutrients. Research found that the FA significantly improved soil nutrient levels, with most nutrient indicators higher than those of FC and GL (P < 0.05), while the above indicators of FC were only slightly higher than GL. Moreover, the effects of agricultural development decreased with soil depth, and mainly occurred within the 0-30 cm soil layer (P < 0.05). With increasing altitude, most of soil nutrients first decreased and then increased and differences in soil nutrients among different land use modes first expanded and then shrank. This may be related to differences in farmland management methods, vegetation coverage, and temperature under different altitude gradient constraints. Especially in middle-altitude areas, the FA not only breaks through the low-temperature limitations of the plateau, but also has the advantage of large-scale development, which is suggested for future agricultural intensification in the plateau.