Effects Of Land Use On Grassland Soil Degradation At A Sub-alpine Site

Cultivating native pasture to plant oat or some other species of grasses is generally recommended as a way to alleviate grazing pressure on native grassland and to improve winter forage production in high altitude regions, however, its effects on soil quality have not be well understood. The aim of this study was to investigate changes in soil organic C dynamics and some soil physical properties induced by cultivations of perennial and annual pasture on native pasture.Compared with native pasture, cultivation of annual pasture resulted in significant reductions of total organic C by 36% in 0~10 cm, 29% in 10~20 cm and 41% in 20~30 cm depths, whereas in perennial pastures total organic C decreased by 18% only in 0~10 cm depth. A significant decrease of C in the coarse organic fraction (0.1 ~2 mm) by 30~ 66% and 21~69% and in young organic fraction (0.05~0.1 mm) by 42~47% and 68~ 78% under annual and perennial pastures, respectively, was found at different depths of up to 30 cm. Stable organic C (< 0.05 mm) decreased by 28-38% due to cultivation only under annual pasture. Cultivation of annual pasture decreased water-soluble organic C by 11 —40% in all the depths and perennial pastures by 16% only in 0~10 cm depth, however, both increased the proportions of water-soluble organic carbon in total soil organic C concentrations. Compared with the native pasture, annual and perennial pastures management significantly decreased microbial C and respired C during 10-d incubation in the 0~10 cm depth but increased the proportions of respired-C in total soil organic C. The above changes in various C pools indicated a decline in soil organic matter concentration and quality, implying that decrease in plant biomass organic C input into soils and increase in microbial activity (soil respiration) contributed to the reduction in total soil organic C under annual and perennial pasture practices in the sub-alpine regions.The changes in water stability of >0.25mm aggregates, particle density, bulk density, porosity and water holding capacity of soils under three different land uses were also evaluated relative to soil physical degradation. Upon cultivation of native pasture, MWD (mean weight diameter) significantly decreased by 12~54% across wet-sieving times, depths and land use changes. WSAP (water-stable aggregate percentage) also decreased due to cultivation. The proportion of aggregates >2~5 mm was higher in the native pasture than in cultivated pasture. The effect of cultivation on soil aggregation (especially onMWD) was more pronounced in the (MO cm depth than in the 10~20 cm and 20-30 cm depths. Particle density of soil responded more to the land use changes than bulk density or porosity. Averaged across land uses, all the physical parameters increased with depth, except WHC (which remained the same with depth). Significant linear relationships were found between changes in total organic C and reported physical properties to the depth of 30 cm, thus suggesting that land disturbances, especially continuous long-term cultivation, would not sustain important soil physical properties in the sub-alpine region of China.Our results indicated that soil system in sub-alpine region was very frangible; soil managements (especially annual pasture) in present study induced serious soil degradation and therefore were unsustainable practices.