| Three field experiments were conducted to study the influences of grassland cultivation, the spread of Ligularia virgaurea and grazing exclusion on the processes of soil carbon and nitrogen in alpine meadows.1. We studied the dynamics of organic carbon (C) and total nitrogen (N) associated with physically-separated soil fractions in a grassland-cultivation sequence in the Qinghai-Tibet Plateau. Concentrations of organic C and N of soil, free and occluded particulate organic matter (OM), and aggregate- and mineral-associated OM in different land uses increased in the order:50-yr cultivation < 12-yr cultivation≤native grassland. The prolonged cropping up to 50 years, markedly affected concentrations of free- and occluded-particulate OM, and mineral-associated OM. After wet-sieving,43% of native grassland soil mass was found in>1-10 mm water-stable aggregates which stored 40% of bulk soil organic C and N; only 16% and 7% of soil mass, containing 16% and 7% of bulk soil organic C and N, was in>1-10 mm water-stable aggregates of soils cultivated for 12 years and 50 years, respectively. This indicated that losses of soil organic C and N following cultivation of native grassland would be largely related to disruption of >1-10 mm size-aggregates and exposure of intra-aggregate OM to microbial attack. Organic C and N concentrations of soil aggregates were similar among aggregate-size fractions (>0.05-10 mm) within each land use, suggesting that soil aggregation process of these soils did not follow the hierarchy model. The increase of the C-to-N ratio of free- and occluded-particulate fractions in the cultivated soils compared to the grassland soil indicated a greater loss of N than C.2. Ligularia virgaurea, a toxic perennial weed, has become a widespread species in the heavily-grazed alpine meadows of the eastern Qinghai-Tibet Plateau over the last few decades. We investigated changes in soil biochemical properties associated with L. virgaurea spread in grazed alpine meadows at three sites. Soil samples were taken at a depth of 0-8 cm,8-16 cm and 16-24 cm recpectively from patches where L. virgaurea was dominant and areas between L. virgaurea patches, with only other native species. Across sites, root biomass, acid-extracted carbohydrate C, organic C and N mineralization, and microbial biomass C and N concentrations in the top 8-cm layer were significantly higher in L. virgaurea patches than in areas between L. virgaurea patches. The increased activities of dehydrogenase,β-glucosidase and phosphatase in the top 8-cm layer under L. virgaurea were mainly functions of soil microbial biomass, whereas urease activity can be enhanced by L. virgaurea through increasing microbial biomass and its functional efficiency compared to the other species. In conclusion, our results indicate significant changes in soil biochemical properties in the top 8-cm layer with the spread of L. virgaurea in alpine meadows of the eastern Qinghai-Tibet Plateau.3. We investigated the changes in soil biochemical properties after grazing exclusion at an alpine meadow site of the Qinghai-Tibet Plateau. Soil samples were taken at a depth of 0-15cm from ungrazed grassland (6 or 8 years after grazing exclusion) and conventionally grazed grassland. The results showed that up to 8-year's of grazing exclusion did not significantly change the soil organic C and N concentrations compared to the grazed grassland. The aboveground biomass in the ungrazed grassland was significantly higher than that in the grazed grassland, but the belowground biomass in the ungrazed grassland was significantly lower than that in the grazed grassland. Soil microbial biomass and activities ofβ-glucosidase, urease and phosphatase were similar between soils from the ungrazed grassland and the grazed grassland. The results suggest that the practice of grazing exclusion in the over-grazed meadow can significantly change the vegetation composition and allocation of primary production into soil, but these changes can not necessarily lead to an alteration of the soil organic C and N storage.
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