Large amounts of terrestrial organic C and N reserves lie in salt-affected environments, and their dynamics are not well understood. This study was conducted to investigate how the contents and dynamics of 'native' organic C and N in sandy soils under different plant species found in a salt-affected ecosystem were related to salinity and pH. Increasing soil pH was associated with significant decreases in total soil organic C and C/N ratio; particulate (0.05-2 mm) organic C, N and C/N; and the C/N ratio in mineral-associated (<0.05 mm) fraction. In addition, mineral-associated organic C and N significantly increased with an increase in clay content of sandy soils. During 90-day incubation, total CO 2 -C production per unit of soil organic C was dependent on pH [CO 2 -C production (g kg -1 organic C) = 22.5 pH -119, R 2 = 0.79]. Similarly, increased pH was associated with increased release of mineral N from soils during 10-day incubation. Soil microbial biomass C and N were also positively related to pH. Metabolic quotient increased with an increase in soil pH, suggesting that increasing alkalinity in the salt-affected soil favoured the survival of a bacterial-dominated microbial community with low assimilation efficiency of organic C. As a result, increased CO 2 -C and mineral N were produced in alkaline saline soils (pH up to 10.0). This pH-stimulated mineralization of organic C and N mainly occurred in particulate but not in mineral-associated organic matter fractions. Our findings imply that, in addition to decreased plant productivity and the litter input, pH-stimulated mineralization of organic matter would also be responsible for a decreased amount of organic matter in alkaline salt-affected sandy soils.

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Ligularia virgaurea, a toxic perennial weed, has become a dominant species in the heavilygrazed alpine meadows of the eastern Qinghai-Tibetan Plateau over recent decades. We investigated changes in soil biochemical properties associated with L. virgaurea spreading in grazed alpine meadows at three sites. Soil and root biomass samples were taken at depths of 0-8 cm, 8-16 cm and 16-24 cm from patches where L. virgaurea was dominant and from 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 while nitrate N concentration was significantly lower in L. virgaurea patches than in areas between L. virgaurea patches. The increased activities of dehydrogenase, β-glucosidase, urease and phosphatase in the top 8-cm layer under L. virgaurea were associated with enhanced soil microbial biomass. Our results indicate a close association between changes in soil biochemical properties in the top 8-cm layer and the spread of L. virgaurea in grazed alpine meadows of the eastern Qinghai-Tibetan Plateau.