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The wetlands on the Qinghai-Tibet Plateau are experiencing serious degradation, with more than 90,000 hectares of marshland converted to wet meadow or meadow after 40 years of drainage. However, little is known about the effects of wetland conversion on soil C stocks and the quality of soil organic carbon (SOC) (defined by the proportion of labile versus more resistant organic carbon compounds). SOC, microbial biomass carbon, light fraction organic carbon (LFOC), dissolved organic carbon, and the chemical composition of SOC in the soil surface layer (0-10 cm), were investigated along a wetland degradation gradient (marsh, wet meadow, and meadow). Wetland degradation caused a 16 % reduction in the carbon stocks from marsh (178.7 ± 15.2 kg C m) to wet meadow (150.6 ± 21.5 kg C m), and a 32 % reduction in C stocks of the 0-10 cm soil layer from marsh to meadow (122.2 ± 2.6 kg C m). Wetland degradation also led to a significant reduction in SOC quality, represented by the lability of the carbon pool as determined by a density fractionation method ( L), and a significant increase in the stability of the carbon pool as reflected by the alkyl-C: O-alkyl-C ratio. C NMR spectroscopy showed that the labile form of C ( O-alkyl-C) declined significantly after wetland degradation. These results assist in explaining the transformation of organic C in these plateau wetland soils and suggest that wetland degradation not only caused SOC loss, but also decreased the quality of the SOC of the surface soil. [ABSTRACT FROM AUTHOR]