Freezing–thawing cycles effect on the water soluble organic carbon, nitrogen and microbial biomass of alpine grassland soil in Northern Tibet

Soil freezing-thawing cycle may substantially influence soil physical properties, microbial activity, and the rates of carbon and nitrogen cycling in soils. In this study, the soil water soluble organic C, N (WSOC, WSON) as well as microbial biomass C, N (MBC, MBN) of two alpine grassland types, alpine meadow and alpine steppe, were investigated after freezing-thawing cycles in a grassland landscape of Northern Tibet. The results showed that with froze at −15°C, the WSOC and WSON contents of alpine grassland soils observably increased after 1 freezing-thawing cycle and decreased after 2 and 4 cycles, and with froze at −25°C, the WSOC and WSON contents showed a gradually increasing trend. The MBC and MBN contents of both alpine meadow and alpine steppe soils generally exhibited a similar “low-high-low” pattern during the process of freezing-thawing cycles. Furthermore, the alpine grassland type had a significant effect on the WSOC and MBC concentrations, and the freeze temperature, freezing-thawing cycle times had significant effects on the WSOC, WSON, MBC, and MBN concentrations.