Responses of soil microorganisms, carbon and nitrogen to freeze thaw cycles in diverse land-use types

Temperatures are predicted to increase in semi-arid regions, which will affect the frequency of some climatic events such as freeze thaw cycles. To understand the impact of recurrent freezing and thawing on soil microorganisms and their environment, three soils from diverse land-use types (grassland, cut grassland and cropland) in the semi-arid area of the Loess Plateau were subjected to two different freeze thaw cycles (FTC) for 48 days (either 4 days each at -2 degrees C and + 3 degrees C for 6 FTC or 8 days each at -2 degrees C and + 3 degrees C for 3 FTC). Nutrient content and mobilization in the soil, and microbial biomass and functional diversity were examined. When subjected to these freeze thaw cycles, soil respiration was low during the -2 degrees C phase and high during the +3 degrees C phase, decreasing in successive cycles, which indicates the rapid response of microbial activity even after long exposure to fluctuating temperatures. Soil respiration stabilized after 32 days in the three soils in the treatment with 3 FTC but continued to decrease in the treatment with 6 FTC. With each successive freeze thaw cycle, microbial biomass C and N decreased gradually in all the tested soils. M the end of the experiment, the 6 FTC treatment had more microbial biomass C and N than the 3 FTC treatment in each soil type, indicating that microbial activity (respiration) was controlled mainly by the rapid freeze thaw cycles rather than microbial biomass. Soil inorganic N concentration declined in all soils subjected to 6 FTC but increased when subjected to only 3 FTC, suggesting that the surviving microorganisms in the treatment with 6 FTC had more potential for using soil N to adapt to the freeze thaw stress. The microbial functional diversity in the three tested soil types in response to the two freeze thaw cycle treatments varied, suggesting differences in the native microbial communities in the soil from various land-use types.