Microbial response to freeze-thaw cycles in tundra and taiga soils

Four tundra and taiga soils were experimentally subjected to three freeze-thaw cycles (5 days each at −5°C and +5°C). After each thaw, there was an initial pulse (<24 h) in microbial respiration. The total amount of C respired in each thaw period was largest during the first cycle and decreased in successive cycles. Three cycles caused a net increase in total respired C relative to the +5°C control in wet meadow tundra soil, a reduction in birch soil, and had no net effect in either alder-poplar or tussock tundra soil. These different patterns apparently resulted from differences in the quality of the soil organic matter and the relative activity of the microbial biomass. Net N mineralization was generally enhanced relative to the +5°C controls in the first cycle, but was inhibited in the third cycle, similar to what was observed with C. Over multiple freeze-thaw cycles, the initial response of C and N mineralization appear to be driven by release from the microbial biomass, while over the longer-term, the response is driven by the reduction in attack on soil organic matter resulting from a reduced microbial population.