Sources of C and N contributing to the flush in mineralization upon freeze–thaw cycles in soils

In mid-latitude climatic regions (35–65°) soils may be subjected to freeze–thaw cycles (FTC) which can occur frequently in late winter and early spring. FTC often results in flushes in C and N mineralization and could therefore be an important factor controlling C and N mineralization rates. Laboratory experiments were carried out to characterize the source of organic matter that becomes available upon FTC. Soils, differing in the quantity and quality of organic matter inputs they had received since 1956, were sampled, pre-incubated to reduce amounts of labile organic matter and subsequently exposed to repeated FTC. Each FTC consisted of 6 h at −2 °C, 16 h at −5 °C, 4 h at +2 °C and 22 h at +5 °C, giving a total of 48 h for each FTC. The contribution of microbial biomass C to the C flush upon FTC was determined by labeling the native microbial biomass with a small amount of 14C-labeled glucose and comparing the specific activity of the C flush upon FTC with that upon chloroform fumigation. Temperature corrected amounts of C and N mineralized in soil incubated at constant temperatures acted as control in the calculation of the flush. FTC increased the amounts of C and N mineralized 2–3 fold. The flush was short-lived and highest in the first four FTC, suggesting that easily decomposable material became available upon FTC and that the size of the freeze–thaw susceptible pool was limited. The C flush was linearly related to organic C, water-soluble organic C, microbial biomass C and basal respiration, but only proportional to the latter two. Labeling the native microbial biomass with a small amount of glucose suggested that microbial biomass C contributed ca. 65% to the C flush upon FTC, while representing only about 5% of microbial biomass C. We have no direct evidence for the source of the remaining 35% of the C flush or for the mechanism of its release. In soils subjected to chloroform fumigation prior to being exposed to FTC organic matter released by fumigation became a more important source to the flush than the microbial biomass, suggesting that labile organic matter is highly susceptible to FTC. From our results, effects of FTC have little consequence for annual C and N budgets, but may need to be taken into account when modeling C and N mineralization during the late winter and early spring period in mid-latitude climatic regions.