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Background  The repeated freeze-thaw events during cold season, freezing of soils in autumn and thawing in spring are typical for the tundra, boreal, and temperate soils. The thawing of soils during winter-summer transitions induces the release of decomposable organic carbon and acceleration of soil respiration. The winter-spring fluxes of CO 2  from permanently and seasonally frozen soils are essential part of annual carbon budget varying from 5 to 50%. The mechanisms of the freeze-thaw activation are not absolutely clear and need clarifying. We investigated the effect of repeated freezing-thawing events on CO 2  emission from intact arable and forest soils (Luvisols, loamy silt; Central Germany) at different moisture (65% and 100% of WHC).    Results  Due to the measurement of the CO 2  flux in two hours intervals, the dynamics of CO 2  emission during freezing-thawing events was described in a detailed way. At +10°C (initial level) in soils investigated, carbon dioxide emission varied between 7.4 to 43.8 mg C m -2 h -1  depending on land use and moisture. CO 2  flux from the totally frozen soil never reached zero and amounted to 5 to 20% of the initial level, indicating that microbial community was still active at -5°C. Significant burst of CO 2  emission (1.2–1.7-fold increase depending on moisture and land use) was observed during thawing. There was close linear correlation between CO 2  emission and soil temperature (R 2  = 0.86–0.97, P < 0.001).    Conclusion  Our investigations showed that soil moisture and land use governed the initial rate of soil respiration, duration of freezing and thawing of soil, pattern of CO 2  dynamics and extra CO 2  fluxes. As a rule, the emissions of CO 2  induced by freezing-thawing were more significant in dry soils and during the first freezing-thawing cycle (FTC). The acceleration of CO 2  emission was caused by different processes: the liberation of nutrients upon the soil freezing, biological activity occurring in unfrozen water films, and respiration of cold-adapted microflora.

Influence of freeze-thaw events on carbon dioxide emission from soils at different moisture and land use