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The carbon-to-oxygen relationship and gas exchange balance, organic carbon to CO2 conversion intensity and efficiency, and their relevance to climate parameters and wood decay fungi were investigated for birch woody debris (WD) in the Mid-Urals mixed pine and birch forests. It was shown that, within the range of temperatures from 10 to 40 °C and relative moisture (RM) of wood of 40% and 70%, aerobic gas exchange was observed in the WD, encompassing the physiologically entwined processes of CO2 emission and O2 uptake. Their volumetric ratio (0.9) confirmed that (1) the WD represents a globally significant CO2 source and appropriate O2 consumer and (2) the oxidative conversion of organic carbon is highly efficient in the WD, with an average ratio of CO2 released to O2 consumed equal to 90%. The balance of carbon-to-oxygen gas exchange and oxidizing conversion efficiency in the WD were not affected by either fungal species tested or by moisture or temperature. However, the intensity of gas exchange was unique for each wood decay fungi, and it could be treated as a climate-reliant parameter driven by temperature (Q10 = 2.0–2.1) and moisture (the latter induced a corresponding trend and value changes in CO2 emission and O2 uptake). Depending on the direction and degree of the change in temperature and moisture, their combined effect on the intensity of gas exchange led to its strengthening or weakening; otherwise, it was stabilized. Aerobic respiration of wood decay Basidiomycetes is an essential prerequisite and the major biotic factor in the WD gas exchange, while moisture and temperature are its climatic controllers only.

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