Contribution of abiotic factors to CO2 emission from soils in the freeze-thaw cycles

Three consecutive freeze-thaw cycles (FTCs) have been performed in a laboratory experiment with lessivated burozems (Haplic Luvisols) under forest and agricultural crops in order to assess the contribution of physicochemical factors to the emission of CO2 from soils in consecutive FTCs. The use of native soils and their sterilized analogues (to exclude the effect of biotic factors) has allowed a quantitative description of physicochemical processes during the FTCs. It has been revealed that the ratio between the biotic and physicochemical factors during the FTCs largely depends on soil solution pH. The proportion of physicochemical factors is low (1.3-9.4%) in the frozen forest soil with low pH (3.9-4.1), while it is no less than 52% in the neutral arable soils, which indicates the prevalent contribution of physicochemical processes to the emission of CO2 from these soils under freezing. CO2 sorption has been observed in the thawed forest soils at an insignificant rate, which we estimate at only 2.3-3.6% of the total CO2 flux from the soil, which confirms the biotic nature of increase in CO2 emission bursts in the acid forest soils under thawing. In the arable soils, along with the enhanced CO2 emission caused by the activation of microbial community, a significant participation of physicochemical processes has been observed under thawing conditions, which makes up 17-20% of the total CO2 flux released during thawing.