Effects of in situ freeze-thaw cycles on winter soil respiration in mid-temperate plantation forests

As an important factor regulating soil carbon cycle, freeze-thaw cycle significantly affects winter soil respiration in temperate regions. However, few in situ studies have been carried out to evaluate the effect of freeze-thaw cycle on soil respiration. Here, a field experiment was conducted to explore the response of winter soil respiration to freeze-thaw cycle and the underlying mechanisms in larch and Chinese pine plantation forests in a mid-temperate region. These results indicated that CO2 emissions during the freeze-thaw period accounted for 18.89–18.94% and 0.79–1.00% of the cumulative winter CO2 emissions and the annual soil CO2 emissions, respectively. Soil respiration rates during the thawing phase were 1.54–3.95 times higher than those during the freezing phase, which was mainly due to the increase of soil microbial biomass upon thawing. This effect declined during the second freeze-thaw cycle compared to the first freeze-thaw cycle due to the exhaustion of resources for microbes. The different responses of soil CO2 flux to freeze-thaw cycle between the two types of forests were mainly because of the difference in the thickness of litter layer, which plays an important role in regulating soil temperature and enzyme activity. These results suggest the intensity and frequency of freeze-thaw cycle strongly affect soil carbon emissions during the freeze-thaw cycle period. Therefore, these factors should be considered in laboratory studies and model simulations under climate change scenarios. The locally weighted polynomial regression analysis indicated that during the first FTC, the changing trends of MBC and MBN were consistent with that of soil CO2 efflux, whereas the changing trends of EOC, EON, enzyme, and NH4+ were opposite to that of soil CO2 efflux. Soil CO2 efflux was significantly correlated with soil temperature during the entire FTC. [Display omitted] •CO2 flux exhibited a small response to FTC due to the exhaustion of resources.•The intensity and frequency of FTC affected the effect of FTC on soil CO2 flux.•The burst of CO2 flux during thawing period was mainly due to the increase in MBC.•The thickness of litter layer affected the response of soil CO2 flux to FTC.