How Winter Soil CO2 Fluxes Respond To Altered Snow Depth In A Temperate Forest Ecosystem

Climate change models suggest that the average temperature of the Earth’s atmosphere will gradually increase 2~5 °C over the next decades. The depth of snow cover may change under global warming scenarios, especially in mid-temperate ecosystems, which may significantly affect soil carbon fluxes in winter. Previous studies have demonstrated that winter soil carbon emissions can account for 3~50% of annual carbon emissions. Therefore, it is important to assess accurately how winter soil CO₂ fluxes respond to altered snow cover depth.

The snow is like a quilt, which has a certain heat preservation effect on the soil. A thin or thick snow cover could change the physical, chemical, and biological properties of the soil by altering the soil temperature and soil moisture, which may influence the cycle of soil carbon. Snowpack effect on the carbon cycle under global warming has become a hot topic in the world. Previous researches have focused on ecosystems with high altitude or latitude, whereas limited studies have been done in temperate forest ecosystems. Temperate ecosystems are deemed to be the main terrestrial carbon sink in the northern hemisphere. Although temperate ecosystems have shorter winters, they also have more frequent variations in the time and amount of snow cover in comparison with arctic and boreal ecosystems. Therefore, it is also important to examine the effects of altered snow depth on winter soil CO₂ fluxes in temperate ecosystems.

To simulate anticipated effects of climate change on winter soil CO₂ fluxes in a temperate forest ecosystem, an artificial snow manipulation experiment (including 50% removal of snowpack, 50% increase of snowpack, ambient snow) was conducted to explore the response of winter soil CO₂ fluxes to altered snow depth in a temperate forest ecosystem.

Our results showed that winter soil CO₂ fluxes ranged from 0.09 µmol m^-2 s^-1 to 0.84 µmol m^-2 s^-1, with a mean at 0.32 µmol m^-2 s^-1 during the whole winter. The cumulative winter soil CO₂ fluxes accounted for annual soil CO₂ fluxes by 5.5%~5.8%. In our study, when the depth of snow cover was less than 7 cm after snow manipulation, snow manipulation had no significant effect on winter soil CO₂ fluxes. This may be mainly attributed to the thermal insulation of the snow. Snow does have a thermal insulation effect on the soil, but only when the thickness of the snow layer reaches approximately 20 cm can the soil be completely insulated. No significant changes in soil nutrients, soil microbial biomass, soil microbial community structure, or soil extracellular enzymatic activity further confirmed that winter soil CO₂ fluxes were not affected by altered snow depth under the condition of low snow cover depth.

Interestingly, when the depth of snow cover reached approximately 13 cm after snow manipulation, snow addition could significantly increase winter soil CO₂ fluxes. The reason is that higher soil microbial activities under snow addition caused by increased soil temperature lead to the increase in winter soil CO₂ fluxes relative to ambient snow. However, our results indicated that these effects were transient and quickly disappeared. These suggested that soil microorganisms could quickly acclimate to fluctuations in snow depth, resulting in no significant change in winter soil CO₂ fluxes between snow addition and ambient snow. Consequently, cumulative winter soil CO₂ fluxes were not affected by changed snow cover depth. To conclude, based on this short-term experiment, our observations suggest that responses of winter soil CO₂ fluxes to snow manipulation is minor in temperate forest ecosystems with low winter precipitation.