Variability in soil CO2 fluxes across a range of forest types and edaphic conditions

Forests play a key role in the global carbon (C) balance. On the one hand, a large amount of C is sequestered in soils, and on the other hand, the forest soils are also a significant source of carbon dioxide (CO₂). Soil respiration includes anaerobic and aerobic microbial respiration, and root respiration which may contribute even more that half of the total soil respiration. Assessment of the contribution of forest soils to CO₂ emissions, in addition to C sequestration, is worth special attention in the context of increasing climate change. To address this field experiments were carried out to assess the CO₂ fluxes of 10 different forest soil types with different tree species (deciduous, coniferous, and mixed) in Poland (using static chamber method). The highest CO₂ emissions were observed for a silty soil under the youngest deciduous forest (12 y.) with a  daily average of 1.66 ± 0.7 g CO₂ m^-2 d^-1. The lowest daily mean CO₂ flux was associated with a sandy soil in a mature stand of a predominantly coniferous forest (0.87 ± 0.3 g CO₂ m^-2 d^-1). Annual averages were in the range 3.21 t C ha^-1 to 6.06 t C ha^-1 for a mature and young forest, respectively. The main factor causing differences in CO₂ emissions could have been the contribution of both trees and soil properties to hydrological conditions. The young forest was covered with trees with a lower root system forest and the young trees could have a lower demand for water resulting in a higher soil moisture content than in a mature forest soil. Different CO₂ fluxes could be also a result of a higher water storage capacity in silty soil in the young forest than that of a sandy soil under mature stand. In addition to water supply, the activity of soil microorganisms is also regulated by C availability which was about 30% lower in sandy soil than in silty soil. The two-yearly measurements showed seasonal variations in CO₂ fluxes depending on the soil type, age and tree species. Regardless of the characteristics of the forest being studied, the highest CO₂ emissions occurred in the summer or spring and the lowest CO₂ emissions were found  in winter as a result of a strong influence of temperature on the biological processes under investigation. The observed seasonality in CO₂ emission may be attributed to changes in soil moisture during the measurement periods since soil water content regulates microbial activity and gaseous diffusion. Statistical analyses, however, imply that temperature could have  a stronger control over CO₂ emissions from the soils studied than soil moisture.

Research was conducted under the project financed by Polish National Centre for Research and Development within of ERA-NET CO-FUND ERA-GAS Programme (ERA-GAS/I/GHG-MANAGE/01/2018) “GHG-Manage”.