https://doi.org/10.1016/j.geoderma.2023.116493

Fairbairn L., Rezanezhad F., Gharasoo, M., Parsons C.T., Macrae M.L., Slowinski, S., and Van Cappellen P. (2023). Relationship between soil CO2 fluxes and soil moisture: Anaerobic sources explain fluxes at high water content. Geoderma Volume 434, June 2023, 116493 https://doi.org/10.1016/j.geoderma.2023.116493

Soil moisture is a known environmental factor influencing carbon dioxide (CO2) emissions and therefore represents an important variable in predictive models. Establishing relationships between soil CO2 emissions and soil moisture has long focused on the role of soil organic carbon mineralization by aerobic respiration. This approach, which generally yields a bell-shaped relationship establishing maximum CO2 production at moderate soil moisture, ignores anaerobic processes as a potential source of CO2. To decouple the effects of soil moisture and O2, we conducted a factorial batch experiment by incubating soil samples at different imposed moisture contents (30%, 45%, 65%, 80%, and 100% water-filled pore space; WFPS) at 25 °C, under both oxic (normal air) and anoxic (N2 atmosphere) headspace conditions. Gas fluxes measured in the oxic incubations show that CO2 fluxes were maximal (31.2 ± 1.8 nmol cm−3 soil hr-1) at moderate moisture content (65% WFPS), as commonly reported. However, contrary to previous models that predict negligible CO2 fluxes under fully saturated conditions due to O2 limitation, substantial fluxes of CO2 (18.1 ± 2.2 nmol cm−3 soil hr-1) were measured at 100% WFPS. In the anoxic treatments, CO2 fluxes rose sharply when the moisture content exceeded 65% WFPS, with values at 100% saturation (21.8 ± 2.2 nmol cm−3 soil hr-1), close to the corresponding fluxes in the oxic incubations. Methane (CH4) fluxes in the anoxic incubations increased over time, ultimately reaching parity with the CO2 fluxes at 100% WFPS. To reproduce the soil moisture dependence of the CO2 fluxes, we propose a kinetic model representing both aerobic and anaerobic CO2 production. Together, the gas flux measurements, porewater geochemistry data, and modeling results indicated that at soil moisture contents approaching saturation (≥90%), anaerobic processes were the major source of CO2 in the oxic incubations. Overall, we conclude that existing models may underrepresent soil CO2 production at high soil moisture by not considering anaerobic reaction pathways releasing CO2.

Highlights
•Soil CO2 fluxes in oxic incubations peak around 65% moisture content.
•Oxic and anoxic incubations exhibit comparable CO2 fluxes at 100% saturation.
•When moisture exceeds 65%, anaerobic CO2 production rises sharply.
•Even in oxic incubations anaerobic CO2 production dominates at 100% saturation.

Keywords
Soil CO2 fluxes
Soil moisture
Heterotrophic respiration
Anaerobic processes

https://doi.org/10.1016/j.geoderma.2023.116493