OOS 47-7
Non-linear response of soil carbon gas (CO2, CH4) flux to oxygen availability
Soil oxygen (O2) concentration can impact the fluxes of carbon dioxide (CO2) and methane (CH4), and contribute large differences in ecosystem carbon (C) storage at local to landscape scales. Soil O2 is increasingly being measured to explore redox dynamics even in drained upland soils, but few studies have established quantitative relationships between gas-phase O2 concentration and soil C fluxes in controlled settings. Though various intrinsic (Michaelis-Menten enzyme kinetics) and extrinsic (gas or substrate transport) mechanisms could lead to highly non-linear relationships between O2 concentration and C gas fluxes, existing laboratory studies have imposed coarse or narrow changes in O2 concentration that necessarily prevent detection of non-linearity. We report on the results of laboratory incubations designed to explore the short-term sensitivity of soil CO2 and CH4 emissions to a wide range of gas-phase O2 concentrations. An organic-rich Histosol was collected from a drained peatland and subjected to seven O2 concentration treatments ranging from 0.03 % - 20 % O2. We compared the fit of the observed C flux response to O2 concentration to linear and non-linear (log) model, and used the coefficient of determination (R2) and distribution of residuals as model performance metrics.
Results/Conclusions
Soil CO2 fluxes increased (P < 0.001) with increasing O2 concentration from 180 ± 5 μg C g-1 d-1 at 0.03 % O2 to 227 ± 16 μg C g-1 d-1 at 20 % O2 whereas soil CH4 fluxes decreased (P < 0.001) from 303 ± 32 ng C g-1 d-1 to 77 ± 11 ng C g-1 d-1 across the same range of O2 concentrations. Net CH4 emission rates were attenuated at higher O2 concentrations most likely due to stimulation of gross CH4 consumption. A log-linear fit outperformed a linear model fit for gas flux response to O2 concentration: log-linear R2 values were 0.49 and 0.70 for CO2 and CH4 flux respectively, in contrast to 0.38 and 0.40 for the linear model. This study demonstrates the potential importance of O2 in regulating soil C gas emissions and supports significant effects of even trace concentrations of O2 in soils.