Continuous Soil Carbon Dioxide and Oxygen Measurements and Estimation of Gradient-Based Gaseous Flux

The magnitude and dynamics of soil gaseous fluxes play critical roles in the global gas balance; yet, these processes are not captured at sufficient temporal resolution by standard methods based on periodic sampling and surface chamber measurements. A novel method for continuous measurement of soil surface gas fluxes based on subsurface CO₂ and O₂ concentration gradient measurements was developed. We tested the gradient-based method under steady- and transient-state soil water content and temperature conditions and compared results with a state-of-the-art surface chamber CO₂ flux system. The new aspects of the method include fast-response sensors installed in the soil profile providing continuous record of concentration gradients coupled with concurrent estimates of water content-dependent gaseous diffusion coefficient enabling calculation of surface gaseous fluxes. Low-cost infrared sensors were used for CO₂ concentration measurements, and galvanic cells for O₂ measurements. An imposed CO₂ concentration gradient in a dry soil column resulted in a quasilinear CO₂ concentration profile and surface CO₂ flux in agreement with chamber-measured fluxes. A series of continuous concentration measurements under variable water content conditions and wetting events showed agreement with surface chamber measurements. Within several days of surface wetting, soil CO₂ concentrations attained 10 mL L⁻¹, one order of magnitude higher than ambient concentrations, whereas O₂ concentrations decreased. The gradient-based approach minimizes soil surface perturbations and provides insights into subsurface soil CO₂ and O₂ dynamics and the distribution and magnitude of soil respiration processes as related to soil environmental factors. The subsurface gradient-based measurement system represents an order-of-magnitude reduction in cost compared with research-grade surface chamber devices.