CO2 production rate maxima in the deeper unsaturated zone of a semi-arid floodplain at Rifle, Colorado

Abstract:

Fluxes of CO₂ from soils are important to understand in order to predict subsurface feedbacks to the atmosphere and responses to climate change. Such fluxes are commonly monitored at the soil surface and generally assumed to largely originate within shallow depths. Relatively little is understood on the depth distribution of CO₂ production below the rhizosphere. We monitored CO₂ fluxes at the soil surface, and measured vertical profiles of vadose CO₂ concentrations, matric potentials, and temperatures at the Rifle Site, a saline semi-arid floodplain along the Colorado River in order to determine the significance of deeper vadose zone respiration. Vadose zone CO₂ profiles exhibit temperature-dependent seasonal variations, and are consistent with CO₂ fluxes measured at the soil surface. The measured vadose zone CO₂ concentration profiles combined with gas diffusion coefficients estimated from soil properties indicated that local maxima in rates of CO₂ production persist in the deeper vadose zone, about 1 m below the rhizosphere, and above the water table (~3.5 m below the soil surface). We hypothesized that water and oxygen activities, nutrient levels, and temperatures remain favorable for microbial respiration throughout the year in the subrhizosphere, unlike overlying drier soils and the underlying poorly aerated aquifer. Using soils and sediments from the field site, the hypothesized existence of deeper subsurface maxima in CO₂ production rate is currently being tested in the laboratory through sediment incubation experiments and in 2.0 m tall vadose zone columns. Initial results from the laboratory support the hypothesized persistence of a subrhizosphere “hot zone” for microbial respiration, partly sustained through seasonal pulses of dissolved and labile organic carbon originating from the rhizosphere. These findings suggest that similar sustained deeper local maxima in respiration rates may occur in many other regions where near-surface conditions are seasonally unfavorable for microbial activity.