DIFFERENTIATING BIOTIC VS. ABIOTIC CO2 IN THE FORMATION OF PEDOGENIC CARBONATE IN AN AGRICULTURAL SITE

Drylands receive little precipitation and must be agriculturally amended and irrigated to grow crops. Aside from loading salts to soils, the irrigation promotes accumulation of secondary calcite. Pedogenic carbonate formation is an enigma in drylands since it affects the soil properties hydrologically and biogeochemically and the carbon cycle. In agricultural sites, HCO₃^- and Ca²⁺ from irrigation water produce both CaCO₃ and abiotic CO₂. Soil respiration also releases a considerable amount of CO₂ and consumes O₂. This research strives to investigate abiotic and biotic CO₂ production processes in agricultural soils while elucidating on the contributions of different sources of CO₂.

We used a series of methods at a pecan orchard in Tornillo, Texas to monitor CO₂ and O₂ concentrations and δ¹³C ratios in agricultural soils. The orchard experiences seasonal flood irrigations from April-October, and the primary source of irrigation water is the Rio Grande River, supplemented by local groundwater from the Hueco Bolson aquifer. We focused on two sites with contrasting soil textures: Pecan_Coarse and Pecan_Fine. For both, we coupled O₂/CO₂ sensors (at 30 and 60 cm depth), deployed soil moisture sensors (at 30, 60, 90, 120, 150 cm depth), and installed soil gas samplers for pCO₂ and δ¹³CO₂ measurements (at 30, 60, and 100 cm depth). Additionally, we collected irrigation water samples for four events, for various analyses including alkalinity, and δ¹³C_DIC.

Concentrations of CO₂ from the sensor measurements match the gas samples for pCO₂ at corresponding depths, validating sensor and gas sampling methods. Soil texture at different depths has different controls for the O₂ vs CO₂ production and transport. Pecan_Fine tends to have higher CO₂ concentrations at 60 and 100 cm in regards to a time series for gas samples. CO₂ concentration increases with depth in all soil profiles, and after each irrigation event, there is a rapid decrease due to gas evacuating the soil zone and into the atmosphere as a result of water occupying the pore spaces. Keeling plot using CO₂ concentrations along with carbon isotopes suggests that Pecan_Fine has more abiotic CO₂ reactions while the Pecan_Coarse predominantly has biotic CO₂.