DIFFERENTIATING BIOTIC VS. ABIOTIC CO2 IN THE FORMATION OF PEDOGENIC CARBONATE IN AN AGRICULTURAL SITE
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 δ13CDIC.
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.