17 Science at the Intersection of Soils, Weathering, Geomorphology, Climate, Ecology and Wildfire—A Tribute to the Careers of Les McFadden and Grant Meyer

Wednesday, 16 March 2022
8:00 AM - 12:00 PM
CBC-A 108

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

Paper number: 17-5
Program: Symposia Sessions
Time: 9:20 AM – 9:40 AM

Authors

Abstract

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 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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 2

doi: 10.1130/abs/2022CD-373583

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