Using Strontium Isotopes in Arid Agricultural Soils to Determine a Sink or Source of CO2

Abstract:

Arid and semi-arid regions of the world are predicted to continue to expand through land degradation and prolonged drought events. Agricultural practices in these drylands degrade soils through elevated salinity, sodicity and alkalinity. Indeed, flood irrigation loads salts onto the soils including carbonate minerals in the form of calcite. Alfalfa and Pecan are salt tolerant and commonly grown in the arid El Paso region, but need irrigation using Rio Grande water with little to no contribution from local ground waters. We hypothesize that the irrigation is loading extra Ca and bicarbonate to soils and anthropogenically enhancing the precipitation of carbonates. We intend to monitor soil CO₂ efflux after irrigation, characterize soil minerals, and combine them to isotopic data of soil, irrigation, and drainage waters to link the sources of Ca and C, kinetics of calcite precipitation, to irrigation events. This will include strontium isotopic analysis to determine the source of calcium in the agricultural fields, U-disequilibrium isotopes to estimate the carbonate ages, and CO₂ efflux to monitor atmosphere-soil exchange. Carbon dioxide emissions are expected to change during flood irrigation when soils are saturated. After irrigation events, evaporative effects increase Ca and dissolved inorganic carbon concentration in soil waters leading to precipitation of calcite and thus elevated CO₂efflux.

Preliminary measurements in the pecan field show a marginally significant difference in CO₂ fluxes before and after irrigation (p=0.07, t-test). Carbon dioxide emissions are lower during moist conditions (0.6 g m^-2hr^-1 CO₂) than those in dry conditions (1.0 g m^-2hr^-1 CO₂). Future C isotope data are needed to identify the source of extra CO₂, biogenic or calcite-precipitation related. A water leachable extraction of alfalfa soils shows ⁸⁷Sr/⁸⁶Sr ratios ranged from 0.7101 to 0.7103, indicating Rio Grande river as a dominant calcium source. Further Sr isotopic analysis of sequential extractions from both alfalfa soils (for calcite fraction), pore waters, irrigation waters, and drainage waters will allow us to quantitatively evaluate the source of calcium in calcite therefore, defining if anthropogenic sources of calcium lead to enhanced CO₂ production in agricultural fields.