A Combined Field, Experimental And Modeling Approach To Understand The Impacts Of Flood Irrigation On Pedogenic CaCO3 And CO2 Dynamics In Drylands

Soil salinization is a global problem affecting 10% of agricultural soils, particularly in irrigated arid-lands. This study investigated salt loading through flood-irrigation, particularly of calcite (CaCO3) salt in the southwest of the United States. To evaluate the impact that flood-irrigation has on CaCO3 buildup and its effect on C-cycling, a natural soil and two agricultural soils were sampled: a pecan orchard and an alfalfa field in the El Paso, TX region. Agricultural soils in this study are flood irrigated by the Rio Grande river and by ground waters in its absence, both of which are oversaturated with calcite. To trace for Ca2+, 87Sr/86Sr characterizations of irrigation water, dust, soil, soil water, and soil amendment samples were conducted to determine controlling sources pedogenic carbonates in agricultural soils. By also characterizing 13C in SOC, carbonates in the soil profiles, DIC, major ions in the irrigation waters, soil waters, and CO2 in soil gases from the highly managed pecan orchard, the relative impact of flood-irrigation and natural processes with mass balance modeling was constrained. Flow-through column experiments of varying textures and flood water salinities helped elucidate physical and abiotic processes that control calcite kinetics and CO2 efflux. This work highlights flood irrigation as the major mechanism of soil salinization and C-cycling in our sample sites. All soils were found to have high salinity and sodicity, are controlled by the quantity/quality of irrigation waters and by variations in soil-texture. Pedogenic carbonate precipitation in agricultural fields is driven by Ca2+ loadings of irrigation practices and not by natural processes. This Dissertation shows a previously unmeasured CO2 flux as a consequence of agricultural calcite formation and propose that these are a function of soil moisture loss in combination with atmospheric pressure pumping. Furthermore, this work suggests that pore-fluid EC can be used successfully a proxy to determine calcite-sourced CO2 efflux. Supplementary work is suggested to determine if CO2 emissions are significant in regional and global carbon budgets