LAUSR

Abstract Natural accumulation of pedogenic carbonates has been well documented but few studies have focused on carbonate formation in agricultural drylands. This study aims to determine accumulation rates of pedogenic carbonates in intensively irrigated soils, and to define key linkages between flood irrigation, salt loading and soil-atmosphere CO 2 exchange in cultivated drylands of the southwestern United States. We used a combination of elemental chemistry (CaO, soil organic and inorganic carbon contents), mineralogy, and U-series ( 238 U- 234 U- 230 Th) disequilibrium dating technique to investigate calcium sources, ages and formation rates of pedogenic carbonates. Study sites include an irrigated alfalfa field near El Paso in western Texas and a non-irrigated natural dryland site on the USDA Jornada Experimental Range of southern New Mexico. Our results showed that large amounts of dissolved calcium and inorganic carbon along with other soluble elements were loaded onto agricultural fields through irrigation waters in El Paso, TX while dust and rainfall were important for salt loading in natural soils of the Jornada. U-series activity ratios, ( 234 U/ 238 U) and ( 230 Th/ 238 U), in bulk soils suggested eolian deposits added U and modified U isotopes in shallow soils at both the irrigated and natural sites. Mobility of 234 U within the soil profile is related to leaching of U (and by inference other soluble ions) and carbonate accumulation at depth. The U-series dating technique in pedogenic carbonates revealed the presence of much younger carbonates at the irrigated site compared to the natural site. Pedogenic carbonate formation rates in the irrigated soils were also much higher than those in the non-irrigated soils, likely a result of influxes from dissolved Ca and inorganic carbon in water used for irrigation. This study demonstrates the potential for agricultural expansion and land use change in drylands to increase rate of pedogenic carbonate accumulation. Such changes may have important implications to global carbon cycling since drylands are forecast to become the most expansive terrestrial biome by mid-century and dryland agriculture is expanding quickly.