Accumulation of Evaporite Minerals and Pedogenic Carbonates in the Agricultural Soils of Southwest US: An Imporant CO2 Efflux from Dryland to Atmosphere?.

Soils along the Rio Grande floodplains have been intensively cultivated for the last 200 years. The Rio Grande river and local groundwater are used for flood irrigation and their high salinity has led to elevated soil salinity, sodicity and alkalinity. We studied accumulation of evaporite minerals and pedogenic carbonates in southern New Mexico and western Texas, and focused on agricultural soils of two major crops, alfalfa and pecan. The dominant controls for the salt accumulation rates are irrigation water chemistry and soil texture. Indeed, clayey soils limit the penetration of irrigation water, resulting in salt precipitation right above these impermeable layers after continuous and intensive water loss through evapotranspiration. Interestingly, the irrigation water is oversaturated with respect to CaCO₃ in the form of calcite, driving the pedogenic carbonate accumulation and release of CO₂. To date, few studies have quantified the production and emission of CO₂ during the development of pedogenic carbonate in dryland agricultural settings. The challenge is to separate the contributions of biogenic CO₂ (from soil respiration) and abiotic CO₂ (from calcite precipitation). We studied such soil-water-gas interaction in a flood-irrigated pecan fields in El Paso, Texas, and characterized the organic matter and carbonates in the soil profiles, dissolved inorganic carbon and major cations in the irrigation and soil waters, and CO₂ in soil gases, as well as CO₂ efflux from land to atmosphere. Using stable carbon isotopes, field measurements, and column experiments, we modelled the dynamics of inorganic and organic carbon, quantified the pedogenic carbonate development (soil C sequestration) and soil-atmosphere CO₂ exchange, and clearly demonstrated that dryland agriculture practices might be very important in land-carbon-climate feedback.