Abstract
Agricultural fields in drylands are challenged globally by limited freshwater resources for irrigation and also by elevated soil salinity and sodicity. It is well known that pedogenic carbonate is less soluble than evaporate salts and commonly forms in natural drylands. However, few studies have evaluated how irrigation loads dissolved calcium and bicarbonate to agricultural fields, accelerating formation rates of secondary calcite and simultaneously releasing abiotic CO2 to the atmosphere. This study reports one of the first geochemical and isotopic studies of such “anthropogenic” pedogenic carbonates and CO2 from irrigated drylands of southwestern United States. A pecan orchard and an alfalfa field, where flood-irrigation using the Rio Grande river is a common practice, were compared to a nearby natural dryland site. Strontium and carbon isotope ratios show that bulk pedogenic carbonates in irrigated soils at the pecan orchard primarily formed due to flood-irrigation, and that approximately 20–50% of soil CO2 in these irrigated soils is calcite-derived abiotic CO2 instead of soil-respired or atmospheric origins. Multiple variables that control the salt buildup in this region are identified and impact the crop production and soil sustainability regionally and globally. Irrigation intensity and water chemistry (irrigation water quantity and quality) dictate salt loading, and soil texture governs water infiltration and salt leaching. In the study area, agricultural soils have accumulated up to 10 wt% of calcite after just about 100 years of cultivation. These rates will likely increase in the future due to the combined effects of climate variability (reduced rainfall and more intense evaporation), use of more brackish groundwater for irrigation, and reduced porosity in soils. The enhanced accumulation rates of pedogenic carbonate are accompanied by release of large amounts of abiotic CO2 from irrigated drylands to atmosphere. Extensive field studies and modelling approaches are needed to further quantify these effluxes at local, regional and global scales.
Highlights
Irrigated agriculture is expanding in drylands to produce crops that meet rising food demands and support local economy[1,2]
This study builds on previous investigation on sources and controls of soil salinity in agricultural soils[12,13,14], and collects new C and Sr isotope data to determine the relative contribution of irrigationinduced reaction (1) to the overall soil CO2 and bulk soil pedogenic carbonate
The soil organic carbon (SOC) contents in both soil profiles are high near ground surface, at ~ 1.5 wt%, and decreases sharply with depth (Fig. S3A)
Summary
Irrigated agriculture is expanding in drylands to produce crops that meet rising food demands and support local economy[1,2]. Soils have been converted to managed agricultural fields along the Rio Grande valley in western Texas and southern New Mexico for more than 100 years[6] This region is a typical dryland system with mean annual precipitation at ~ 16–25 cm and annual potential evapotranspiration at ~ 194 cm[7,8]. Large areas of cropland are inundated by diverting Rio Grande river water through canals and by pumping groundwater from deep aquifers Such flood irrigation is not a water-conservative method and has led to high evaporative water loss especially during hot and dry summers, salt accumulation, and a decrease in soil permeability, quality and p roductivity[8,9,10,11,12,13,14]. This study builds on previous investigation on sources and controls of soil salinity in agricultural soils[12,13,14], and collects new C and Sr isotope data to determine the relative contribution of irrigationinduced reaction (1) to the overall soil CO2 and bulk soil pedogenic carbonate
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