IS PEDOGENIC CARBONATE A SIGNAL AND PRECURSOR OF ELEVATED SOIL SALINITY IN IRRIGATED AGRICULTURAL FIELDS OF DRYLANDS?

Along the Rio Grande valley in western Texas, soils are developed on floodplain sediments and have been frequently irrigated to grow crops like pecan, cotton, and alfalfa. Soils here become saline and sodic after 100 years of cultivation. In a pecan orchard at Tornillo, TX, USA, our previous studies have shown that soil texture, as the master variable, has controlled the soil moisture dynamics (especially the evaporation to transpiration ratios), dictated salt buildup rates, and impacted tree sizes and crop yields. Indeed, soils with finer particles are associated with smaller pecan trees, due to higher soil salinity exceeding the pecan trees’ tolerance levels, periodic anoxic conditions, and physical barriers that limit root development. Pedogenic carbonate is less soluble than salts, and thus precipitate before evaporite salts. For example, in natural aridlands, pedogenic carbonate commonly forms to govern the soil structure and is an important soil inorganic carbon stock globally. In the pecan orchard site due to irrigation, we have observed accelerated accumulation of pedogenic carbonate, a different form of “salt”. Is such secondary calcite a precursor and a signal of elevated soil salinity? To better link the water, salts, and carbon in different soil substrates, our team collected 100 soil cores in this small pecan orchard, and analyzed salinity, pH, soil inorganic and organic carbon contents (SIC, SOC), and particle sizes of three soil depths (shallow (0-10 cm), mid (50-60cm) and deep (90-100 cm)). The results showed that both soil salinity (as electrical conductivity, EC) and pedogenic carbonate contents (as SIC) correlated significantly with soil texture (as sand%), but higher coefficients and thus stronger correlations were observed for SIC vs. sand% than EC vs. sand%. The relationship between soil EC and SIC, however, was complex, controlled by solubility of halite, gypsum, and calcite, and also by irrigation water chemistry, soil texture and soil depth. This study helped to quantify pedogenic carbonate accumulation rates, driven by irrigation at aridland soils, and more importantly, better predict how secondary calcite accumulation will drive more salt accumulation.