A New Method for Ultra-Low Sulfate Extraction and a Pilot Study in Arid Soils

Sulfur in soils is a crucial nutrient for all organisms including plants. Sulfate (SO42-), a water soluable anion, is a good source of this important element. Because of the importance of sulfur for agriculture, the use of sulfate has been well studied in temperate climate zones, and it is generally accepted that microbes play a critical role in controlling the availability of sulfate. Organically bound sulfur, such as sulfate esters and carbon-bonded sulfur, compounds that are generated and decomposed by microbes, are essential sulfur sources for plants under these conditions. For arid environments, knowledge of sulfate turnover in soils by microbes and plants is much more limited. It is not known whether sulfate turnover by microbes in desert soils is more or less critical than in temperate climates. El Paso, Texas is located in the Chihuahuan Desert, an arid environment. It is also surrounded by agricultural areas, which receive sulfate through application of fertilizers and irrigation waters; is located in proximity of White Sands National Park, a source of gypsum dust; and receives water from two aquifers, Mesilla and Hueco Bolson that have relatively high sulfate concentrations. These contrasts make El Paso a unique site for sulfur cycling studies. Theoretically, it is possible to trace the sulfate turnover by measuring changes in the sulfur and oxygen isotope of sulfate. The challenge for this approach is that sulfate in arid soils exists only in low concentration, which renders accurate determination of isotope compositions difficult. The objectives of this study were 1) to develop a new method to extract low sulfate from soils for isotope analysis and 2) to carry out a pilot study on microbial turnover of sulfate in soils from nearby White Sands National Park, El Paso Texas and the Indio Mountains. So far, methods for the extraction of sulfate from solutions with low sulfate concentrations relied on techniques that employed ion exchange resins. This approach requires large volumes of the solution from which sulfate is to be extracted, and only works for solutions with low ion strength. Thus, sulfate extraction from high-chloride, low-sulfate solutions was not possible. My now proven technique uses the classical approach of collecting sulfate as barium sulfate, followed by a chelator-assisted dissolution-reprecipitation step which returns a pure barium sulfate sample. The method can be used on solutions with high chloride levels, up to the equivalent of chloride encountered in seawater (\u3e500 mM), and enables extraction of sulfate from samples with sulfate concentrations as low as 0.03 µM and volumes below of 50 ml. The application of the developed technique to soils from a site on the ‘Lost Dog Trail’ (El Paso), and a location in the Indio Mountains revealed that arid soils have indeed extremely low sulfate content. The observed values for the Indio Mountains are ~0.5 µmol of sulfate per gram of soil. No sulfate was recovered from the Lost Dog Trail samples, suggesting a sulfate content of less than 0.1 µmol of sulfate per gram of soil for this site. The sulfur isotope composition of the sulfate obtained from the Indio Mountains is almost identical to the isotope composition of sulfate found near the White Sands National Park, indicating that transport of dust from this site could be a major sulfur source to this region. This input could be in form of dry deposition, or happen during rain events. In order to obtain insight if biological sulfur cycling takes place in the investigated arid soils, a strong rainfall event was mimicked. The soils were covered with water that was amended with sulfate (~210 µM, approximately 3 times the typical value for rainwater over North America) and labeled with 18O-enriched water. Decreasing sulfate concentrations and an enrichment in 18O of sulfate revealed that microbial sulfur cycling takes place after rainfall events, and that oxygen isotopes of sulfate can be successfully applied to trace soil sulfur cycling in arid soils