SOURCES AND ISOTOPIC CHARACTERISTICS OF VADOSE-ZONE NITRATE IN ARID REGIONS

Large quantities of nitrate exist within the vadose zone in many arid regions where distributed ground-water recharge is limited. In some cases, this nitrate appears to have accumulated over long periods of time (thousands to millions of years) by natural processes that could include atmospheric nitrate deposition or bacterial nitrification of reduced N. The reduced N may be present as a result of rock weathering, biologic nitrogen fixation and recycling, atmospheric deposition of ammonium and organic N, or from other sources. Natural vadose-zone accumulations of nitrate may be augmented by anthropogenic nitrate as a result of land-use change, and both may enter ground water as a result of climate change, flooding, irrigation, or artificial recharge. Stable isotopic analyses of N (¹⁵N:¹⁴N) and O (¹⁸O:¹⁷O:¹⁶O) indicate a variety of natural and anthropogenic sources of vadose-zone nitrate in hyper-arid to semi-arid regions such as the Atacama Desert, Mojave Desert, and High Plains region.

Atmospheric nitrate has relatively low δ¹⁵N, high δ¹⁸O, and large positive Δ¹⁷O (Δ¹⁷O=δ¹⁷O – 0.52 x δ¹⁸O) resulting from non-mass-dependent O isotope effects of photochemical reactions involving ozone. Nitrate formed by bacterial nitrification of reduced N has variable δ¹⁵N, relatively low δ¹⁸O, and Δ¹⁷O near zero. Natural nitrate salt deposits in some of the extreme hyper-arid deserts have atmospheric isotopic signatures indicating long-term accumulation of atmospheric nitrate deposition in the absence of biologic cycling. Natural vadose-zone nitrate in more moderate arid and semi-arid regions typically has O isotopic signatures indicating bacterial nitrification or mixtures of bacterial and atmospheric sources. Natural and anthropogenic occurrences of bacterial nitrate may or may not have distinctive N isotopic signatures depending on locality and N source. Although the database is limited and variability is common, stable isotopes have contributed to identifying the sources of vadose-zone nitrate, understanding links between vadose-zone nitrate and ground-water nitrate, and documenting changes in nitrate fluxes in arid regions caused by land-use change.