Tracing sources of soil nitrate using the dual isotopic composition of nitrate in 2 M KCl-extracts

Soils comprise a critical interface between the atmosphere, lithosphere, hydrosphere and biosphere, and play a major role in the cycling of nitrogen (N), an element crucial to plant growth. Isotope techniques constitute a powerful tool to study the origin and fate of N compounds (e.g. NO(3)(-)) within the environment including soils. The objective of our study was to test the usefulness of the isotope composition of soil NO(3)(-) extracted with 2 M KCl (soil NO(3)) as a tool to investigate the origin and fate of NO in the environment. Specifically issues related to repeat extractions, crop type, length of fertilization, and soil depth were addressed. Soils from four contrasting agricultural management regimes were sampled. Within the relatively confined study area (4 ha), the isotopic compositions of soil NO(3) differed markedly due to management treatments (up to 6 and 17 parts per thousand for delta(15)N and delta(18)O, respectively), but were repeatable among replicate plots (+/-1 parts per thousand). Differences in both delta(15)N and delta(18)O values were observed between legume and non-legume treatments, as well as fertilized versus non-fertilized treatments, which were larger than the variability observed between replicate plots. Differences in the isotopic composition of extractable soil nitrate were not limited to the surface layer, but also occurred within deeper soil layers. This study indicates that the analysis of the natural abundance stable isotope composition of soil NO(3) may provide a promising additional tool for tracing the origin and fate of NO(3)(-) in the soil zone. (C) 2011 Elsevier Ltd. All rights reserved.