Oxygen exchange with water alters the oxygen isotopic signature of nitrate in soil ecosystems

Abstract

Combined oxygen (O) and nitrogen (N) stable isotope analyses are commonly used in the source determination of nitrate (NO3−). The source and fate of NO3− are studied based on distinct O and N isotopic signatures (δ18O and δ15N) of various sources and isotopic effects during NO3− transformation processes, which differ between sources like fertilizer, atmospheric deposition, and microbial production (nitrification). Isotopic fractionation during production and consumption of NO3− further affects the δ18O and δ15N signal. Regarding the δ18O in particular, biochemical O exchange between O from NO3− and H2O is implicitly assumed not to affect the δ18O signature of NO3−. This study aimed to test this assumption in soil-based systems. In a short (24 h) incubation experiment, soils were treated with artificially 18O and 15N enriched NO3−. Production of NO3− from nitrification during the incubation would affect both the 18O and the 15N enrichment. Oxygen exchange could therefore be studied by examining the change in 18O relative to the 15N. In two out of the three soils, we found that the imposed 18O enrichment of the NO3− declined relatively more than the imposed N15–NO3− enrichment. This implies that O exchange indeed affected the O isotopic signature of NO3−, which has important implications for NO3− source determination studies. We suggest that O exchange between NO3− and H2O should be taken into consideration when interpreting the O isotopic signature to study the origin and fate of NO3− in ecosystems.