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

Combined oxygen (O) and nitrogen (N) stable isotope analyses are commonly used in the source determination of nitrate . The source and fate of are studied based on distinct O and N isotopic signatures (δ¹⁸O and δ¹⁵N) of various sources and isotopic effects during transformation processes, which differ between sources like fertilizer, atmospheric deposition, and microbial production (nitrification). Isotopic fractionation during production and consumption of further affects the δ¹⁸O and δ¹⁵N signal. Regarding the δ¹⁸O in particular, biochemical O exchange between O from and H₂O is implicitly assumed not to affect the δ¹⁸O signature of . 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 . Production of 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 declined relatively more than the imposed enrichment. This implies that O exchange indeed affected the O isotopic signature of , which has important implications for source determination studies. We suggest that O exchange between and H₂O should be taken into consideration when interpreting the O isotopic signature to study the origin and fate of in ecosystems.