The oxygen isotope composition of nitrate generated by nitrification in acid forest floors

The oxygen isotope composition of nitrate is used increasingly for identifying the origin of nitrate in terrestrial and aquatic ecosystems. This novel isotope tracer technique is based on the fact that nitrate in atmospheric deposition, in fertilizers, and nitrate generated by nitrification in soils appear to have distinct oxygen isotope ratios. While the typical ranges of delta O-18 values of nitrate in atmospheric deposition and fertilizers are comparatively well known, few experimental data exist for the oxygen isotope composition of nitrate generated by nitrification in soils. The objective of this study was to determine delta O-18 values of nitrate formed by microbial nitrification in acid forest floors. Evidence from laboratory incubation experiments and field studies suggests that during microbial nitrification in acid forest floor horizons, up to two of the three oxygen atoms in newly formed nitrate are derived from water, particularly if ammonium is abundant and nitrification rates are high. It was, however, also observed that in ammonium-limited systems with low nitrification rates, significantly less than two thirds of the oxygen in newly formed nitrate can be derived from water oxygen, presumably as a result of heterotrophic nitrification, It can be concluded from the presented data that the delta O-18 values of nitrate formed by microbial nitrification in acid forest floors typically range between +2 and +14 parts per thousand, assuming that soil water delta O-18 values vary between -15 and -5 parts per thousand. Hence, oxygen isotope ratios of nitrate formed by nitrification in forest floors are usually distinct from those of other nitrate sources such as atmospheric deposition and synthetic fertilizers and, therefore, constitute a valuable qualitative tracer for distinguishing among these sources of nitrate. A quantitative source apportionment appears, however, difficult because of the wide range of delta O-18 values, particularly for atmospheric nitrate deposition and for nitrate from microbial nitrification. Copyright (C) 2001 Elsevier Science Ltd.