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

The oxygen isotope composition of nitrate is used increasingly for identify ing 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 isoto pe composition of nitrate generated by nitrification in soils. The objectiv e of this study was to determine delta O-18 values of nitrate formed by mic robial 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 tw o 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 nitri fication rates, significantly less than two thirds of the oxygen in newly f ormed nitrate can be derived from water oxygen, presumably as a result of h eterotrophic nitrification, It can be concluded from the presented data tha t the delta O-18 values of nitrate formed by microbial nitrification in aci d forest floors typically range between +2 and +14 parts per thousand, assu ming that soil water delta O-18 values vary between -15 and -5 parts per th ousand. 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, consti tute a valuable qualitative tracer for distinguishing among these sources o f nitrate. A quantitative source apportionment appears, however, difficult because of the wide range of delta O-18 values, particularly for atmospheri c nitrate deposition and for nitrate from microbial nitrification. Copyrigh t (C) 2001 Elsevier Science Ltd.