FIG. 3.

Accumulation of ¹⁵N in ²⁹N₂ (open circles) and ³⁰N₂ (filled circles) during parallel anoxic incubations of marine sediment amended with ¹⁵N-ammonium and unlabeled nitrate (same experiment as Fig. 1) (A), ¹⁵N-nitrate (B), and ¹⁵N-ammonium and ¹⁵N-nitrate (C). Percentages of labeling with ¹⁵N-ammonium and ¹⁵N-nitrate were 30 to 80% and 90%, respectively. During all experiments, unlabeled ammonium was generated through ammonification. Examples are from May 2000, Skagerrak S9, 1- to 3-cm depth, and are representative of two or more replicates.