Fate of Ammonium in a Gulf Coast Estuarine Sediment

Abstract The loss of 15 N labeled (NH 4 ) 2 SO 4 applied uniformly to estuarine sediment was investigated under laboratory conditions. The recovery of 15 N labeled NH + 4 ‐N in the NH + 4 , NO − 2 + NO − 3 , and organic N pools was determined at depth within the sediment cores. Concentrations of NH + 4 ‐N generally increased with depth, with largest gradients near the surface. There was significant ammonification in underlying anaerobic sediment with a consistent increase in concentration of NH + 4 being observed with increasing time of incubation. Surface sediment became depleted in 15 NH + 4 ‐N with time of incubation due to nitrification. Nitrogen‐15 losses occurred in the surface layer predominately from nitrification‐denitrification of the NH + 4 . Net NH + 4 incorporation into the organic N fraction (immobilization) was greater in the aerobic surface sediment than in the underlying anaerobic sediment. In contrast, after 70 d of incubation, anaerobic sediment partitioned 65 ± 9% of the added 15 N in the inorganic NH + 4 ‐N pool and 27 ± 7% in the organic N pool. Total 15 N losses increased with time of incubation and ranged from 9 to 26% following the addition of 3, 6.5, and 17 mg 15 N excess kg −1 dry sediment.

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