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Fate of Ammonium in a Gulf Coast Estuarine Sediment¹

ABSTRACT

The loss of ¹⁵N labeled (NH₄)₂SO₄ applied uniformly to estuarine sediment was investigated under laboratory conditions. The recovery of ¹⁵N 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 ¹⁵NH⁺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 ¹⁵N in the inorganic NH⁺₄-N pool and 27 ± 7% in the organic N pool. Total ¹⁵N losses increased with time of incubation and ranged from 9 to 26% following the addition of 3, 6.5, and 17 mg ¹⁵N excess kg^–1 dry sediment.

Key Words: Louisiana • labeled N • NH⁺₄ volatilization

¹ Contribution from the Laboratory for Wetland Soils and Sediments and Nuclear Science Center, Louisiana State Univ., Baton Rouge, LA 70803.

² Assistant Professor, Louisiana State Univ. Senior author is now at Irrigation Research Inst., Tatura Victoria 3616, Australia.

Received for publication July 15, 1985.