Denitrification and nitrogen immobilization as affected by organic matter and different forms of nitrogen added to an anaerobic water-sediment system

The effects of wheat straw and different forms of N on denitrification and N immobilization were studied in an anaerobic water-sediment system. The water-sediment system was supplemented with various combinations of wheat straw and ¹⁵N-labelled and unlabelled (NH₄)₂SO₄ or KNO₃, and incubated anaerobically at 30°C for 10 days. ¹⁵N-labelled and unlabelled NO _inf3 ^sup- , NO _inf2 ^sup- , NH _inf4 ^sup+ , and organic N were determined in the water-sediment system. The gases evolved (N₂, CO₂, N₂O, and CH₄) were analyzed by gas chromatography at regular intervals. Larger quantities of ¹⁵N₂–N and organic ¹⁵N were formed in wheat straw-amended systems than in non-amended systems. Trends in CO₂ production were similar to those of N₂–N evolution. The evolution of N₂O and CH₄ was negligible. Denitrification processes accounted for about 22 and 71% of the added ¹⁵NO _inf3 ^sup- –N in the absence and presence of wheat straw, respectively. The corresponding denitrification rates were 3.4 and 12.4 g ¹⁵Ng^-1 dry sediment day^-1. In systems amended with ¹⁵NO _inf3 ^sup- –N and ¹⁵NO _inf3 ^sup- +NH _inf4 ^sup+ –N without wheat straw, 1.82 and 1.58%, respectively, of the added ¹⁵NH _inf3 ^sup- –N was immobilized. The corresponding figures for the same systems supplemented with wheat straw were 5.08 and 4.10%, respectively. Immobilization of ¹⁵NO _inf4 ^sup+ –N was higher than that of ¹⁵NO _inf3 ^sup- –N. The presence of NO _inf3 ^sup- –N did not stimulate NH _inf4 ^sup+ –N immobilization.