Distribution of reduced products of 15N-labelled nitrate in anaerobic soils

15NO3− was added to two soils, and the distribution of labelled N in the organic, inorganic and gaseous N pools was measured after anaerobic incubation for 6 days at 30°C with or without addition of glucose. Samples were incubated under He to enable determination of total gaseous N emission, or under a mixture of Ar and N2 (75.5:24.5) in order to determine labelled gaseous N emission by the arc redistribution method. The ArN2 atmosphere was transferred to an evacuated arcing vessel to give a pressure of 58.5 kPa and O2 was added to give atmospheric pressure (101 kPa). The arcing efficiency was 14.1 ± 0.3%, producing 2.1 ± 0.1 mg NOxN for isotope-ratio analysis. N2O and N2 accumulated in both soils. The maximum concentration of N2O in an alkaline clay occurred much earlier (days 1–2) and was approximately one half of the peak concentration measured in an acidic clay loam (days 3–4). N2O was not detected in the incubation vessels at day 6. The production of NO2− + N2O + N2 at day 6 did not equal the total loss of nitrate. Total recovery of labelled N at day 6 was 98–100%, distributed between organic (1.0–5.8%), ammonium (1.2–1.5%), nitrite + nitrate (0–0.8%) and N2 (93–96%) pools. Addition of glucose (100 μg C g−1 soil) had no effect on the evolution of N2 or N2O during incubation, or on the distribution of labelled N at day 6. Total loss of nitrate or total production of N2 at day 6 was predicted by a simple zero order model based on measured changes in the size of the labelled pools and the ratio of mean total-to-mean labelled nitrate.