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

(NO3-)-N-15 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 degrees 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 N-2 (75.5:24.5) in order to determine labelled gaseous N emission by the are redistribution method. The Ar-N-2 atmosphere was transferred to an evacuated arcing vessel to give a pressure of 58.5 kPa and O-2 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 N-2 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 + N-2 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 N-2 (93-96%) pools. Addition of glucose (100 mu g C g(-1) soil) had no effect on the evolution of N-2 or N2O during incubation, or on the distribution of labelled N at day 6. Total loss of nitrate or total production of N-2 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.