Contribution of denitrification and autotrophic and heterotrophic nitrification to nitrous oxide production in andosols

Abstract

To quantify the contribution of denitrification and autotrophic and heterotrophic nitrification to N₂O production in Andosols with a relatively high organic matter content, we first examined the effect of C₂H₂ concentrations on N₂O production and on changes in mineral N contents. The optimum C₂H₂ concentration for inhibiting autotrophic nitrification was 10 Pa. Secondly, and Andosol taken from an arable field was incubated for 32 days at 30°C at 60, 80, and 100% water-holding capacity with or without the addition of NH ⁺₄ or NO ^sup-_inf3 (200 mg N kg^-1), and subsamples collected every 4–8 days were further incubated for 24 h with or without C₂H₂ (10 Pa). At 60 and 80% water-holding capacity with NH ⁺₄ added, 87–92% of N₂O produced (200–250 μg N₂O−N kg^-1) was derived from autotrophic nitrification. In contrast, at 100% water-holding capacity with or without added NO ^sup-_inf3 , enormous amounts of N₂O (29–90 mg N₂O−N kg^-1) were produced rapidly, mostly by denitrification (96–98% of total production). Thirdly, to examine N₂O production by heterotrophic nitrification, the Andosol was amended with peptone or NH ⁺₄ (both 1000 mg N kg^-1)+citric acid (20 g C kg^-1) and with or without dicyandiamide (200 mg N kg^-1). Treatment with citric acid alone or with citric acid+dicyandiamide suppressed N₂O production. In contrast, peptone increased N₂O production (5.66 mg N₂O−N kg^-1) mainly by denitrification (80% of total production). However, dicyandiamide reduced N₂O production to 1.1 mg N₂O−N kg^-1. These results indicate that autotrophic nitrification was the main process for N₂O production except at 100% water-holding capacity where denitrification became dominant and that heterotrophic nitrification had a lesser importance in the soils examine.