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

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 degrees C at 60, 80, and 100% water-holding capacity with or without the addition of NH4+ or NO3- (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 NH4+ added, 87-92% of N2O produced (200-250 mu g N2O-N kg(-1)) was derived from autotrophic nitrifiction. In contrast, at 100% water-holding capacity with or without added NO3-, 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 hetero-trophic nitrification, the Andosol was amended with peptone or NH4+ (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 auto-trophic 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.