CONTRIBUTION OF DENITRIFICATION AND AUTOTROPHIC AND HETEROTROPHIC NITRIFICATION TO NITROUS-OXIDE PRODUCTION IN ANDOSOLS

To quantify the contribution of denitrification and autotrophic and he terotrophic nitrification to N2O production in Andosols with a relativ ely 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 nitrificati on was 10 Pa. Secondly, and Andosol taken from an arable field was inc ubated for 32 days at 30 degrees C at 60, 80, and 100% water-holding c apacity 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 2 4 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)) w as derived from autotrophic nitrifiction. In contrast, at 100% water-h olding capacity with or without added NO3-, enormous amounts of N2O (2 9-90 mg N2O-N kg(-1)) were produced rapidly, mostly by denitrification (96-98% of total production). Thirdly, to examine N2O production by h etero-trophic nitrification, the Andosol was amended with peptone or N H4+ (both 1000 mg N kg(-1))+citric acid (20 g C kg(-1)) and with or wi thout dicyandiamide (200 mg N kg(-1)). Treatment with citric acid alon e or with citric acid+dicyandiamide suppressed N2O production. In cont rast, peptone increased N2O production (5.66 mg N2O-N kg(-1) mainly by denitrification (80% of total production). However, dicyandiamide red uced N2O production to 1.1 mg N2O-N kg(-1). These results indicate tha t auto-trophic nitrification was the main process for N2O production e xcept at 100% water-holding capacity where denitrification became domi nant and that heterotrophic nitrification had a lesser importance in t he soils examine.