Simultaneous effects of increasing levels of glucose and oxygen partial pressures on denitrification and dissimilatory nitrate reduction to ammonium in repacked soil cores

Dissimilatory nitrate reduction to ammonium (DNRA) and its importance in comparison to denitrification were studied in soil samples artificially repacked to control water potential and porosity, and incubated for 72 h. Labelled nitrate (100 mg N.kg(-1) dry soil, 21.8 % N-15 in excess) and increasing levels of glucose-C (250, 500 and 1000 mg glucose-C.kg(-1) dry soil) were initially added to the soil samples to obtain increasing glucose-C/nitrate-N ratios of 2.5, 5 and 10, which were then subjected to different O-2 partial pressures (0, 0.5, 1.0 and 2.0 % (v/v)). The results confirmed the good reproducibility of the experimental conditions using this method. Denitrification, rather than DNRA, was the dominant process in all the treatments developed during this experiment: N2O production in the presence of acetylene varied from 4.9 (glucose-C/nitrate-N = 2.5; 2 % O-2) to 103.6 % (glucoseC/nitrate-N = 2.5; 0 % O-2) of the original nitrate whereas DNRA varied from 1.8 (glucose-C/nitrate-N = 2.5; 2.0 % O-2) to 24.6 % (glucose-C/nitrate-N = 10; 1.0 % O-2) of the original nitrate. This work demonstrated that under these conditions, DNRA activity was less sensitive than denitrification to an inhibitory effect by O-2 and reinforced the idea that carbon is the main driving factor regulating nitrate distribution between denitrification and DNRA. (C) Elsevier, Paris.