Kinetics of nitric oxide consumption in tropical soils under oxic and anoxic conditions

The kinetics of nitric oxide consumption in four tropical soils were studied under oxic and anoxic conditions in a flow-through system in the laboratory. Under anoxic conditions the soils had a very high affinity for NO, resulting in KM values of 0.02–0.27 ppmv NO (equivalent to 0.04–0.50 nM NO in the aqueous phase). These KM values were lower than literature values for NO consumption by denitrifying bacteria. Under oxic conditions the kinetics of NO consumption in the tropical soils were completely different, exhibiting KM values higher than 1.7 ppmv. These higher KM values were similar to literature values for NO consumption by aerobic heterotrophic bacteria. Thus, the tropical soils studied seem to contain two different NO consumption activities which can be distinguished by their kinetics and which predominate under aerobic and anaerobic conditions, respectively. However, it was not possible to quantify the contribution of each process to total NO consumption under natural conditions. Under aerobic conditions NO turnover kinetics were positively correlated with soil respiration, N mineralisation and soil organic carbon, whereas under anaerobic conditions they were positively correlated with potential and actual denitrification rates and pH.