DENITRIFICATION IN SOIL AGGREGATES ANALYZED WITH MICROSENSORS FOR NITROUS-OXIDE AND OXYGEN

Denitrification in soil microenvironments is difficult to study by conventional techniques. By use of O-2 and N2O microsensors, it was possible to study the spatial distribution of oxygen respiration and denitrification in soil aggregates kept at field capacity. Aggregates with diameters of 20 to 25 mm often contained anaerobic, centers but exhibited only low rates of denitrification due to a limiting availability of electron donors. Addition of organic matter such as clover leaves or tryptone to the surface of the aggregates greatly stimulated respiratory activity and caused anoxia and high rates of denitrifying activity within the organic hot-spot and in the underlying soil. Due to imbalance between N2O production and N2O reductase activity, the creation of a hot-spot led to an initial accumulation of N2O even when acetylene was absent. Denitrification within the center of a hot-spot rapidly became limited by the diffusional supply of NO3-, even when the underlying soil contained 5 mM NO3-, and such hot-spot centers depleted of NO3- served as sinks for N2O while N2O concentrations of up to 400 mu M were building up immediately below. The bulk N2O production rate of the soil was thus the result of a net production in some microenvironments and simultaneous net consumption in other microenvironments.