DETERMINATION OF THE SIMULTANEOUS PRODUCTION AND CONSUMPTION OF SOIL NITRITE USING N-15

Traditionally nitrite is viewed as a transient intermediate in a numbe r of processes. Recent studies have shown that NO2- can occur in relat ively high concentrations in soil and may be the source of concentrati ons in river water above EU guidelines. Nitrite has the potential to b e formed by several processes. Although there is evidence that both ox idative and reductive processes contribute to the presence of NO2- in soil, the relative contribution of these processes to soil NO2- flushe s is not known. The development of sensitive techniques for determinin g the N-15 content of soil NO2- has facilitated new studies on its tur nover. In our study, the rates of NO2- production and consumption in s oil were determined, using N-15-labelled NO2- and isotope dilution equ ations. Portions of soil with a pulse of (NO2-)-N-15 were incubated in sealed jars for 24 h. Periodically, NO2- and NO3- concentrations and their N-15 content were determined by destructive sampling during the experiment. The he concentration and N-15 content of the NO2- pool dec lined rapidly during the first 6 h of the incubation, becoming more st able thereafter. Concentrations of NO3- increased between 0 and 20 h, as did the N-15 content of this pool. Calculated rates of NO2- product ion and consumption indicated the speed with which NO2--N can be metab olised within the soil. Consumption rates were 5 times greater than pr oduction rates during the first 6 h of incubation. Generally > 50% of NO2--N consumed was recovered in the NO3- pool, indicating that nitrif ication was the major NO2--consuming process. It was not possible to c alculate the relative contribution of reductive processes to NO2- cons umption. The validity of using the isotope dilution equations is discu ssed, and in particular the significance of the recycling of NO2--N fr om the NO3- pool. The stabilising of NO2- production and consumption r ates, of NO2- and NO3- N-15 contents, and of NO2--N concentrations ind icated that recycling occurred between 20 and 24 h. The rapid metaboli sm of NO2- in soil suggests that the NO2- in river water may not be wh olly soil-derived. The processes responsible for the presence of NO2- in river water need further investigation. The techniques used in this study should be readily applied to water and sediment samples.