Determination of the simultaneous production and consumption of soil nitrite using 15N

Traditionally nitrite is viewed as a transient intermediate in a number of processes. Recent studies have shown that NO2− can occur in relatively high concentrations in soil and may be the source of concentrations in river water above EU guidelines. Nitrite has the potential to be formed by several processes. Although there is evidence that both oxidative and reductive processes contribute to the presence of NO2− in soil, the relative contribution of these processes to soil NO2− flushes is not known. The development of sensitive techniques for determining the 15N content of soil NO2− has facilitated new studies on its turnover. In our study, the rates of NO2− production and consumption in soil were determined, using 15N-labelled NO2− and isotope dilution equations. Portions of soil with a pulse of 15NO2− were incubated in sealed jars for 24 h. Periodically, NO2− and NO3− concentrations and their 15N content were determined by destructive sampling during the experiment. The concentration and 15N content of the NO2− pool declined rapidly during the first 6 h of the incubation, becoming more stable thereafter. Concentrations of NO3− increased between 0 and 20 h, as did the 15N content of this pool. Calculated rates of NO2− production and consumption indicated the speed with which NO2−N can be metabolised within the soil. Consumption rates were 5 times greater than production rates during the first 6 h of incubation. Generally > 50% of NO2−N consumed was recovered in the NO3− pool, indicating that nitrification was the major NO2−-consuming process. It was not possible to calculate the relative contribution of reductive processes to NO2− consumption. The validity of using the isotope dilution equations is discussed, and in particular the significance of the recycling of NO2−N from the NO3− pool. The stabilising of NO2− production and consumption rates, of NO2− and NO3− 15N contents, and of NO2−N concentrations indicated that recycling occurred between 20 and 24 h. The rapid metabolism of NO2− in soil suggests that the NO2− in river water may not be wholly 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.