Production and Loss of Nitric Oxide from Denitrification in Anaerobic Brookston Clay

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Nitric oxide, nitrous oxide, and nitrite ion production was measured in a Brookston clay column undergoing anaerobic denitrification. A flow system method was used whereby argon carrier gas continuously stripped soil gases from the column, allowing steady-state rates to be obtained. Over several days the temporal change in rates of these gases and NO2− followed a pattern of increase and decay which may be expected of a reaction proceeding by several consecutive steps. The method permits observation of the relatively large net production rate of NO, which is normally not observed in static systems based on head space analysis of gaseous denitrification products. In the first several hours after the onset of anoxic conditions, the net rate of NO production, fNO, increased sharply to a maximum (∼1 × 10−10 mol of N/g of soil per min), paralleling the rapid increase in NO2− level, and then followed a more gradual decline extending over approximately 45 h. A similar but less pronounced pattern was observed for N2O, with net rates of production being considerably less than for NO. The ratio [NO-N]/[N2O-N] decreased with time from ∼2.5 at 6 h to ∼2.0 at 45 h. Estimated rates of N2 production appeared to be initially high, decreased rapidly within a few hours, and then gradually increased with time after the establishment of anaerobic conditions.

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