Nitrous Oxide Production and Reduction during Denitrification as Affected by Redox Potential

Abstract

Abstract Nitrous oxide evolution from soils may contribute to partial destruction of the ozone layer in the stratosphere. Knowledge of factors affecting the N 2 O/N 2 evolution ration from soils is important in properly assessing the hazard associated with N 2 O evolution. Previous investigations demonstrated that air‐dry soils incubated in the laboratory under conditions to cause denitrification had an initial high N 2 O/N 2 evolution ratio which rapidly decreased and approached zero. It was not possible to determine whether the observed time effect on the gaseous evolution was related to concurrent changes in the redox potentials in the soil. Experiments were conducted to measure the production, reduction, or both, of N 2 O at various redox potentials in incubation containers. Equal weights of soil and KNO 3 solution were added to the incubation containers which were placed in a temperature‐controlled waterbath shaker. The container was evacuated and refilled with helium. Small amounts of oxygen were periodically injected into the container to achieve the desired redox potential. Gas samples were withdrawn daily and analyzed for N 2 O, N 2 , O 2 , and CO 2 on a gas chromatograph. Nitrate‐nitrogen concentrations in solution were analyzed at the beginning and end of the incubation and were found to range from approximately 200 to 400 mg/liter during the incubation. There was essentially no denitrification at redox potentials of 300, 350, or 400 mV as determined by gas analyses and measurement of nitrate‐nitrogen. When no oxygen was added to the system, the redox potential decreased and poised at approximately 200 mV throughout the incubation. At a redox potential of 200 mV, the N 2 O concentration increased, reaching a maximum at approximately 3 days, and decreased thereafter, reaching zero in 5 to 7 days after initiation of incubation. Thus, the previously observed time dependency of N 2 O evolution was repeated in these experiments and the results cannot be attributed to change in redox potential with time. The data also demonstrate that N 2 O can be reduced more rapidly than NO 3 ‐ , even in the presence of relatively high NO 3 ‐ concentrations.