Nitrous oxide production from soil experiments: denitrification prevails over nitrification

Nitrous oxide is produced in soils and sediments essentially through the processes of nitrification and denitrification, although several rival processes could be competing. This study was undertaken in order to better understand the controlling factors of nitrification, denitrification and associated N2O production as well as the contribution of these two processes to the average N2O production by soils and sediments. With this aim, soil and sediment samples were taken in contrasting periods and different land use types, each time at different depths (upper and lower soil horizons). They were incubated in separate batches in specific conditions to promote denitrification and nitrification: (1) a complete anaerobic environment adding KNO3 for the denitrification assay and (2) an aerobic environment (21 % O2) with addition of NH4Cl for the nitrification assay. Potentials of nitrification and denitrification were determined by the rates of nitrate either reduced (for denitrification) or produced (for nitrification). Overall, denitrification potential varied from 70 to 2,540 ng NO3—-N g−1 dry soil h−1 and nitrification potential from 30 to 1,150 ng NO3−-N g−1 dry soil h−1. Nitrous oxide production by denitrification was significantly (P < 0.05) greater in topsoils (10–30 cm) than in subsoils (90–110 cm), ranging, respectively, from 26 to 250 ng N2O-N g−1 dry soil h−1 versus 1.5 to 31 ng N2O-N g−1 dry soil h−1, i.e., a mean 19.5 versus. 6.0 % of the NO3− denitrified for the upper and lower horizons, respectively. Considering the N2O production in relation with the nitrate production (e.g., nitrification), no significant difference (P < 0.05) was found in the soil profile, which ranged from 0.03 to 6 ng N2O-N g−1 dry soil h−1. This production accounts for 0.21 and 0.16 % of the mean of the NO3− produced for the top and subsoils, respectively. On the basis of the average production by both top- and subsoils, N2O production by denitrification is clearly greater than by nitrification under the measurement conditions used in this study, from 30- to 100-fold higher. Such a high potential of N2O emission must be taken into account when reducing nitrate contamination by increasing denitrification is planned as a curative measure, e.g. in rehabilitation/construction of wetlands.