Nitrous oxide production by denitrification and nitrification in temperate forest, grassland and agricultural soils

Nitrous oxide is produced in soils by biological denitrification and nitrification. To improve the fundamental understanding of the processes leading to N₂O fluxes from soils, the production of N₂O from denitrification and nitrification in spruce forest, beech forest, riparian grassland, coastal grassland and an agricultural field were studied. Samples were taken at a high and a low position along a topographic gradient in each site in the spring and autumn when the largest N₂O fluxes were expected. They were incubated after being amended with N, and C₂H₂ was used as biological inhibitor to distinguish nitrification and denitrification. The N₂O production in the low landscape position varied between 32 and 121 ng N cm^?3 h^?1 in the riparian grassland. 9 and 26 ng N cm^?3 h^?1 in the coastal grassland, and 135 and 195 ng N cm^?3 h^?1 in the agricultural field which was 10–100 times more than in the high positions where rates ranged between 3 and 5 ng N cm^?3 h^?1, 0.3 and 0.4 ng N cm^?3 h^?1, and 7 and 10 ng N cm^?3 h^?1, respectively. These differences almost certainly arose because the soil in the low positions was wetter and contained more organic matter. In the two forests N₂O production was less than 1 ng N cm^?3 h^?1, strongly inhibited by O₂, and not influenced by landscape position. Nitrification contributed to more than 60% of total N₂O production in the riparian grassland. In the agricultural field nitrification produced 13–74% of the total N₂O in the low position, and 10–88% in the high position. Denitrification was the dominant source of N₂O in the coastal grassland except at the low position in the autumn where nitrification produced 60% of the total N₂O. In the two forests where the soil had small nitrification potentials denitrification was the only source of N₂O. In the other sites nitrification and denitrification potentials were large and of identical magnitude. The results emphasize the need to separate nitrification and denitrification at the process level and to recognize topography at the field scale when modelling N₂O effluxes from soil.