Denitrification and nitrous oxide production in sediments of the Wiske, a lowland eutrophic river

Abstract

Denitrification rate and nitrous oxide production were investigated in sediments of the River Wiske, a lowland, nutrient-rich calcareous tributary of the River Swale, NE England. Slurries from samples taken on two occasions at the most downstream site were used to compare potential denitrifying activity at various depths; this activity decreased by about 40% between the 0–1 and 1–2 cm layers. Minimum and maximum values for 5-cm depth cores sampled at the same site from visually different sediments differed by a factor of four. Denitrification rate in cores measured monthly during 1996 showed the lowest values in winter, a peak in late spring and a subsequent decrease through summer and autumn. Nitrous oxide production showed a similar trend and accounted for 0–100% of total nitrogen gases produced by nitrate reduction, with an average value of 42%. Denitrification rate typically ranged over one order of magnitude on passing down the river. The relative importance of various environmental factors on denitrification rate was assessed by laboratory experimental studies, using previous data for nitrate and new studies on temperature and organic carbon. Temperature showed a different effect according to whether assays were carried out with cores or slurries; denitrification rate showed an increase up to 30°C, irrespective of the season of sampling. Addition of organic substrates had no detectable effect on denitrification rate, but did lead to a significant decrease in the proportion of nitrous oxide formed as a result of nitrate reduction. Limitations in current methodologies for assaying denitrification are discussed.