EMISSION OF N2O, NITRIFICATION AND DENITRIFICATION IN A EUTROPHIC LAKE SEDIMENT BIOTURBATED BY CHIRONOMUS-PLUMOSUS

Emission of nitrous oxide and rates of nitrification and denitrificati on were determined in sediment from a eutrophic lake in southern Swede n. Effects of bioturbation on nitrogen transformation were investigate d by incubating sediment at 10 and 15 degrees C in a continuous flow-t hrough system containing ca 2000 tube-dwelling larvae of Chironomus pl umosus L. (Chironomidae, Diptera) m(-2). N2O emission was found to be independent of nitrate concentration (33 to 267 mu M) in the overlying water at both 10 and 15 degrees C. It is shown that C. plumosus larva e significantly enhanced the emission of N2O from the sediment at 15 d egrees C, the highest emission recorded under these conditions was 0.4 5 +/- 0.2 mu mol N m(-2)h(-1). Since released N2O was proportional to the rate of nitrification, it is suggested to originate mainly from th is process. Bioturbation stimulated the rate of nitrification by a fac tor of 1.8 at 15 degrees C. However, no enhancement of nitrification b y chironomids was observed at 10 degrees C. In contrast to N2O emissio n and nitrification, denitrification in both bioturbated and non-biotu rbated cores was significantly correlated to the concentration of nitr ate in the overlying water. The stimulation of denitrification by biot urbation at 10 and 15 degrees C was 3.5- and 4.3-fold respectively. De nitrification rates were mainly accounted for (98%) by changes in the concentration of nitrate in the water, the water temperature and the b iomass of C. plume sus larvae. C. plumosus larvae were found to decrea se the N2O:N-2 ratio at both temperatures. This was explained by the w ater pumping activity of the animals, mobilizing released N2O to denit rifiers in or close to the animal burrows. The ratio of N2O:N-2 did no t exceed 0.5% in any of the treatments.