Seasonal variation in denitrification rates and nitrous oxide fluxes in intertidal sediments of the western Wadden Sea

Abstract

Variation m denitnfication rates, nitrous oxide fluxes and nitrate concentrabon of the overlying water were studied over 1 yr in sedlments from the intertidal zone in the western Wadden Sea, The Netherlands. Denitrification rates were highest in winter and early spring, while in summer the rates were low. Maximum rate was 55 ,urn01 N m-2 h' ; minimum rate was 1 ~imol N m-2 h-' N20 fluxes were from the sediment to the water column in winter and early spring (maximum 0.6 pm01 N m-2 h' ) , while in summer fluxes from the water column to the sediment were recorded (maxlmum -0.3 wmol N m-2 h-' ) . For marine systems this is the first hme that N20 influxes into the sediment over a considerable part of the year are reported. On an annual basis the denitrification rate and N20 flux were 110 mm01 N m-' yr-l and 0.3 mm01 N m-2 yr-l respectively. Denitrification rates and NO; fluxes were coupled to the NOT concentration in the overlying water, as confirmed by NOT enrichment experiments. INTRODUCTION The presence of nitrate, and the limited oxygen penetration as a consequence of mineralization, make Denitrification is an important process in the nitrogen sediments of coastal systems favourable sites for denicycle. In the absence of oxygen, nitrate and nitrite are trification. The aim of this research was to study seaused as ternunal electron acceptors for respiration and sonal variations in denitrification rates and N 2 0 fluxes are converted to nitrous oxide and/or nitrogen gas across the water/sediment interface in a coastal system, (Fenchel & Blackburn 1979, Knowles 1982): as a function of NOT availability, temperature, and oxygen penetration. NOT --t NO; --t N 2 0 + N2 The ratio of N 2 0 to nitrogen gas production is controlled by factors such as oxygen concentration (Betlach & Tiedje 1981, J ~ r g e n s e n et al. 1984), presence of hydrogen sulfide (Snrensen et al. 1980), NO: concentration ( k n g & Nedwell 1987), availability of organic matter and composition of the microbial population ( k n g & Nedwell 1985). Denitrification is a process which has the potential to counteract the increasing nitrogen load to coastal systems. It can, however, result in the emission of N 2 0 to the atmosphere, where it is involved in global warming (greenhouse effect; Wang et al. 1976) and destruction of the ozone layer (Liu et al. 1977, Crutzen 1981). The global input of N 2 0 to the atmosphere is estimated to be 14 Tg N yr-', and the contribution of oceans and estuaries is ca 14 % (Seiler & Conrad