Denitrification is a critical process regulating the removal of bioavailable nitrogen (N) from natural and human-altered systems. While it has been extensively studied in terrestrial, freshwater, and marine systems, there has been limited communication among denitrification scientists working in these individual systems. Here, we compare rates of denitrification and controlling factors across a range of ecosystem types. We suggest that terrestrial, freshwater, and marine systems in which denitrification occurs can be organized along a continuum ranging from (1) those in which nitrification and denitrification are tightly coupled in space and time to (2) those in which nitrate production and denitrification are relatively decoupled. In aquatic ecosystems, N inputs influence denitrification rates whereas hydrology and geomorphology influence the proportion of N inputs that are denitrified. Relationships between denitrification and water residence time and N load are remarkably similar across lakes, river reaches, estuaries, and continental shelves. Spatially distributed global models of denitrification suggest that continental shelf sediments account for the largest portion (44%) of total global denitrification, followed by terrestrial soils (22%) and oceanic oxygen minimum zones (OMZs; 14%). Freshwater systems (groundwater, lakes, rivers) account for about 20% and estuaries 1% of total global denitrification. Denitrification of land-based N sources is distributed somewhat differently. Within watersheds, the amount of land-based N denitrified is generally highest in terrestrial soils, with progressively smaller amounts denitrified in groundwater, rivers, lakes and reservoirs, and estuaries. A number of regional exceptions to this general trend of decreasing denitrification in a downstream direction exist, including significant denitrification in continental shelves of N from terrestrial sources. Though terrestrial soils and groundwater are responsible for much denitrification at the watershed scale, per-area denitrification rates in soils and groundwater (kg Nkm � 2 � yr � 1 ) are, on average, approximately one-tenth the per-area rates of denitrification in lakes, rivers, estuaries, continental shelves, or OMZs. A number of potential approaches to increase denitrification on the landscape, and thus decrease N export to sensitive coastal systems exist. However, these have not generally been widely tested for their effectiveness at scales required to significantly reduce N export at the whole watershed scale.

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Denitrification across landscapes and waterscapes: a synthesis.

The use of 13 C natural abundance (δ 13 C) to follow C input to soil has gained widespread acceptance. However, inorganic C present in the soil as carbonates will interfere with the measurement of soil organic 13 C unless removed or excluded from measurement. We report a simple and convenient HCl-fumigation method to remove inorganic C from soil. Soil samples are weighed in Ag-foil capsules, arranged on a microtiter plate, wetted with water to approximately field capacity, and placed in a desiccator containing a beaker with concentrated (12 M) HCl. The carbonates are released as CO 2 by the acid treatment in 6 to 8 h. The soil samples are then dried at 60°C prior to isotope determination. The advantages of the HCl-fumigation method to remove inorganic C from the soil are that: (i) no water soluble C will be lost from the soil; (ii) a large number of samples can be processed simultaneously; (iii) the removal of inorganic C is rapid and complete; and (iv) the method could also be used to determine both organic and inorganic C content in the soil. A potential disadvantage, however, is that the HCl fumigation changed the 15 N natural abundance of soil N.

Acid fumigation of soils to remove carbonates prior to total organic carbon or CARBON-13 isotopic analysis

Denitrification, the reduction of the nitrogen (N) oxides, nitrate (NO3-) and nitrite (NO2-), to the gases nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2), is important to primary production, water quality, and the chemistry and physics of the atmosphere at ecosystem, landscape, regional, and global scales. Unfortunately, this process is very difficult to measure, and existing methods are problematic for different reasons in different places at different times. In this paper, we review the major approaches that have been taken to measure denitrification in terrestrial and aquatic environments and discuss the strengths, weaknesses, and future prospects for the different methods. Methodological approaches covered include (1) acetylene-based methods, (2) 15N tracers, (3) direct N2 quantification, (4) N2:Ar ratio quantification, (5) mass balance approaches, (6) stoichiometric approaches, (7) methods based on stable isotopes, (8) in situ gradients with atmospheric environmental tracers, and (9) molecular approaches. Our review makes it clear that the prospects for improved quantification of denitrification vary greatly in different environments and at different scales. While current methodology allows for the production of accurate estimates of denitrification at scales relevant to water and air quality and ecosystem fertility questions in some systems (e.g., aquatic sediments, well-defined aquifers), methodology for other systems, especially upland terrestrial areas, still needs development. Comparison of mass balance and stoichiometric approaches that constrain estimates of denitrification at large scales with point measurements (made using multiple methods), in multiple systems, is likely to propel more improvement in denitrification methods over the next few years.

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Methods for measuring denitrification: diverse approaches to a difficult problem

Both terrestrial and marine forces drive underground fluid flows in the coastal zone. Hydraulic gradients on land result in groundwater seepage near shore and may contribute to flows further out on the shelf from confined aquifers. Marine processes such as tidal pumping and current-induced pressure gradients may induce interfacial fluid flow anywhere on the shelf where permeable sediments are present. The terrestrial and oceanic forces overlap spatially so measured fluid advection through coastal sediments may be a result of composite forcing. We thus define “submarine groundwater discharge” (SGD) as any and all flow of water on continental margins from the seabed to the coastal ocean, regardless of fluid composition or driving force. SGD is typically characterized by low specific flow rates that make detection and quantification difficult. However, because such flows occur over very large areas, the total flux is significant. Discharging fluids, whether derived from land or composed of re-circulated seawater, will react with sediment components. These reactions may increase substantially the concentrations of nutrients, carbon, and metals in the fluids. These fluids are thus a source of biogeochemically important constituents to the coastal ocean. Terrestrially-derived fluids represent a pathway for new material fluxes to the coastal zone. This may result in diffuse pollution in areas where contaminated groundwaters occur. This paper presents an historical context of SGD studies, defines the process in a form that is consistent with our current understanding of the driving forces as well as our assessment techniques, and reviews the estimated global fluxes and biogeochemical implications. We conclude that to fully characterize marine geochemical budgets, one must give due consideration to SGD. New methodologies, technologies, and modeling approaches are required to discriminate among the various forces that drive SGD and to evaluate these fluxes more precisely.

Groundwater and pore water inputs to the coastal zone

The burial of organic matter (OM) in marine sediments represents the major link between “active” surface pools of carbon in the oceans, atmosphere, on land, and in marine sediment, and carbon pools that cycle on much longer, geologic time scales (i.e., carbon in sedimentary rock, coal, and petroleum deposits). It also plays some role in controlling atmospheric CO2 and O2 on these long time scales because in a highly simplified fashion OM burial in sediments can be thought of in terms of the balance between primary production and respiration on land and in the oceans.

Preservation of organic matter in marine sediments: controls, mechanisms, and an imbalance in sediment organic carbon budgets?

Iron and manganese cycling in different sedimentary environments on the North Sea continental margin

Benthlc nltrogen cycling was studled In different areas of the southeastern North Sea in August 1991 and February 1992 Main attention was focused on the coupling between nltrlficatlon and demtnfication Additionally, extensive measurements of sediment-water exchange of lnorganlc nitrogen compounds and of the penetration of oxygen Into the sediment were made Denltnfication rates ranged from 0 to 8 pm01 N m-' h' , whlle nitnfication rates ranged between 5 and 26 pm01 N m-2 h ' Although the oxic layer of the sedlrnent was thnne r in August, nltnfication was hlgher dunng that period, indicating the importance of ammonlum availabihty rather than oxygen penetration Except in the deposltlonal area in the German Blght, denitrlflcation was completely dependent on nitnfication In general, however, the coupllng between the 2 processes was incomplete On average, 3 and 25 % of the produced nitrate was denltr~fied In August and February respectively The bulk of the produced nltrate diffused upward (along the steepest concentratlon gradient) to the nitrate-depleted overlylng water and was unavailable for denitnficatlon, part~cularly in August It 1s postulated that the coupllng efficiency is indirectly influenced by the nitrate concentratlon in the overlylng water

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Nitrogen cycling in North Sea sediments: interaction of denitrification and nitrification in offshore and coastal areas

Denitrification rates in continental shelf sediments of the North Sea were measured by the simultaneous application of the acetylene inhibition technique (AIT) and the isotope pairing method (IPM). Nitrification was the predominant source of nitrate in these sediments. Both techniques were able to detect coupled nitrification-denitrification activity. Denitrification rates measured by the AIT were only measurable during the first 60 min of the incubation. Rates obtained by the AIT in this period ranged between 5.4 and 7.3 pm01 N m" h-' Continued incubation of acetylene amended sediment cores resulted in decreasing denitrification rates, suggesting that accumulated nitrous oxide was consumed despite the presence of acetylene. Denitrification rates obtained by the application of the IPM ranged between 9.8 and 13.2 pm01 N m-' h-' Constant production of single-labelled ('"N1'N) relative to double-labelled (lSN''N) dinitrogen indicated a homogeneous mixlng of both isotopic nitrate species through the denitnfication zone. The present results suggest that previously published data severely underestimated denitnfication rates in North Sea continental shelf sediments. Concomitant measured fluxes of nitrate and ammonium across the sediment-water interface reveal that in these sediments most of the deposited nitrogen is returned to the water column as ammonium and nitrate.

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Denitrification rates as measured by the isotope pairing method and by the acetylene inhibition technique in continental shelf sediments of the North Sea

Oxygen pore water profiles in North Sea sed~ment s were measured with microelectrodes during 2 contrasting seasons The measurements were conducted In a wide variety of sediments, including non-depositional areas on the southern shelf as well as depositional areas in the Skagerrak All measurements were performed within minutes on board at In sltll temperature The curvature of oxygen profiles in sandy sediments on the southern shelf Indicated the presence of a surface layer characterised by enhanced diffusion The occurrence of enhanced diffusion was related to sedimentological and seasonal differences Quantitative evaluat~on of the pore water profiles by a diffusion-reaction model indicated that the effective diffusion coeff~c~ents In a 02 to 16 mm subsurface layer were 15 to > l 0 0 times higher than the ~nolecular diffusion coefflcient Highest effective diffusion coefficients were reported for non-depositional sediments characterised by low diffusive oxygen fluxes Oxygen profiles in sediments of the depositional area of the Skagerrak indicated constant diffusivity throughout the sediment column Diffusive fluxes cdlculated from profiles ranged from 52 to 89 ~nlnol m-2 d-' in August 1991, and were between 08 and 62 mm01 nl ' d-' in February 1992 Seasonal differences in sediments locatt!d In the Skagerrak area were minor It is proposed that near-bottom tidal currents induce enhanced diffus~on transport processes in the upper millimetres of the sandy sediments of the southern North Sea, while less energetic hydrodynamical conditions in the depos~tional area of the Skagerrak favour sediment-water exchange based on molecular diffusion only Biogeochemical implications of the enhanced diffusivity close to the sediment-water interface are discussed

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Oxygen pore water profiles in continental shelf sediments of the North Sea: turbulent versus molecular diffusion

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

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L. Lohse

Papers

Iron and manganese cycling in different sedimentary environments on the North Sea continental margin

Nitrogen cycling in North Sea sediments: interaction of denitrification and nitrification in offshore and coastal areas

Denitrification rates as measured by the isotope pairing method and by the acetylene inhibition technique in continental shelf sediments of the North Sea

Oxygen pore water profiles in continental shelf sediments of the North Sea: turbulent versus molecular diffusion

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