Wetlands, denitrification

Denitrification is typically the dominant pathway of nitrate ranoval in wetlands. Thus, the nitrate reduction rates presented in Tables 8.10 and 8.11 represent denitrification rates. Rates are presented on areal basis (Table 8.10) and as first-order rate constants (Table 8.11). Nitrate reduction rates reported for constructed wetlands are in the range of 3-1,020 mg N m day In most wetlands, denitrification rates are limited not only by nitrate concentration but also by hydraulic retention time (or contact time of nitrate with anaerobic zones) and diffusion/mass flow of nitrate from aerobic zones to anaerobic sites (Martin and Reddy, 1997). Denitrification rates are usually higher in soils receiving steady loading of nitrate than in soils receiving low or negligible nitrate levels (Cooper, 1990 Gale et al., 1993). 

Table 1.5 shows the importance of denitrification in wetlands on a global scale. Further, agricultural wetlands are important sources of NH3 which is emitted by volatilization of ammoniacal-N in the floodwater ... 

Baker CJ, Maltby E. 1995. Nitrate removal by river marginal wetlands factors affecting the provision of a suitable denitrification environment. In Hughes JMR, Heath-waite AL, eds. Hydrology and Hydrochemistry of British Wetlands. Chichester Wiley, 291-313. 

Reddy KR, Patrick WH, Jr., Lindau CW. 1989. Nitrification-denitrification at the plant root-sediment interface in wetlands. Limnology and Oceanography 34 1004-1013. 

Davis, J. L., Nowicki, B., andWigand, C. (2004). Denitrification in fringing salt marshes ofNarragan-sett Bay, Rhode Island, USA. Wetlands 24, 870—878. 

Denitrification can be limited by carbon availability when O2 is absent and NO3 is abundant. Additions of glucose stimulated denitrification in 11 of 13 agricultural soils that were presumably fertilized (Drury etaL, 1991). Similar observations have been made in water columns (Brettar and Rheinheimer, 1992), marine sediments (Slater and Capone, 1987), river sediments (Bradley et aL, 1995), aquifers (Smith and Duff, 1988 Obenhuber and Lowrance, 1991), wastewater treatment wetlands (Ingersoll and Baker, 1998), and forested wetlands (DeLaune et aL, 1996). Tiedje (1988) proposed that the major influence of carbon on in situ denitrification is to promote anaerobic conditions. 

DeLaune R. D., Boar R. R., Lindau C. W., and Kleiss B. A. (1996) Denitrification in bottomland hardwood wetland soils of the Cache River. Wetlands 16, 309-320. 

Groffman P. M. (1994) Denitrification in freshwater wetlands. Curr. Topics Wetland Biogeochem. 1, 15-35. 

Seitzinger S. P. (1994) Linkages between organic matter mineralization and denitrification in eight riparian wetlands. Biogeochemistry 25, 19-39. 

The wetland/stream/river/estuary/shelf region provides a continuum with substantial capacity for denitrification. Nitrate is commonly found, there is abundant organic matter, and sediments and suspended particulate microsites offer anoxic environments. In this section we discuss denitrification in the stream/river/estuary/shelf continuum. Although there are several specific studies of denitrification in wetlands, the role of wetlands in Nr removal at the watershed scale needs to be better understood. 

Sink Contaminants are transformed to biologically unavailable forms within the system. For example, wetlands can convert nitrate to N2 gas through a biological reaction called denitrification (this process is discussed in detail in Chapter 8). 

Wetlands can function as a sink, source, or transformer for nutrients. In addition to functioning as sources or sinks, wetlands act as transformers through processes such as denitrification, methanogenesis, and the microbial breakdown of organic matter. 

Nitrogen is exported from wetland systems via ammonia volatilization under alkaline conditions, where unionized ammonia in its gaseous form can be transported from soil and water column to the atmosphere. Another gaseous loss of nitrogen includes nitrous oxide and nitrogen produced during denitrification and emitted into the atmosphere. 

FIGURE 8.5 Oxidation and reduction reactions of nitrogen in wetlands. Numbers 1-7 refer to pathways of nitrogen reactions. 1 = ammonification 2 = immobilization 3 = nitrification 4 = denitrification 5 = dissimilatory nitrate reduction to ammonia 6 = dinitrogen fixation and 7 = ammonia volatilization. 

Pathway 7 shows ammonia volatilization, which is a physicochemical process controlled by the pH of the environment. There are many biotic processes that can alter the pH of a wetland including photosynthesis and denitrification. An alkaline pH favors the presence of unionized ammonia, whereas acidic or neutral pH favors that of ionized ammonia. Loss of nitrogen due to volatilization is insignificant at pH <7.5, but it dramatically increases at pH >7.5. 

Nitrification in wetlands is restricted to aerobic zones of soil and water column or under drained soils conditions, where ammonium is oxidized to nitrate. Nitrification reaction supports denitrification by supplying heterotrophs with nitrate as their electron acceptor. In a broader sense, nitrification is defined as the conversion of organic or inorganic compounds from reduced state to a more oxidized state. Three groups of microorganisms are capable of oxidizing ammonium under aerobic conditions ... 

Martin and Reddy, 1997). These values are lower than the values reported for mineralization, suggesting that oxygen and ammonium availability limits nitrification. Measurement of oxygen concentration in the water column or Eh values in soils can provide a reliable indication of potential nitrification. Measurement of nitrate in aerobic zones of wetlands may underestimate overall nitrification, as some of the nitrate can diffuse into anaerobic zones and is lost through denitrification. However, in well-aerated soils, nitrification potentials are determined by measuring accumulation of nitrate over time. 

In continental shelf sediments, up to 67% of the N2 formation was found to be due to anaerobic ammonium oxidation with nitrate (or possibly nitrite) and only 33% of the N2 production was due to denitrification (Thamdrup and Dalsgaard, 2002). At present this reaction is not reported in wetland soils. Anaerobic ammonium oxidation is regulated by... 

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