Coupled nitrification-denitrification

An, S.M., and Joye, S.B. (2001) Enhancement of coupled nitrification-denitrification by benthic photosynthesis in shallow estuarine sediments. Limnol. Oceanogr. 46, 62-74. 

Risgaard-Petersen, N. (2003) Coupled nitrification-denitrification in autotrophic and heterotrophic estuarine sediments on the influence of benthic algae. Limnol. Oceanogr. 48, 93-105. 

Nitrifying bacteria are traditionally considered to be obligate aerobes they require molecular oxygen for reactions in the N oxidation pathways and for respiration. They are reputed to be microaerophiles, however, who thrive best under relatively low oxygen conditions. Microaerophily may be important in interface environments such as the sediment water interface and in the oxygen minimum zones of the ocean. The role of oxygen in sedimentary nitrification and coupled nitrification/ denitrification is discussed above in the section on nitrification in sediments. 

Risgaard-Petersen, N. (2003). Coupled nitrification-denitrification in autotrophic and heterotrophic estuarine sediments On the influence of benthic microalgae. Limnol. Oceanogr. 48, 93—105. Risgaard-Petersen, N., Rysgaard, S., Nielsen, L. P., and Revsbech, N. P. (1994). Diurnal variation of denitrification and nitrification in sediments colonized by benthic microphytes. Limnol. Oceanogr. 

Hamersley, M. R., and Howes, B. L. (2005). Couple nitrification-denitrification measured in situ in a Spartina alternijlora marsh with a (NH4+)-N-15 tracer. Mar. Ecol. Prog. Ser. 299, 123-135. 

The isotope pairing approach was developed to quantify what fraction of measured denitrification was fueled by substrates in the water column, versus coupled nitrification-denitrification (Nielsen, 1992). In this approach, is... 

Because nitrification occurs only under aerobic (or microaerobic) conditions and denitrification under anaerobic conditions, the two processes are spatially separated. However, if the sites where these processes occur are sufficiently close together, NO transport and consumption are very rapid and the overall process is considered to be coupled nitrification-denitrification. On the basis of a literature review, Seitzinger (1988) concluded that nitrification is generally the major source of NOj" for denitrification in river, lake, and coastal sediments. The same is likely to be true of non agricultural soils that are largely dependent on mineralization and atmospheric deposition for fixed nitrogen. 

The response of denitrification to certain environmental factors varies according to how strongly the process is coupled to nitrification. An increase in O2 penetration into sediments can stimulate coupled nitrification-denitrification by enhancing NOj" availability (Rysgaard et al., 1994), but simultaneously depress water column-supported denitrification because of the increased distance that NOj" must diffuse to reach the anaerobic layer (Cornwell et al., 1999). Risgaard-Petersen et al. (1994) reported that a photosynthesis-induced increase in O2 penetration into an estuarine sediment doubled the coupled nitrification-denitrification rate, but reduced the amount of water column-supported denitrification by 50%. Because the contribution of coupled nitrification-denitrification to total denitrification at this site was small (—16%), the net effect of deeper O2 penetration was to reduce... 

Classical coupled nitrification-denitrification requires NH, organic carbon, aerobic conditions, and anaerobic conditions. It involves three distinct populations of microorganisms, some of which are heterotrophic and others autotrophic. As a result, regulation of the process is rather complex and the relative proportions of NOj", NO, N2O, and N2 as end products varies widely with environmental and ecological conditions. 

Alternative pathways for denitrification have been proposed that do not involve the classical enzyme systems, but their existence has only recently been confirmed or investigated in detail. In the absence of O2, a number of elements and compounds have the potential to oxidize NH , including Mn(II), Mn02, NO, NO2 and NO2. Because anaerobic oxidation of NH directly to N2 bypasses the multiple oxidation steps required for coupled nitrification-denitrification... 

Consequently, authors have developed the model of coupled nitrification/denitrification for continental shelf sediments to estimate the spatial distribution of denitrification throughout shelf regions in the North Atlantic basin. Using data from a wide range of continental shelf regions, authors found a linear relationship between denitrification and sediment oxygen uptake (Figure 20). 

This relationship suggests that approximately 13% of the N incorporated into phytoplankton in shelf waters is eventually denitrified in the sediments via coupled nitrification/denitrification, assuming a C N ratio of 6.625 1 for phytoplankton. The model calculated denitrification rates compare favorably with rates reported for several shelf regions in the North Atlantic. 

Authors calculate that a total of 143 x lO molN/yr. is removed via coupled nitrification/denitrification on the North Atlantic continental shelf. This estimate is expected to underestimate total sediment denitrification because it does not include direct denitrification of nitrate from the overlaying water. The rate of coupled nitrification/denitrification calculated is greater than the N inputs from atmospheric deposition and river discharge combined (Figure 22). 

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