Anaerobic soils, denitrification

As discussed in Chapter 5, in submerged soils nitrification occurs in aerobic sites at the iloodwater-soil and root-soil interfaces. Denitrification occurs upon diffusion of the NO, to the anaerobic bulk soil. Denitrification is favoured over dissimilatory reduction to NH4+(NO, -> NO2 NH4+) because of the large ratio of available carbon to electron acceptors in submerged soils. Denitrification is likely to proceed completely to N2 with little accumulation of N2O because of the very large sink and therefore steep concentration gradient of O2, and because carbon is less likely to be limiting (Section 5.1). 

Exchange of dissolved nitrogen species between the soil and water column support several nitrogen reactions. For example, nitrification in aerobic soil layer is supported by ammonium flux from the anaerobic soil layer. Similarly, denitrification in anaerobic soil layer is supported by nitrate flux from the aerobic soil layer and water column (see Chapter 14 for discussion on transport processes). 

FIGURE 8.57 Schematic showing the flux of nitrate nitrogen from aerobic soil layer to anaerobic soil layer and overlying water column, and denitrification of nitrate in anaerobic soil layer. 

Carbon acts as the electron donor for denitrification. The availability of carbon often limits denitrification in anaerobic microsites in non-submerged soils. As a result, the reaction does not go to completion and the intermediaries NO2 and N2O accumulate. Completion of the reaction may also be hindered by low pH. But under uniformly anaerobic conditions NO3 as electron acceptor is more likely to be limiting than carbon as electron donor because NO3 is not regenerated. 

In a soil-water system under anaerobic conditions, no significant degradation of naphthalene was observed after 45 d. Under denitrification conditions, naphthalene (water concentration 700 pg/L) degraded to nondetectable levels in 45 d. In both studies, the acclimation period was approximately 2 d (Mihelcic and Luthy, 1988). 

N2 under anaerobic conditions, a process called denitrification (Fig. 22-1). These soil bacteria use N03 rather than 02 as the ultimate electron acceptor in a series of reactions that (like oxidative phosphorylation) generates a transmembrane proton gradient, which is used to synthesize ATP. 

Denitrification The reduction of nitrates to nitrites and finally to nitrous oxide or even to molecular nitrogen catalyzed by facultative aerobic soil bacteria working under anaerobic conditions. 

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. 

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. 

---