Losses from denitrification

The oceanic N inventory largely depends on the balance between N inputs from terrestrial systems and N2 fixation versus N losses from denitrification. Based on current estimates, the oceanic N budget is far from balanced (e.g., oceanic denitrification far exceeds N2 fixation Codispoti, 2007 Codispoti et al., 2001 Chapter 1 by Gruber, this volume). A better understanding of the geographical distribution and limitations on marine N2 fixation is required to understand N cycling and oceanic N budgets under present, past, and future oceanic scenarios. 

The interest in gaseous losses of nitrogen from soil is now extensive and includes the well established community of soil scientists concerned with losses of fertilizer-applied nitrogen by nitrification and denitrification. More recently, interest in ammonia losses from plants and soil has been stimulated by the very large emissions from intensive cattle production in the Netherlands and their... 

The reduction of nitrate via nitrite to ammonia occurs in soils at low rates3. If it were possible to enhance the reduction of N03 to NH3 and its subsequent incorporation into SOM (reaction 4), large N losses resulting from denitrification and leaching could be presented so that the economy of N use would be improved. 

The major modes of nitrogen losses from the Bay of Bengal are sedimentary denitrification and burial. Note that the estimate for sedimentary denitrification (3 Tg N year ) is based on only one measurement at the outer shelf off Chennai (Naik, 2003), while the burial in sediments was computed from the carbon burial using a C N value of 10. 

Additionally, the anammox reaction mentioned earlier results in the reduction of N02 with the production of N2 gas. Recent direct tracer evidence from both marine sediments (Thamdrup and Dalsgaard, 2002) (see Chapter 6, Devol, this volume) and several anoxic water column systems (Hamersley et al., 2007 Kuypers et al., 2005) has shown that the anammox pathway, rather than conventional denitrification, can be a significant and, at times, predominant source of N2 production. Future studies of combined N loss from reefal environments should also consider this pathway. 

In the short term, the most significant temporary sink for dissolved nutrients is uptake by phytoplankton. This is followed by remineralisation, which returns nutrients to solution or loss from the system by burial in sediments or export to ocean waters. These processes effectively separate N, P and Si because reminer-alisation returns them to solution at different rates (Officer Ryther, 1980). In particular, Si dissolves slowly from shell material, and denitrification leads to the loss of biologically available nitrogen from the system as nitrogen gas. 

The dominant product from denitrification is N2. This process, therefore, represents a reversal of nitrogen fixation, returning fixed nitrogen from the biosphere to the pool of atmospheric N2. The release of N20 also constitutes a loss of fixed nitrogen, as the main fate of N20 in the atmosphere is photodecomposition by ultraviolet (UV) radiation with N2 and O atoms as products. The release of NO or N02, in contrast, does not lead to a loss of fixed nitrogen, because these compounds are oxidized further to HN03, which is then returned to the earth surface by wet and dry deposition. 

Denitrification, the bacterial reduction of nitrate to N2, is the major mechanism of fixed N loss from the ocean, occurring both in the water column and in sediments when the oxygen concentration is low (<5 rM) (Figure 2). Denitrification strongly discriminates against the heavier isotope, pro-... 

Point and nonpoint sources, precipitation, and biological N fixation are the major inputs to wetlands, whereas N losses from wetlands include biotic (denitrification) and abiotic (volatilization) reactions and surface/subsurface flows. Wetlands often function as effective reservoirs of N. 

Nitrification-denitrification reactions are the major pathway for N loss from wetland ecosystems to the atmosphere. The process involves a series of sequential microbial processes that include mineralization of organic nitrogen to ammonium, oxidation of ammonium to nitrate, and denitrification of nitrate to nitrous oxide or dinitrogen gas. Nitrification-denitrification in wetlands occurs primarily in two zones the aerobic-anaerobic interface at the surface of the flooded soil or sediment, and the oxidized rhizosphere of wetland plants (Figure 16.6). 

The main nitrogen losses from marshes are caused by denitrification and detrital export and sedimentation sinks. Denitrification has been reported to be the major cause of nitrogen loss from marshes. Although the potential for denitrification is very high in the reduced marsh soil, the level of in situ denitrification depends on the availability of nitrate. Nitrate must be either formed through nitrification of ammonium or supplied from an extrinsic source such as tidal input, wet deposition, or groundwater flow. 

The excesses of nitrogen application over crop uptake in the individual years from 1977 to 1986 were read from Figure 4 of Sylvester-Bradley et and subjected to the rules. Neither the leaching nor the denitrification losses seemed particularly large (Table 4), given that these were aggregate values for ten years, and the amount of nitrogen that was remineralized and then leached seemed very unlikely to be important. 

K. A. Smith and J. R. M. Arah, Losses of Nitrogen by Denitrification and Emissions of Nitrogen Oxides from Soils, The Fertiliser Soeiety, 1990, Proeeedings No. 299. 

---