Rates of denitrification

Therefore the rate of denitrification is limited by the supply of NO3 rather than carbon, and proceeds almost completely to N2. 

Figure 6.19 shows the concentration profiles of O2, Fe +,NH4+ and N03 near a root calculated with this model for realistic flooded soil conditions and realistic rates of O2 release (last section) Figure 6.20 shows the corresponding fluxes of O2 out of the root and NH4+ and NO3 in. The amount of N denitrified in 10 days in the calculations corresponds to about 10% of the NH4+ initially in the volume of soil influenced by the root. This is of the order of maximum rates of denitrification reported in the literature for rice in flooded soil, indicating that the model parameter values are indeed realistic. 

Estimates of denitrification rates range from 54 to 345 xmol/m2 per hour in streams with high rates of organic matter deposition, 12 to 56 xmol/m2 per hour in nutrient-poor oligotrophic lakes, and 42 to 171 xmol/m2 per hour in eutrophic lakes (62). Rudd et al. (64) reported an increase in the rate of denitrification from less than 0.1 to over 20 xmol/m2 per hour in an oligotrophic lake when nitric acid was added in a whole-lake experimental acidification. This result suggests that freshwater denitrification may be limited by N03" availability. In deep muds of slow-flowing streams, the process can effectively reduce N03" concentrations in... 

Seitzinger, S.P., and Nixon, S.W. (1985) Eutrophication and the rate of denitrification and N2O production in coastal marine sediments. Limnol. Oceanogr. 30, 1332-1339. 

Recently, Devol et al. (2006) have used on-deck and in situ incubation of water samples with added N03 to determine the rate of denitrification by measuring the rate of production. Although these were not true tracer experiments, as the in vitro N03 concentrations were increased 5 iM over the in situ concentration of 25 pM, it is likely that rates were not stimulated by the amendment. In the Arabian Sea ODZ the of the N2 typically increased by 0.4-0.8%o. The average... 

Denitrification occurs in environments that are deficient in O2. An increase in the ocean s nitrate content will drive higher export production (neglecting, for the moment, the possibility of phosphate limitation). When this increased export production is oxidized in the ocean interior, it will drive more extensive O2 deficiency. This will lead to a higher global rate of denitrification, which, in turn, will lower the ocean s nitrate content. Thus, the sensitivity of denitrification to the O2 content of the ocean interior generates a hypothetical negative feedback that may, like the N2 fixation-based feedback, work to stabilize the nitrate content of the ocean (ToggweUer and Carson, 1995). 

If soils with large nitrate concentrations become wet and anaerobic conditions develop, the rate of denitrification is greatly increased. Denitrification represents a loss of fixed nitrogen capital, and the emitted nitrous oxide (N2O) may contribute to an enhancement of Earth s greenhonse effect. 

The release of N20 in this process is enhanced under certain conditions, such as low pH, low temperatures, and high nitrate concentrations these factors inhibit, or at least do not favor, the last step of the preceding sequence of reactions. As in the case of nitrification, agricultural management practices may influence both the total rate of denitrification and the ratio of nitrous oxide to N2 produced. 

Figure 21. Model predicted rates of denitrification coupled to sediment nitrification for various continental shelf regions in the North Atlantic basin a) denitrification rates per unit area (mmol N as N2 perm /day), andb) N removal by region (mol N x j e.g.. Nova Scotia/Hattras...

Rate of denitrification — 13% from manure N rate Annual flux — 20528 tons... 

One approach of kinetics is to describe the dependence of reaction rates on reactant concentrations. For instance, the rate of phosphate fixation depends at least partly on the amount of fertilizer added, and the rate of denitrification (the conversion of soil nitrogen, usually nitrate to N2 and N2O) depends on soil solution nitrate concentrations. Kinetics relates reaction rates and reactant concentrations by means of the reaction order and the reaction rate constant. The denitrification rate (—ANO /At) is presumably related to soil nitrate concentration by... 

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