Nitrogen transformation processes are inevitable in nature, but despite following the best
management practices, it is hard to eliminate all the N2O production from denitrification in soils
or sediments. Some mitigation options that have been proposed to target denitrification include the application of Cu fertilizer, SOM management, and the application of lime to regulate pH and enhance the efficiency of N2OR enzyme. A more efficient way to mitigate N2O is proposed to be strategies targeting both nitrification and denitrification simultaneously, preferably by controlling the amount of reactive N released in soil (Baggs, 2011). Some of the possible
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mitigation techniques to reduce denitrification or N2O production through denitrification are
discussed in this section of review chapter.
2.6.1 Managing N application and transformations on farm
The traditional methods for reducing N2O emissions include restricting the application of inorganic N fertilizers on farms or increasing the N-use efficiency of the applied fertilizer. The N-use efficiency has been enhanced by performing the following practices.
A major source of anthropogenic N2O emission is the use of nitrogen fertilizers in agriculture. As a substantial proportion of applied fertilizers are emitted in the form of N2O, better targeted fertilizer applications, which reduce the availability of nitrogen to
microorganisms, can substantially decrease N2O emissions. Possible strategies should include
reducing the amount of fertilizer and applying it at the cropping time when demand for N is highest and preferably in dry conditions when leaching losses would be minimal. Using slow-
release fertilizers and avoiding N in NO3– form, which is likely to produce large emissions
through denitrification especially under high rainfall condition, would also be useful. Similarly, improved land drainage and better management practices to limit anaerobic conditions in soils (for example, land compaction and excessive wetness) could reduce denitrification rates and,
thus, N2O emissions.
The use of NH4+ based fertilizer rather than NO3– under mild, moist conditions to restrict
the supply of NO3–for denitrification could lower N2O emission from denitrification. The use of
nitrification inhibitor to slow down the oxidation of NH4+ to NO3– might also have the potential
to reduce N2O emission (Ledgard & Luo, 2008). Dobbie & Smith (2003) reported that DCD was
effective in reducing N2O emissions when it was applied with urea and ammonium sulphate from an intensively managed grassland site in the UK.
The practice of restricted grazing has been proposed as one of the options to reduce N2O emissions on farm. Studies in New Zealand farms have demonstrated nearly 60 % reduction in
N2O emission due to less excretal deposition and lower soil compaction when grazing animals
were kept in house during late-autumn-winter compared with year-round farm grazing
75 2.6.2 Biochemical changes in soil
One of the options for mitigating N2O released through denitrification is by enhancing the
reduction of N2O to N2 (Richardson et al., 2009). The advancement in the biochemical and
molecular sciences has developed our understanding of the enzymology, chemistry, and microbiology of the denitrification process. This advancement may enable us to develop mitigation techniques by manipulating the physiochemical conditions of soil, thus influencing the physiology and activity of denitrifying bacteria ensuring as far as possible the reduction of
N2O to N2 (Thomson et al., 2012). The inactivity of N2OR enzyme under high O2, low pH or
lower availability of Cu ions results in high N2O production during denitrification.
One of the possible strategies to mitigate denitrification in soil could be the application of biochar. It has been observed that biochar application facilitates the reduction of N2O to N2 by both its liming effect and by enhancing the electron availability to denitrifiers, which helps
completing the denitrification process to emit N2 as the end product (Cayuela et al., 2013). With
biochar application, decreases in N2O emission by 10 % to 90 % have been observed in 14
different agricultural soils.
A set of management practices is needed that could either lower the N2O emission from soil or increase the N2O reduction to N2. Management practices such as the application of crop
residue, liming, increasing Cu availability, high SWC, biochar application, may enhance N2O
reduction to N2. Higher N2O production from some soils may be due to its poor capacity of
reduce N2O to N2.
2.6.3 Plant breeding and genetic engineering
One of the options to reduce the application of active N to agricultural soils is growing cereal or pasture crops to fix atmospheric N2 to sustain growth and yield. Beatty & Good (2011) have discussed the prospects of engineering cereal crops and bacteria to establish conditions in the roots of plants to support the activity of either the symbiotic or free living bacteria to fix
atmospheric N2 and to increase the affinity of bacteria to the engineered plant roots. Another
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provide anaerobic conditions to support the activity of this enzyme. In a similar approach, greater
reduction of N2O to N2 has been achieved by inoculating soybean plants with rhizobium bacteria
possessing nosZ gene to reduce N2O to N2 (Sameshima-Saito et al., 2006). In that case, boosting
the soils’ capacity to reduce N2O has been proposed as a tool to mitigate N2O. Sameshima-Saito
et al. (2006) have demonstrated that the Bradyrhizobium japonicum USDA110 carrying nosZ
gene has the capacity to reduce even a low concentration of N2O. Inoculating the legume plants
with this bacterium might enhance its capacity to reduce N2O to N2 (Hénault & Revellin, 2011).