Inhibiting nitrous oxide emissions

Dr Selai Letica from AgResearch at Invermay is concerned about the nitrous oxide emissions that contribute to greenhouse gases.

Nitrous oxide

Greenhouse gases warm the atmosphere to make the Earth a pleasant place to live. Scientists are concerned that too many greenhouse gases may warm the atmosphere too much and cause global warming – upsetting the delicate balance of nature by changing the ecosystem needs for many organisms.

Nitrous oxide (N₂O) is the fourth largest contributor to global warming. On a molecular basis, it has a global warming potential 23 times greater than methane gas, the third largest contributor, and 310 times greater than carbon dioxide, the second largest contributor after water vapour. Natural and managed soils (agricultural) are the largest sources of atmospheric N₂O.

The release of nitrous oxide (N₂O)

The release of N₂O occurs during the nitrogen cycle. Atmospheric nitrogen (N₂) enters the soil-plant system via legume fixation. This fixed N may then be released into soil through plant death and decay or by recycling via grazing animals. Through a process that involves other soil bacteria, this fixed N may be converted from ammonium to various other nitrogen compounds (such as nitrate), some of which are highly mobile and susceptible to loss to the environment. This conversion of ammonium to nitrate is the process of nitrification. Nitrate that is not taken up by plants or lost to leaching may undergo the process of denitrification in which bacteria using these compounds reduce them back to N₂. It is during these microbial processes – nitrification and, particularly, denitrification – that N₂O is released as a byproduct.

In agricultural settings, nitrogen levels in soil can be high because of poorly timed or excessive applications of fertilisers or effluent. Animal urine patches are also ‘hot spots’ containing relatively high levels of ammonium – or nitrate forms of nitrogen. High nitrogen levels can lead to the release of more N₂O.

Selai’s research

The focus of Selai’s research is to reduce N₂O emissions.

Selai suggests a two-pronged approach to reducing N₂O emissions:

• Slowing down or inhibiting the natural production of nitrogen compounds (where N₂O is a byproduct). Selai’s focus is on inhibiting the denitrification process of the nitrogen cycle.
• Making the nitrogen cycle so efficient that there is little release of the byproduct N₂O. N₂O is only released when compounds are not fully converted to the next stage.

Nitrification inhibitors to slow down nitrous oxide emissions

One aspect of her research involves interfering with the nitrogen cycle to stop or slow down the production of nitrogen compounds that can lead to emissions of potentially harmful N₂O.

This is through the use of nitrification inhibitors (such as DCD – dicyandiamide). These are chemicals that are applied to soils to slow the microbial conversion of ammonium to nitrate during the nitrification stage of the cycle. As a result, there is less leaching because ammonium adheres (sticks) to the soil, and there is little denitrification because of the lack of conversion to nitrate. With fewer conversions from ammonium to nitrate (nitrification) and then from nitrate back to N₂ (denitrification), N₂O emissions are reduced.

Selai’s research investigates how nitrogen inhibitors work on different farms. The warmer the soil, the faster the inhibitor will break down and therefore needs to be applied more frequently. The inhibitor chemicals are also mobile, so high rainfall might result in high losses from the soil. Inhibitors therefore work better in colder, drier climates. Other factors being investigated for their effects on these inhibitors include nitrogen levels in the soil and levels of compaction of the soil under different stock types (dairy or beef cows or sheep).

The use of DCD – dicyandiamide was voluntarily suspended in 2013 when very small traces of residue were unexpectedly detected in milk. Although the residue posed no food safety risk, the Ministry for Primary Industries notes that international dairy customers expect New Zealand products to be residue-free. Ongoing research led by AgResearch scientists has identified a new inhibitor that shows similar efficacy to DCD, but without the same risks. The inhibitor is undergoing longer-term field trials.

Other farming strategies to reduce nitrous oxide

Several management practices can be implemented to limit N₂O emissions.

• Reducing stock time on paddocks. This can be done using stand-offs – a pad where cows, for example, can stand for periods of time. The pad collects the faeces and urine (containing nitrogen) so that it does not go onto the paddocks in a concentrated form.
• Keeping stock (and fertiliser) off saturated soil. Saturated soils are conducive to the denitrification process. This is because bacteria don’t have access to oxygen for growth and reproduction so they use nitrate instead. In the process, N₂O is released. Avoiding the addition of nitrogen compounds (such as urine and fertiliser) reduces these emissions. Well drained paddocks also help limit denitrification and therefore N₂O emissions.
• Avoid compacting soil. Compacted soil contributes to anoxia (a lack of oxygen). Compacted soils occur when numbers of animals in one area are too high. If the soil is wet through rain or irrigation, the anoxia problem is compounded. The denitrification process is reduced by keeping stock off susceptible areas.
• Farmers can use tools such as the online Overseer® Nutrient Budgeting tool to calculate the nutrient needs of their farm. This helps farmers determine fertiliser needs and possible risk for greenhouse gas emissions.