Nitrous Oxide

Nitrous oxide (N₂O) is a powerful GHG with a global warming potential 265 times greater than carbon dioxide (CO2) and has a lifespan of 100 years. Over the last 150 years, atmospheric concentrations of N₂O have reached unprecedented levels, increasing from 270 parts perbillion (ppb) to 335 ppb. View Global N₂O Levels

Agriculture and Nitrous Oxide

Agriculture produces 90% of N₂O emissions in Ireland.The main contributions are:

through the application of synthetic fertilizers on pastures - 38%
the deposition of animal excreta during grazing - 23%
manure management - slurry storage and spreading - 14%

N₂O is produced in the soil through microbial processes which utilize nitrogen (N) from the above agricultural practices. The rate of N lost from pastures in the form of N₂O is stimulated by environmental conditions which favour microbial activity such as:

wet soil conditions
rainfall events
high soil temperatures

Where multiple factors which promote N₂O production exist in one space, hotspots of N₂O occur where the potential for N₂O losses greatly increases.

Mitigation of N₂O

To mitigate against N₂O emissions in agricultural systems potential N₂O hotspots must first be indentified and treat these areas accordingly. Potential hotspots of N₂O within agricultural landscapes could be water-trough areas and gateways i.e spaces that cows may spend more time congregating. Reducing the number of gateways per paddock and/or locating gateways on well-drained soils could reduce N₂O emissions.

Improved soil management

Farmers can play a central role in mitigating against N₂O emissions through improved soil management. Conditioning soil using:

zeolites that work by removing heavy metals from soils which can negatively affect plant growth. These can improve crop yield through increased nutrient and water use efficacy of plants
lime to adjust soil pH levels to an optimum level for plant growth, improve soil structure and increase rates of infiltration.

Fertiliser and manure management

By actively avoiding spreading N fertilizer and farm effluent/manure near potential hotspots of N₂O such as gateways or water troughs, farmers can limit high N losses from managed pastures. Where possible, nitrification inhibitors and protected urea should be applied to gateway areas and other potential hotspot areas, to reduce N loss and the availability of N for N₂O production.

Protected urea is urea which has been treated with a urease inhibitor which prevents the conversion of urea to ammonium. Studies investigating the performance of protected urea versus calcium ammonium nitrate (CAN) have shown the same yield output with lower N₂O emissions (Fig. 1) (Forrestal et al., 2017).

Figure 1: The effect of fertilizer type and nitrogen (N) rate on annual dry matter year. Fertilizer treatments include calcium ammonium nitrate (CAN), urea and urea + the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT). Graph adopted from Forrestal et al. (2017).

Synthetic fertilizer alternatives

An alternative to synthetic fertilizers is the use of legume crops. Legumes contain symbiotic rhizobia bacteria that fix N from the air in the soil and release that N slowly, meaning plants can optimize the N inputs more effectively for growth. By incorporating legumes into cropping systems, farmers can reduce loading rates of N fertilizer applications and therefore lower N₂O-N losses.

Nitrates Directive

The Nitrates Directive (91/676/ECC) is primarily concerned with improving water quality which is complimented by a reduction in atmospheric N₂O emissions (Leahy et al., 2004). Each EU member state is required to create National Nitrates Action Programmes (NAPS) which highlight the rules for the management and application of livestock manures and other fertilizers. Irelands third NAP came to an end in 2017 whereby farmers were limited to using 170 kg N per hectare per year from livestock manure and the last date for spreading synthetic fertilizers is the 14^th September.