Inhibition of nitrification to mitigate nitrate leaching and nitrous oxide emissions in grazed grassland: A review

Purpose: Climate change is arguably the biggest environmental challenge facing humanity today. Livestock production systems are a major source of greenhouse gases that contribute to climate change. Nitrous oxide (N₂O) is a potent greenhouse gas with a long-term global warming potential 298 times that of carbon dioxide (CO₂). Nitrate (NO₃⁻) leaching from soil causes water contamination, and this is a major environmental issue worldwide. Agriculture is identified as the dominant source for NO₃⁻ in surface and ground waters. In grazed grassland systems where animals graze outdoor pastures, most of the N₂O and NO₃⁻ are from nitrogen (N) returned to the soil in the excreta of the grazing animal, particularly the urine. This paper reviews published literature on the use of nitrification inhibitors (NI) to treat grazed pasture soils to mitigate NO₃⁻ leaching and N₂O emissions. Materials and methods: This paper provides a review on: ammonia oxidisers, including ammonia oxidising bacteria (AOB) and ammonia oxidising archaea (AOA), that are responsible for ammonia oxidation in the urine patch areas of grazed pastures; the effectiveness of NIs, such as dicyandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP), in inhibiting the growth and activity of ammonia oxidisers; the efficacy of DCD and DMPP in reducing NO₃⁻ leaching and N₂O emissions in grazed pastures; additional benefits of using NI in grazed pasture, including increased pasture production, decreased cation leaching and decreased NO₃⁻ concentrations in plants; and major factors that may affect the efficacy of NIs. Results and discussion: Research from a number of laboratory and field studies have conclusively demonstrated that treating grazed pasture soils with a NI, such as DCD, is an effective means of reducing NO₃⁻ leaching and N₂O emissions from grazed livestock production systems. Results show that N₂O emissions from animal urine-N can be reduced by an average of 57 % and NO₃⁻ leaching from animal urine patches can be reduced by 30 to 50 %. The NI technology has been shown to be effective under a wide range of soil and environmental conditions. The NI technology also provides other benefits, including increased pasture production, reduced cation (Ca²⁺, Mg²⁺ and K⁺) leaching and reduced NO₃⁻ concentration in pasture plants which would reduce the risk of NO₃⁻ poisoning for the animal. Conclusions: The use of NIs such as DCD to treat grazed pasture soil is a scientifically sound and practically viable technology that can effectively mitigate NO₃⁻ leaching and N₂O emissions in grazed livestock production systems