A review of plant options for mitigating nitrous oxide emissions from pasture-based systems

In grazed pasture-based systems, a high percentage of nitrogen (N) consumed by grazing animals is voided in concentrated urine patches that are the key sources of nitrous oxide (N2O) emissions. N2O is a potent greenhouse gas (GHG) and the development of mitigation strategies for reducing N2O emissions from these patches is an ongoing focus of international research. Plants can help mitigate N2O emissions through reducing urinary-N excretion by minimising the total amount of N consumed by the animal and/or by increasing the partitioning of excreta N into dung relative to urine. Plants can also affect the N2O emission factor (the proportion of urinary N emitted as N2O) by influencing nitrification and denitrification rates in soil. This paper reviews the current understanding of the impact of plant species on N2O emissions from urine patches and assesses the potential of two promising plant species to reduce GHG emissions from New Zealand dairy farming systems. The review confirms that different plant species can result in different N2O emissions from urine patches, and recent New Zealand studies suggest that plantain and fodder beet show particular potential for grazed systems. None of the existing studies have provided conclusive evidence of the mechanisms by which these plants reduce N2O emissions. However, based on the literature review, we suggest that a key mechanism by which plants such as plantain affect the N2O emission factor is through root exudates that inhibit nitrification and/or increase available C and thus N immobilisation. Plant species that reduce the N concentration of urine but not the total amount of urine N, may not reduce total N2O emissions as the results on the effect of urinary N-loading rate on the N2O emission factor are inconclusive. A modelling assessment suggests that incorporating 20% of the diet as diverse pasture (including plantain) and 10% as fodder beet could potentially reduce N2O emissions from New Zealand dairy farms by 12-15%, and total biological GHG emissions (i.e. methane plus N2O) by 2-3%.