Potential hotspot areas of nitrous oxide emissions from grazed pastoral dairy farm systems

Nitrous oxide (N2O) is a potent greenhouse gas with a global warming potential 265–298 times greater than carbon dioxide (CO2) and causes destruction of stratospheric ozone. In soil, N2O is produced through the process of incomplete microbial denitrification or as a by-product of nitrification. Agricultural soils are the main source of N2O emissions globally. Total N2O emissions from dairy grazed farm systems can be dominated by large emissions within a small area (hotspots). Typically, N2O hotspots are areas with high stocking density, high excretal inputs (resulting in high soil N), and situations when soil water filled pore space is elevated. Different pasture grazing systems can be used on dairy farms, including year-round low input dairy systems and higher input systems with some animal confinement component. Potential N2O hotspot areas can be categorized into the following: areas of manure accumulation, storage, and spreading; areas of high stocking intensity leading to soil compaction and high inputs of urine and dung; cultivation and grazing of forage crops; and landscape features including topography, riparian areas, and soil property effects. High input systems can lead to a greater potential for N2O emission hotspots. To demonstrate the effect of hotspot zones on the calculation of total farm N2O emissions, a model was developed and used to assess the N2O emissions from a New Zealand case-study farm. Emission factor (EF3) values for cow urine in the gateway and water-trough areas were measured on the case-study farm and were both found to be about five times that of the rest of the paddock. Using these values for the total farm emissions calculation, it was found that gateways could be significant hotspots for N2O emission with 3.2% of the farm area contributing 9.4% of the total farm N2O emissions. Knowledge of the significance of hotspot zones would enable more accurate calculation of total farm emissions and more efficient targeting of N2O mitigation strategies. There is a paucity of studies which specifically examine hotspots of N2O emissions from farm-scale features and the full magnitude of the emissions from possible hotspot areas and their contributions to the total farm emissions require further investigation.