Effect of different irrigation systems on nitrous oxide emissions from urine applied to pasture soil

Abstract

Nitrous oxide (N₂O) is one of the important greenhouse gases (GHGs) that contributes to climate change and depletion of the ozone layer. Nitrous oxide is produced by nitrification and denitrification processes in soils. In New Zealand, the agriculture sector produces the largest proportion of the total GHG emissions and the largest source of N₂O emissions is from agricultural soils. New Zealand’s commitment to the Paris Agreement 2015 is to reduce all GHG emissions to 30 percent below 2005 levels, by 2030. Increased GHG emissions have an effect on climate change which is a threat to many countries in the world and especially the South Pacific Islands. These islands have already been affected by climate change where coastal agricultural land is submerged due to sea-level rises. As agriculture in New Zealand is the main emitter of GHGs, and because of the impact it has on the South Pacific Islands, more research is required to reduce emissions. Therefore, the objectives of this research are to: 1) quantify the effect of irrigation systems (spray vs roto-rainer vs flood) on N₂O emissions from urine applied to pasture soil and, 2) determine the relationship between N₂O emissions and soil nitrifier and denitrifier population abundance as affected by different irrigation systems. A field trial was carried out at Lincoln University Research Dairy Farm over a period of 135 days (late summer to early winter) to assess the effect of the different irrigation systems on nitrous oxide emissions from urine applied to pasture soil. The soil used was Templeton sandy loam/Paparua (Udic Haplustepts). The treatments included three irrigation systems: spray, roto-rainer and flood, each with control (water) and/or urine (700 kg N/ha) treatments. Soil samples were also collected from companion soil blocks and analysed for soil mineral N, nitrifier and denitrifier population abundance. The results from this research showed that there was no significant difference between the irrigation systems on total N₂O-N emissions but the different irrigation systems affected the temporal pattern of the N₂O-N emissions. The irrigation systems did not significantly affect the AOB, AOA or denitrifier abundance, which helps to explain the similar total N₂O-N emissions between the irrigation treatments. The emission factor (EF3) values for these irrigation treatments (2% for spray, 3% for roto-rainer, and 2% for flood) were higher than the New Zealand’s specific EF3 value (1%). This is probably a result of the warm soil temperatures combined with moist soil conditions under irrigation, as supported by the observed positive relationship between soil temperature and N₂O-N emissions. Relationships between N₂O-N emissions and soil water content or water-filled pore space (WFPS) were weak because most of the N₂O-N emissions occurred at moisture contents below field capacity. Importantly, considerable amounts of N₂O-N can be emitted from urine patches in irrigated pasture despite the soil water content being below field capacity.