Nitrous oxide is New Zealand’s second most important agricultural greenhouse gas after methane, representing 22% of agricultural emissions and 11% of total greenhouse gas emissions.
Research of more than 1200 emission factors and data measured from 139 field experiments over the past decade produced the new finding on discharges from urine, dung and fertiliser.
The study showed emissions from sheep are about half those of beef cattle.
It also found urine emissions are higher for gentle slopes than for steeper slopes.
The new calculation cut total sheep emissions in 2017 by 10.6%, total beef cattle emissions by 5% and dairy cattle emissions by 1.4%.
AgResearch scientist Tony van der Weerden said the new calculations are a significant change for hill-country farm emissions.
“For example, nitrous oxide emission factors from excreta deposited by sheep will be reduced by two-thirds while emission factors from beef cattle will be reduced by one-third.”
Emissions from dung and urine on pasture are the single largest contributor to nitrous oxide emissions in NZ.
In 2017 those emissions from animal excreta contributed the equivalent of 6.6 million tonnes of carbon dioxide in the agriculture sector, equivalent to 17% of agricultural emissions.
The impact hill country slopes have on urine emissions is thought to be a combination of lower soil microbial activity and lower soil moisture content relative to low slopes, he said.
“Under these conditions, smaller amounts of nitrous oxide are produced and released into the atmosphere.”
Lower urine emissions in sheep compared to cattle are believed to be caused by the lower volumes produced, which might lead to greater use of the excreted nitrogen by plants.
“However, this theory requires further investigation.”
According to Beef + Lamb there are more than 5500 sheep and beef farms on hill country, representing about half of all sheep and beef farms. About 1000 are on hard hill country in the North Island.
The new emission figures were considered by a panel of agriculture emissions experts who recommended they be applied to the 2020 national greenhouse gas inventory, which will report emissions from 1990 to 2018.
Manaaki Whenua Landcare Research was a partner in the research commissioned by the Ministry for Primary Industries.
