Agricultural emissions of greenhouse gases

 

This project aims to estimate emissions of the greenhouse gases methane (CH4) and nitrous oxide (N2O) from New Zealand’s pastoral farming systems, and to estimate the effectiveness of different approaches to reducing these emissions.

The problem

Pastoral agriculture accounts for about half of all greenhouse gases emitted by New Zealand (in ‘CO2-equivalent’). Most of this is methane emitted by livestock.

This agricultural proportion is about six times higher than the average for developed countries (7.5%), which raises unique questions for New Zealand about how to limit future emissions.

  • How can we assess agricultural emissions accurately?
  • How can these emissions be reduced without harm to the economy?

There needs to be better ways to accurately estimate the greenhouse gases produced by agriculture - methane (CH4) and nitrous oxide (N2O).There also needs to be ways to measure and verify the effectiveness of new techniques that attempt to reduce emissions.

The solution

For CH4 emitted directly by ruminant livestock no techniques to sustainably reduce emissions have yet been established. The CH4 is generated largely in the rumen (first stomach) and “eructated” through the mouth and nostrils. This research focuses on better understanding the role of diet and dietary additives in determining how much CH4 is emitted. This is done in collaboration with AgResearch where NIWA’s role is in developing ways to estimate CH4 emission through unobtrusive sampling and analysis of “breath”.

Agricultural N2O accounts for about one sixth of New Zealand’s CO2-equivalent emissions. It is a by-product of microbial degradation of animal excrement (mainly urine) and of nitrogen fertilisers in pasture soils. Recent innovations use “nitrification inhibitors” added to the fertiliser that slow down the N2O production. The reduced N2O fluxes can be determined using sophisticated and specialised instruments to measure N2O in air downwind of the paddock (“micro-meteorology”) in order to verify the slow-down. Measuring N2O fluxes is nevertheless very challenging, and other measurement methods by collaborators at Landcare Research provide complementarity. Micro-meteorological techniques are also available to verify prospective, but so far elusive, approaches to reduce CH4 emissions.

The result

NIWA and AgResearch have jointly pioneered the “SF6 tracer technique” to measure daily CH4 emissions by grazing livestock in NZ. This involves mounting light-weight breath-sampling equipment on each animal (see photo above) and subsequently analysing the sample in the laboratory. While complex and technical to implement, the SF6 tracer technique remains the only method available to measure CH4 emitted by individual freely grazing animals.

Recent developments in instrumentation are enabling new approaches in micrometeorology that will permit emission fluxes and reductions in fluxes to be determined to higher confidence. Computer automation of both air sample collection and analysis by gas chromatography results in field experiments being less demanding of staff time, even to the point of being completely unattended in some cases, and with better quality control.

A new diode laser instrument can measure N2O to much higher precision and with much higher frequency than was previously achievable. A new instrument based on “cavity ring-down” laser technology promises an improved CH4 measurement capability. All of these techniques are presently being deployed and tested in New Zealand’s pastoral landscapes.

Page last updated: 
22 February 2014
Samples of breath being collected for gas analysis from grazing cows (Keith Lassey)