Agricultural emissions – can we measure and mitigate?

Agricultural emissions are linked to intensive farming, with dairy cattle and road transport the largest contributors to the growth in emissions since 1990.

Can these be measured and mitigated?

Currently, the Overseer nutrient budgeting model can provide a reliable indication of greenhouse gas emissions at farm level.

But scientists are of the opinion that it is difficult for any system to go out and measure emissions on every farm. We should be relying on robust science that is able to develop equations that could calculate emissions based on factors such as stocking rates, dry matter eaten and fertiliser use. The primary sector partnership – He Waka Eke Noa – is planning to discuss options with farmers in the next few months.

We need to design a system that can bring a positive change and outcome that protects our environment and economy. The system should be flexible enough to adapt to future changes, a blend of new technology, such as vaccines and inhibitors, and current practices.

Over the past 20 years, our farmers have improved the emissions efficiency of production by about one per cent a year. This good work can be kept up by implementing good farming principles.

Where the emissions come from

Carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) are the three main greenhouse gases affecting the climate by warming the planet.

The dairy sector contributes 46 per cent of New Zealand’s agricultural greenhouse gas emissions, with an average dairy farm emitting 9.6 tonnes of GHG/ha/year.

The sheep and beef sector is next, contributing 43.6 per cent of our nation’s agricultural greenhouse gas emissions, with current estimates showing that an average sheep and beef farm emits about 2.8 tonnes of GHG/ha/year.

Emissions from dedicated cropping and horticulture contribute a small percentage of New Zealand’s total biological emissions.

Methane, although considered short-lived, once emitted into the atmosphere causes a lingering warming effect for a long time after the methane itself has gone. One tonne of biological methane traps approximately 33 times more heat than a tonne of carbon dioxide over a 100-year period.

About 80 per cent of our country’s total nitrous oxide emissions come from urine patches on paddocks. One of the recent reports from the government indicated that the nitrous oxide emissions have increased by almost half since 1990.

Nitrous oxide gas generally comes from conversions in the soil by microbes of nitrogen in fertilisers, urine and dung. When soils become anoxic, nitrate can be sequentially reduced to nitrous oxide and inert nitrogen. This is called de-nitrification.

Methane emissions are higher on farms with higher stocking rates and higher dry matter consumption. Some of the options to reduce methane are lowering replacement rates, reducing the dry matter feed per cow, and lowering stocking rates.

Minimising human induced erosion and maintaining good soil quality are essential for maintaining soil ecosystem services such as nutrient and water buffering, productive capacity, assimilating waste and minimising impacts of sediment and other contaminants on waterbodies.

Other good practices include optimum cultivation, increasing greater plant diversity in swards, avoiding over grazing and heavy grazing under wet weather leading to compaction, avoiding under or over-fertilisation, practicing appropriate use of pesticides and other agrochemicals, managing pasture to maintain complete soil cover and careful application of farm dairy effluent to avoid saturation and optimise organic matter.

The options for reducing nitrous oxide could be reducing nitrogen inputs through judicious use of fertilisers, using low nitrogen feeds and improving pasture quality.

Complementing these research programmes is a new long-term national soil carbon study conducted at 500 locations around New Zealand, which will benchmark soil carbon stocks in different land uses and then monitor how those stocks are changing with time. Initial data will provide a national-scale picture of how much soil carbon is currently stored and, later, whether those stocks are increasing or decreasing.

• Bala Tikkisetty is a sustainable agriculture advisor (technical) at Waikato Regional Council. Contact him on 0800 800 401 or bala.tikkisetty@waikatoregion.govt.nz.