Mitigating greenhouse gases

Australia’s natural forests and planted forests are regarded as important ecosystems that can contribute to the mitigation of rising greenhouse gas concentrations. Recent political focus has been directed to the greater mitigation opportunities from conserving tropical forests in neighbouring countries, which undoubtedly has merit. However, this does not detract from the greenhouse gas benefits of either conserving Australia’s natural forests or establishing new forests on agricultural land for timber, pulp or biodiversity ends.

So far, the debate has centred solely upon the greenhouse gas benefits of forests actively taking carbon dioxide (CO2) out of the atmosphere and sequestering it as biomass carbon. But there are other important greenhouse gases, such as nitrous oxide (N2O) and methane (CH4), that occur at smaller concentrations in the atmosphere but have a greater global warming potential (GWP) than carbon dioxide because of their ability to trap radiative heat and their persistence.

Nitrous oxide has a global warming potential 320 times greater than CO2, whereas methane is 21 times more potent. Both these gases can be emitted into the atmosphere or taken out of the atmosphere by soils, including forest soils. So, it is possible that Australia’s natural forests and planted forests could provide greenhouse gas benefits other than through carbon sequestration alone.

Preserving existing forests secures huge quantities of carbon locked up or sequestered in both the tree biomass and the forest soils. Planting new forests establishes new biological carbon sinks. While the trees are growing rapidly they remove large amounts of carbon dioxide from the atmosphere, actively sequester carbon in their woody biomass and eventually in the soils beneath.

These concepts and benefits are now widely accepted and provide one of the cornerstone opportunities to the imminent carbon trading system for Australia. The long-term benefit of carbon sequestration in these new forests does admittedly depend on the end-use of that wood and the on-going use of that land for tree production. Whereas, converting marginal agricultural land back to native woodlands or forests can provide multiple benefits of enhanced habitat and biodiversity, carbon sequestration and water catchment security.

Increased production of methane and nitrous oxide in Australia represents about 25 per cent of the national greenhouse gas emissions, and the vast majority of these emissions come from the agricultural sector, through ruminant cattle and the use of nitrogen fertilisers. Bacterial processes in the soil are responsible for the production of nitrous oxide following nitrogen fertiliser additions. The same processes can produce nitrous oxide in forest soils as well, albeit in smaller amounts.

The importance of soils, and soil bacterial processes, does not stop at the production nitrous oxide, because other soil bacteria can produce or consume methane. The most important sources for methane entering the atmosphere are wetland or submerged soils, where “methanogens” are dominant and produce large methane emissions, contributing up to 115 Mt CH4 per year (natural systems only).

The opposing process of methane consumption is dominant in drier soils, where “methanotroph” bacteria can oxidise, consume and remove up to 30 Mt CH4 from the atmosphere every year, and most of this uptake occurs in undisturbed forest soils.

So, things are looking good for additional greenhouse gas benefits of conserving natural forests. However, recent measurements in natural eucalypt forest systems of NSW (Eucalyptus delegatensis - Alpine Ash), VIC (E. regnans - Mountain Ash) and WA (E. marginata - Jarrah) have shown that even though these forest soils are effective and important sinks for methane, removing up to 150kg CO2e (carbon dioxide equivalent) per hectare per year, they also produce small amounts of N2O through soil nitrification processes.

The greater global warming potential of N2O, when compared with methane, means that the benefit of methane uptake in natural forest soils is offset by simultaneous emissions of nitrous oxide, on a carbon dioxide equivalent basis (CO2e). However, all is not lost when considering such greenhouse gases in Australia’s natural forests. The amount of methane taken up by undisturbed Australian native forest soils is similar to that measured in European or North American forest soils, but the amount of nitrous oxide emitted from Australian forests are much less because of atmospheric nitrogen pollution and deposition in the heavily industrialised regions of the northern hemisphere.

Just because there are no greenhouse gas benefits from considering N2O and CH4 in natural forests, does not mean there are none from planting of trees on previously agricultural land.

First, there is the undeniable benefit of removing methane emissions associated with sheep or cattle in pasture grazing systems. This can be a huge greenhouse gas benefit. In New Zealand, converting grazed pasture to forests has been estimated to provide a methane benefit of more than 2,200kg CO2e per hectare per year and similar benefits could be expected in Australia.

Second, recent field measurements have shown that nitrous oxide emissions from legume-grass pastures can be an order of magnitude greater than from the same land under a pine or eucalypt plantation. This greenhouse gas benefit could reduce emissions by a further 1,100kg CO2e per hectare per year. In the same study, it was shown that methane uptake was approximately doubled after planting trees on previously grazed pasture, which removed an additional 5-10kg CO2e per hectare per year from the atmosphere.

These soil-based greenhouse gas benefits are brought about by a reduction in nitrogen additions (from legumes, excreta and urine), improved nutrient cycling and a drier, less compacted soil profile. All in all, planting trees on legume-grass pastures could lead to a greenhouse gas benefit of more than 3.3 ton CO2e per hectare per year, and a third of this comes from soil processes.

The magnitude of these soil-based benefits depends greatly upon the quality and productivity of the previous pasture. A poor quality pasture will have produced little nitrous oxide, therefore the greenhouse gas benefit of converting it to trees will be less than that for a highly productive, legume-grass pasture.

Such benefits should be considered alongside the carbon sequestration benefits of actively growing plantations, which could be as much as 20.0 ton CO2e per hectare per year in tree biomass alone. Reduced methane and nitrous oxide emissions and increased methane uptake following the conversion of pasture to tree plantations could therefore contribute something like another 12 per cent to carbon sequestration benefit of planting trees.

Accounting for greenhouse gases other than carbon dioxide will not increase the benefit of natural forest systems, but then again their benefit is obvious and strong enough already, as they provide the greatest biological stores of carbon on the planet, and a multitude of biodiversity, amenity and water quality services. Whereas, accounting for nitrous oxide and methane when planting new forests on agricultural or pastoral land can greatly enhance the greenhouse benefits, beyond those from carbon sequestration alone.

This may sound like a platform for singing the praises of planting trees or the plantation industry itself, but it is not. It is simply an attempt to inject some science into the debate on the greenhouse gas benefits of various land-based industries and other natural ecosystems, whilst bringing attention to the potential benefits (or not) of considering nitrous oxide and methane, that until recently have been largely ignored by policy, industry and researchers alike.