Mitigating greenhouse gases

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.