Can Soils Soak Up Greenhouse Gases?
At the collapsed climate change negotiations in The Hague, the impasse was over carbon sinks used to offset greenhouse gas emissions. The US wants soil to be accepted as one of those sinks, but their critics say it's just hot air, and another way to avoid reducing the use of fossil fuel.
Richard Aedy: Last week in Ottowa, there was a discreet little meeting. We were there, so were the Canadians, it's their capital. But the really important representatives were from the US and the EU. The meeting was an attempt to move things forward after the failure at The Hague, and it didn't work out.
The sticking point's the same: the umbrella group of countries led by the US want to be allowed to offset some emissions with greenhouse sinks. The Europeans want the emissions cut. Most debates focused on forests, but increasingly, scientists are talking about another form of sink: the soil; the idea of using soils to soak up carbons being widely attributed to Rattan Lal, Professor of Soil Science at Ohio State University. At the moment, the world soils don't act as a carbon sink, they act as a carbon source. Ever since we invented farming, soils have been losing carbon, about 100-billion tonnes so far.
Rattan Lal says the main culprit is one of our oldest technologies, the plough.
Rattan Lal: When it opens up the soil it also mixes oxygen in the soil, and that leads to oxidation of carbon riches in the soil, that's one consequence of ploughing. Another consequence of ploughing is that it decreases the structural stability of the soil so that it becomes more susceptible to erosion by water and by wind. And as the erosion happens, it removes the light fraction of the soil, which is organic matter content. So ploughing in fact is a double jeopardy: it oxidises more carbon and it promotes more erosion, and by both processes, the carbon is depleted.
Richard Aedy: To put back this carbon, we have to stop ploughing, or plough much less. Ploughs not only dig a furrow through the soil, they turn it over, which is what does the real damage when it comes to oxidising carbon. Ploughing in a way that doesn't turn the soil, or not ploughing at all, is called 'reduced tillage'. It's an approach that goes hand in hand with preventing soil erosion, using plenty of mulch and manure and planting crops to enhance the soil.
Professor Lal says applying these techniques around the world would sequester a lot of carbon.
Rattan Lal: The potential of carbon sequestration on a global scale is about 0.7-billion tonnes, 0.6-billion tonnes to 1-billion tonnes per year. That's quite a lot of carbon, considering that the amount of carbon increases in the atmosphere at about 3-billion tonnes every year. So there is a lot of potential, but to realise that potential we really have to have a global adoption of improved agricultural systems which is very difficult, especially in developing countries.
Richard Aedy: That's an understatement, to say the least. Ploughing kills weeds and many farmers in developing countries have no alternative way of doing that. Farmers in rich countries can use a herbicide to do the same job, which explains why Monsanto, the maker of Round-up, is so enthusiastic about the idea. But Monsanto isn't the only cheerleader; many scientists, including Ian Noble, Chief Executive of the CRC for Greenhouse Accounting, say sequestering carbon in soil offers a distinct advantage.
Ian Noble: Carbon stored in the soil is actually much safer for reversibility than for example trees and other vegetation. However there are some risks. Say for example the farmer has been carrying out say, a minimum tillage, for 10, 15 years or more, and then for one reason or another they go back to deep ploughing, even for a few years, that can release a lot of the carbon that's been stored. So any monitoring system has to be accurate enough to detect that, and make sure that that's properly accounted for.
Richard Aedy: These issues of monitoring and compliance are important. They're expensive to do, but if they're not done, it's impossible to know how much carbon is being stored in the soil. If it becomes possible to trade in carbon credits, some farmers will be able to sequester carbon as a cash crop, and the money will be there to make sure they're doing it properly. Ian Noble says farmers already have reasons to be interested in minimum tillage.
Ian Noble: Minimum tillage was not done originally for Kyoto purposes, it was done because it was good agricultural practice, and we'd like to encourage that practice throughout, not just the Annex 1 countries, the developed countries, but also through the developing countries. So it is a very worthwhile thing to do for many different purposes.
Richard Aedy: It sounds almost too good to be true, and as we've heard, compliance will be a real issue. Even Professor Rattan Lal, the man who's done most to popularise the idea, doesn't believe soil sequestration's a panacea.
Rattan Lal: It's an important strategy, it's cost effective, but it is not the only thing that can solve the problem. Eventually in the long run, we must find alternatives to carbon based fuel.
Alexandra de Blas: Rattan Lal, Professor of Soil Science at Ohio State University; and Richard Aedy prepared that report.
- Professor Rattan Lal
- School of Natural Resources
Ohio State University
- Professor Ian Noble
- Cooperative Research Centre for Greenhouse Accounting
CRC for Greenhouse Accounting
GPO Box 475
Canberra ACT 2601
- Alexandra de Blas
- Natasha Mitchell
- Richard Aedy