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12 Jun 2013, 15:20 Robin Webster Managing forests so that wood can be burnt for energy could release large amount of carbon from the soil and increase greenhouse gas emissions, according to a new study. The government plans to ramp up the amount of power the country gets from bioenergy . In theory, generating electricity from plants, trees and crops can be carbon negative . But there’s a complicated argument going on about whether using wood to generate electricity might lead to more, or less, greenhouse gas emissions. Now, a new study from Dartmouth College has added to the controversy. It shows that harvesting forests more intensively disturbs the carbon in the soil – releasing more carbon dioxide into the atmosphere. What is soil carbon? Forests are carbon stores. About a third of the world’s global soil carbon is stored in forest soil, and around half of that is in soil derived from minerals or rocks . Disturbing the soil could mean that the carbon is released into the air as carbon dioxide. But collecting measurements on mineral soil carbon is a labour-intensive process – which means there isn’t much data out there. As a result, many carbon monitoring systems assume that mineral soil carbon is not affected by the way a forest is managed. Assessing what happens to mineral soil carbon According to the new paper, published in the journal ofGlobal Change Biology-Bioenergy, this isn’t right. Reviewing a number of recent studies in North America, it says that harvesting and then replanting forests may lead to “significant and long-term carbon losses in the mineral soil”. In one case, a forest that was clear-cut and then replanted in Nova Scotia still showed a 50 per cent loss in mineral soil carbon 30 years later. Another study recorded a “significant” decline in mineral soil carbon eight years after a hardwood forest in the United States was cut down. Co-author Professor Andrew Friedland tells Eurekalert : “Our paper suggests the carbon in the mineral soil may change more rapidly, and result in increases in atmospheric carbon dioxide, as a result of disturbances such as logging … [it] suggests that increased reliance on wood may have the unintended effect of increasing the transfer of carbon from the mineral soil to the atmosphere. So the intended goal of reducing carbon in the atmosphere may not be met.” Some caveats There are some caveats. First, this is a complicated area. The studies reviewed don’t all agree about how much soil carbon is lost when a forest is cut down. The paper also concentrates on what happens when forests are chopped down and regrown – it doesn’t look in detail at what different sorts of forest management could mean. The biomass industry argues whole trees won’t be used as a source of bioenergy anyway. Instead, it says it will use by-products from forestry like sawdust, bark and smaller trees known as thinnings. The study doesn’t address what extracting these other products from the forest could mean for mineral soil carbon. But Friedland tells Carbon Brief: “…we can say that there are suggestions that each of these different forest removal approaches will have different implications for the amount of carbon that gets mineralised, or released, from the soil to the atmosphere”. What does it mean? The authors conclude: “This debate [on how to manage forests, and the use of wood for bioenergy] must include mineral soil carbon.” Including even a “moderate forest soil carbon loss” can have a significant impact on calculating whether burning wood to make bioenergy will lead to a reduction in greenhouse gas emissions, they conclude.   The paper is clearly not the last word on the bioenergy debate – but it brings a new issue into what is already a pretty involved argument. In the future, a significant amount of wood burnt in UK power stations could come from North America – so how those forests are managed will matter. Continue reading →