Organic soils are a source of greenhouse gases
An intensive monitoring program has, for the first time in Denmark, quantified the loss of carbon and emissions of methane and nitrous oxide from organic soils managed by agriculture.
Cultivation of organic soils – that is, drained peat soils with at least 20 percent organic matter in the plough layer –accelerates degradation of the peat. This will affect the greenhouse gas balance of agriculture. The reasons for this, and the importance of management of organic soils for emissions of carbon dioxide, methane and nitrous oxide, will be discussed by senior scientist Søren O. Petersen from the Faculty of Agricultural Sciences, Aarhus University, at the annual Plant Congress in Herning 11-13 January 2011.
- Based on measurements during a 14-month period we have calculated an annual greenhouse gas balance for each of eight locations. The balance was always positive, that is, all of these organic soils were net sources of greenhouse gases, says Søren O. Petersen.
The relative importance of carbon dioxide (CO2), methane and nitrous oxide, however, varied. The emission of CO2 was generally the largest source, but also the most uncertain part of the budget. This number is calculated as a difference between two much larger components, the fixation of CO2 via photosynthesis and the degradation of soil organic matter and plant respiration.
First complete greenhouse gas balance
The intensive measurement program, which has been completed in close collaboration with the National Environmental Research Institute between August 2008 and October 2009, constitutes the first annual study of greenhouse gas balances for Danish organic soils.
- As support for Denmark’s documentation to the United Nations on soil carbon stocks, and for national action plans on land management, there was a need for a special focus on cultivated organic soils. This effort was also important, because the organic matter in many situations will be degraded under conditions with oxygen limitation, where two other greenhouse gases, methane and nitrous oxide, can be formed.
When soil organic matter is degraded, nitrogen is released, and this can lead to emission of nitrous oxide. Nitrous oxide has a global warming potential that is 298 times greater than that of CO2, explains Søren O. Petersen. Nitrous oxide emissions are particularly high on arable soil where both nitrogen from soil organic matter and nitrogen from fertilizers may stimulate emissions.
In saturated parts of the soil degradation of organic matter will lead to formation of methane, which has a global warming potential that is 25 times higher than that of CO2. However, methane can also be degraded by specialized bacteria in the upper, unsaturated soil. Groundwater level is therefore an important control of methane emissions across the year. In organic soils all three gases – CO2, nitrous oxide and methane – can be important for the total greenhouse gas balance.
- Our results confirm that factors such as groundwater level, nitrogen status and vegetation are key controls of emissions of CO2, methane and nitrous oxide. Reconstruction of wetlands may therefore influence the overall greenhouse gas balance, Søren O. Petersen points out.
