Integrated Assessment of the European and North Atlantic Carbon Balance

Schulze et al., 2009

Content

• Large-scale study of the effects of land-use on greenhouse gas emissions, using aircrafts, measurement towers, gas exchange measurements at the soil surface, and biomass and soil carbon inventories.

Findings

• The net CO₂ balance of land can be calculated in the following way: Plants absorb carbon for photosynthesis and release part thereof through respiration. Some of the net carbon embodied in plants is harvested or lost through fires, while litter and manure add to it. In the case of forests, a share of the carbon remains above surface in the plants, but most eventually ends up in the soil. Most of this is released back to the atmosphere through microbial respiration, while a minor part is washed away. As a result, only a fraction of the carbon originally absorbed by plants is fixed in soil humus for the long term. Furthermore, calculation of the net greenhouse gas balance of land requires the inclusion of other significant greenhouse gases, such as methane.
• Based on current land-use practices, forests and grasslands are net greenhouse gas sinks. The net balance is best for forests. However, this is only because forests accumulate carbon above ground (and this carbon can be released through harvest). The advantage of grasslands is that they bind more carbon in soil. Croplands are net greenhouse gas sources. Among the reasons for this is that fertilizer use engenders nitrous oxides and that plowing aerates the soil, thus increasing microbial populations which break down carbon and emit CO₂.
• The greenhouse gas balance of EU agriculture is problematic. While croplands emit 11 terra grams of carbon per year, methane (CH₄) emissions from agriculture are equivalent to 28 terra grams of carbon and agricultural emissions of nitrous oxides (NO₂) are equivalent to 70 terra grams of carbon. Taken together, this makes 109 terra grams of carbon equivalents - an amount that corresponds to the entire net CO₂-absorption of forests. As a result, the EU's land surface is a very small greenhouse gas sink (5 terra grams of carbon per year). The EU’s ecosystem only makes a marginal contribution to balancing the EU’s greenhouse gas emissions from fossil fuels, which approximate to 1060 terra grams of carbon per year.
• Land use practices greatly affect land’s greenhouse gas balance. Croplands can be managed as a carbon sink. The effect of forestation on climate change is unclear as new forests typically supplant grasslands. It is more important to prevent the replacement of old forests with plantations of fast-growing trees that can be quickly harvested. This would release the carbon accumulated in the old trees, while the young trees would extract carbon from the soil for their growth, which would be released again upon harvest. Interestingly, unmanaged forests continue to be carbon sinks when they grow old (over several hundreds of years) though they grow less quickly.
• Draining and extracting peat releases substantial amounts of greenhouse gases (20 terra grams of carbon). Restoring peatland tends to result in net savings of greenhouse gases, but the initial excess emission from destroying peatland cannot be undone.
• Very hot and dry summers, which will become more frequent with climate change, reduce land’s carbon absorption as plants scale down their photosynthesis. The 2003 drought caused land to become a net carbon source that canceled out the equivalent of five years of net carbon absorption by land.

Comment

• Among many other insights, the study shows that the climate effect of agriculture cannot simply be measured by the release of greenhouse gases. Land should be a much stronger greenhouse gas sink to compensate for man's (partly inevitable) emissions.