Human activities are having a major impact on the carbon balance of forests in the northern hemisphere, according to new research carried out in the framework of the EU-funded CarboEurope project. Writing in the journal Nature, an international team of researchers, led by Federico Magnani of the University of Bologna, shows that we are affecting the carbon balance of northern forests both directly through forest management activities, and indirectly through nitrogen pollution. During photosynthesis, trees take up CO2 (carbon dioxide) from the atmosphere and release oxygen back into the atmosphere. However, trees also respire, a process which returns CO2 back to the atmosphere. Furthermore, decomposing organic matter in the soil of forests also releases CO2. When conditions are right, forests absorb more CO2 than they release, making them effective carbon sinks. Researchers are working hard to understand the factors which control the balance between CO2 absorption and release. In this latest study, scientists investigated the carbon balance of temperate and boreal forests in western Europe and North America. As expected, they found that forest management activities accounted for a large proportion of the variations in carbon balance in the forests studied. When an area of forest is disturbed, it typically acts as a carbon source for some years before becoming a carbon sink again. However, once the effects of forest management and disturbance had been accounted for, the main factor driving carbon sequestration by forests was found to be nitrogen deposition caused by human activities. 'As a result of pollution of the atmosphere by active nitrogen from the internal combustion engine, from factories, and from intensive agriculture, the whole planet receives an annual dose of what can be regarded as nitrogen fertiliser,' explained Professor Magnani. 'It comes for example in rain, snow and fog.' By matching CO2 uptake to nitrogen deposition, the researchers were able to reveal that for every kilogram of nitrogen that rains down onto forests, an extra 400kg of carbon is absorbed from the atmosphere. 'This dual effect of humans on forests - causing release of carbon by management, and increasing uptake of carbon by nitrogen pollution - is but one illustration of the way anthropogenic influences impact on the global environment in ways that were not intended,' commented Professor Magnani. 'Forests, often viewed as pristine ecosystems, are in reality deeply conditioned by mankind.' As Peter Högberg of the Swedish University of Agricultural Sciences points out in an accompanying article, the study raises a number of important questions. 'Should forests be fertilised with nitrogen to sequester more atmospheric CO2?' he asks. 'And should strategies to reduce levels of CO2 emissions include forest fertilisation to produce more wood products to replace fossil fuels, or to replace concrete as a building material (large amounts of CO2 are generated during concrete production)?' However, he warns that further research is needed to understand the wider environmental impacts of artificially fertilising our forests.