How is deforestation related to soil erosion?

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After burning fossil fuels, clearing forests is considered to be the most important source of atmospheric carbon (CO2), the most important engine of global climate change. An international team of researchers has now found that the CO2-Release through deforestation is not as negative as previously assumed. The soil erosion that sets in on the cleared areas carries away carbon-rich topsoil. However, this is partly stored as sediment terrestrially or in water bodies. In combination with the extensive formation of new soil at eroded sites, this process leads to the binding of atmospheric CO2. Dr. Thomas Hoffmann, private lecturer at the University of Bonn and since 2015 employee at the Federal Institute for Hydrology, is one of the authors of a scientific article in the journal "nature climate change" on this topic, which is also relevant to questions about the sediment balance of waters.

When forest disappears, carbon dioxide is released, especially during slash-and-burn operations. We are not only talking about the carbon that is bound in wood - large amounts of carbon are also stored in the soils of healthy forests. So if forest soils are converted into fields, then regular cultivation of the arable land releases carbon dioxide. This process of land use change began about 7000 years ago when humans began to settle down and practice agriculture. In the meantime, the global forest area has shrunk in favor of the agriculturally used area. The clearing of the forests and the onset of agricultural use lead directly to an increase in soil erosion. The rate of soil erosion of agricultural land is often ten times higher than that of slow erosion under natural forests.

The soil eroded by erosion, together with the associated nutrients (mainly carbon, nitrogen and phosphorus), is stored for a long time at the foot of slopes, in large, extensive floodplains and in lakes. "This storage of carbon in sediments and the formation of new soil at the eroded sites act as a sink for carbon," says Thomas Hoffmann, "that is, in eroding landscapes, more carbon is bound from the atmosphere than in forested landscapes".

On the basis of a comprehensive database on soil carbon and the modeling of the soil carbon balance, the research team succeeded for the first time in quantifying the global temporal course of carbon release and storage and in analyzing the influence of humans. Since the beginning of arable farming, the natural balance between new land formation and degradation has been disturbed, which has led to the cumulative storage of carbon on the land surface. The research group's balance sheet shows the strong overestimation of CO2-Emissions through land use change, because around a third of the carbon is only shifted through the erosion-related processes, not as CO2 released.

The study makes an important contribution to understanding the long-term changes in the global carbon balance in the atmosphere: While short-term fluctuations in land biomass have a direct impact on CO2-Carbon storage occurs via erosion-related sediments on time scales of hundreds to thousands of years. The soil erosion caused by humans in the Holocene led to a build-up of terrestrial carbon stores, the retention time of the carbon in these sediment stores and the release as CO2 depends on how the respective areas are used. How humans control the water balance also plays a role.

In this sense, there is a bracket to the tasks of the BfG to consider the sediment and material balance of waters. This includes, among other things, research activities of the BfG on topics such as "Climate change and land-use-induced changes in sediment inputs in federal waterways".

The link to the article in "nature climate change" is: http://www.nature.com/nclimate/journal/v7/n5/full/nclimate3263.html

Bank demolition with alluvial clay deposits (Kananaskis River, Canadian Rocky Mountains)