The influence of C02 emissions from burning of fossil fuels on the global climate is intensified by the release of C02 from changes in land use (see Fig. 5.2.6). Changes in land use amount to about 20% of global C02 emissions - the trend is increasing. Man influences the global C balance by:
• changing forests into agricultural and grazing systems. The biomass of the forest is used as firewood or it is destroyed in slash-and-burn clearing;
• mechanical working of soils of natural grasslands and of converted forest ecosystems. Mechanical working of prairie soils leads over a period of about 50 years to a loss of about
50% of the original natural C contents (Fig. 5.4.1; Matson et al. 1997) due to decomposition of easily mineralisable C compounds. It is unclear why decomposition of soil carbon stops at 50%, despite further working. It has been shown that the black soils of steppes and prairies were formed, to a large extent, from charcoal and soot particles as a result of vegetation burning about 5000 years ago (Schmidt et al. 1999). This black carbon (BC) is resistant to decomposition, even with mechanical working. Less intensive ploughing, the so-called low tillage cultivation, leads to renewed accumulation of soil C. This is caused by accumulation of an easily decomposable humus fraction which, however, can be easily mineralised again after a change of management (deep ploughing following low tillage).
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