Global warming will get worse as agricultural methods accelerate the rate of soil erosion, which depletes the amount of carbon the soil is able to store, a United Nations' Environment Programme report said on Monday.
Soil contains huge quantities of carbon in the form of organic matter. which provides nutrients for plant growth and improves soil fertility and water movement. The top meter of soil alone stores around 2,200 billion tonnes of carbon, which is three times the level currently held in the atmosphere, said the UNEP Year Book 2012. (...)

The article:

http://www.reuters.com/article/2012/02/13/us-unep-soil-idUSTRE81C13J20120213

"The benefit of soil carbon":
http://www.unep.org/yearbook/2012/pdfs/UYB_2012_CH_2.pdf

From UNEP website:
http://www.unep.org/newscentre/Default.aspx?DocumentID=2667&ArticleID=9027&l=en

UNEP Year Book 2012:
http://www.unep.org/yearbook/2012/