Here is one of the best SOM articles I have floating around in my fertility folder: http://www.fao.org/3/a-a0100e.pdf

In it is this statement found on page 14....

....."Root turnover also constitutes an

important addition of humus into the soil,

and consequently it is important for carbon

sequestration. In forests, most organic matter is added as superficial litter. However, in

grassland ecosystems, up to two-thirds of organic matter is added through the decay

of roots (Quideau, 2002)."