Why does organic matter increase in the A HORIZON in soil?

Question

im doing a report, and iv found that organic matter is limited in the O horizon then like 50 percent in the A horizon and still relatively high in the B horizon.. why is this, i thought it would be high in the o horizon/

p.s this has been studied on a soil slope (catena)

Answer 1

Only on an areal basis (like kg/ha) could you have more organic matter in the A horizon than in the O.  If you have more on a percent dry weight basis, you need to recheck your methodology! By definition, the O has to have more organic matter (% dry weight basis) t han the A.  On an areal basis, there are many situations that can produce this situation.  For example our good friends the earthworm species can produce that effect in hardwood forest environments.  In grasslands it is common to have a very thin O, if any, and a deep A horizon because the fibrous root system of the grasses supports a huge microbial biomas, the senesced roots, etc, but on top of the ground the senesced biomass is not resistant to decomposition. When I mow my lawn, the grass clippings don't build up into a thick O horizon because that green grass is so attractive to bacterial decomposition.  Hope this helps!