Litter layers forest floor

The decomposition of tree leaves is not entirely confined to the litter layer on the forest floor. Leaves and needles are invaded by bacteria and fungi even as they grow these microorganisms may be either pathogens or saprophytes. ... 

Fauna also influence soil carbon cycling. Bioturbation mixes and aerates soil, physically breaks down litter, creates flow paths for water in soil, and can reduce surface litter stocks and enhance erosion (Bohlen et al., 2004). For example, along a gradient of European earthworm (Lumbricus terrestris) colonization in a deciduous forest of northern Michigan, earthworms are associated with a decrease in litter-layer thickness, apparently mixing some forest floor organic matter into the mineral soil. Thus, fauna can create spatial patterns in SOM stocks. 

Information on the land biota is also tentative. Because most land plants accumulate woody parts for an extended period of their life, the overall amount of mercury in the total land biomass is proportionally greater than that in water biota. The huge amount of litter produced in forests (10 tons/year) (25) carries a large amount of the immobilized mercury to the forest floor. From there, it is incorporated into humus and finally into the soil. For this reason the uppermost soil (Ao and Ai horizons) is enriched in mercury by a factor of 2 to 4 with respect to that in underlying layers. To a smaller extent the same situation applies to agricultural land and pastures. The removal of mercury from soil by cropping represents barely 5% in 1000 years, which means that agricultural soil is really not in equilibrium with respect to natural soils. 

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