The majority of studies that have looked at the rates of dead material production have concentrated on litter - small-diameter twigs, leaves, flowers, fruits and bud scales. But dead material also includes large-diameter dead wood (standing and fallen), deceased roots or material sloughed off roots below-ground (which, as described below, can be a considerable amount), dead animals and the excrement of their live relatives. Less is known about these because they can be harder to quantify since large dead bodies (tree or animal) and hidden roots are difficult to quantify and, in the case of animals, generally they make up a small proportion of the annual input of dead material to the decomposer cycle. Nevertheless, such components as animal remains can be locally important; a dead deer or an excess of insect bodies will provide a large and concentrated input of nitrogen and other nutrients to the soil (see Box 7.2). Using a wider definition of what constitutes dead material than just 'litter' leads to a problem of terminology. Detritus is often used but strictly this could also mean rock fragments. The most all-encompassing term is necromass, the total mass (weight) or volume of dead organisms or their parts in an area or volume at a given time. As this decomposes it merges into the humus remains found on and in the soil, at which point it is easier to talk about the soil organic matter, all the dead organic debris no matter what its stage of decomposition comprising a cellular fraction and amorphous humus (Swift et al, 1979).
There is a major difference between the total amount of a nutrient stored in a soil and its availability. For example, in soils with abundant mor humus, such as in boreal forests, the total amount of nitrogen stored is high but because turnover is low, availability is low. In the tropics, the total nitrogen pool in the soil is low but availability can be high due to rapid turnover.
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