Without decomposition the dead material in a forest would physically swamp the field and ground-layer vegetation, prevent new seedlings establishing and, most importantly, lock up nutrients to such a degree that woodland processes would grind to a halt. Decomposition is the breakdown by physical (abiotic) and biological means of organic material in and above the soil. The organic substances involved include plant material ranging from large woody trunks to leaves and shed parts, dead animals and their excrement. The process of converting the organic matter into the final humus is called humification. Along the way, as complex organic components are broken down, the nutrients are mineralized (transformed from organically bound nutrients to simple gases such as ammonia and carbon dioxide or soluble (ionic) forms) and released into the soil water where they can be absorbed by decomposers and green plants. As described in Chapter 1, in any ecosystem nutrients are recycled usually with very little input from outside, so any major bottle-neck in the cycle will affect all future activity, including plant growth.
The major gaseous product of decomposition is CO2 so decomposition is crucial in influencing the amount of carbon locked up in forests, a matter of increasing importance when looking at the implications of climate change scenarios and the large amount of carbon currently stored in the soil (see Chapter 11). This chapter will examine how litter and other dead material is decomposed and by what, what controls the processes, what is left at the end, and take a detailed look at the value and decomposition of woody material.
It is tempting when standing in a forest to see the vegetation as the main 'keystone' component, the most crucial aspect of the forest, and therefore the part that controls the running of the forest. However, ecologists are increasingly recognizing the importance of multiple links between the above- and below-ground communities (see Wardle, 2002), which can be crudely equated with the twin processes of plant growth (plus associated herbivory) and decomposition. Such linkages are demonstrated in the two extreme categories of humus normally recognized (Ponge, 2003) - mull and mor (see Section 2.2.2). Mull humus is associated with the most fertile soils (typical of grasslands and temperate deciduous woodland) characterized by a large number of burrowing animals, which intimately mix the decomposing organic matter into the mineral soil. This tends to lead to rapid decomposition and turnover of nutrients available to plants. Plants grow well and produce easily decomposed (labile) litter. So here we have a positive feedback loop where the nutrient-rich and easily decomposed litter leads to the development of mull by encouraging an abundant soil fauna and the mull in turn helps soil fertility and the growth of nutrient-rich plants. At the other extreme mor humus consists of deposits of largely unde-composed organic matter built up on top of the underlying mineral soil. This tends to develop under harsh cold climates or over poor acidic parent rock, where conditions discourage the larger soil fauna and most decomposition is carried out by fungi. This severely limits the speed of decomposition, leading to organic matter build-up and leaving the vegetation short of nutrients (maximum conservation of organic matter and minimum release of nutrients). In response to the harsh climate and lack of nutrients, the plants are usually evergreen and slow-growing and tend to put more investment into defences such as lignin and tannins (see Section 7.5.2 below), making the litter difficult to decompose. Moreover, before the leaves eventually fall, a greater proportion of useful nutrients are reabsorbed back into the plant. Both these factors combine to produce a very recalcitrant (decomposition-resistant) and often toxic litter which further decreases the diversity of soil fauna, further hindering the release of nutrients.
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