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Microalgae Freshwater plants Amphibious plants Macroalgae Sea grasses Grasses Sedges Mangroves Broad deciduous tree leaves Shrubs Conifers

Broad perennial tree leaves i ■ i m

0.0001

0.0001 0.001 0.01 0.1 Decomposition rates (day-1)

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001 0.01 0 Phosphorus(

001 0.01 0 Phosphorus(

for phosphorus in freshwater communities, and either nitrogen or phosphorus in marine communities).

Daufresne and Loreau (2001) developed a model that incorporates both mutualistic and competitive relationships and posed the question 'what conditions must be met for plants and decomposers to coexist and for the ecosystem as a whole to persist?' Their model showed that the plant-decomposer system is generally persistent (both plant and decomposer compartments reach a stable positive steady state) only if decomposer growth is limited by the availability of carbon in the detritus - and this condition can only be achieved if the competitive ability of the decomposers for a limiting nutrient (e.g. nitrogen) was great enough, compared to that of plants, to maintain themselves in a state of carbon limitation. When decomposers were not competitive enough, they became nutrient-limited and the system eventually collapsed. Daufresne and Loreau (2001) note that the few experimental studies so far performed show bacteria can, in fact, outcompete plants for inorganic nutrients.

In contrast to terrestrial plants, the bodies of animals have nutrient ratios that are of the same order as those of microbial biomass; thus their decomposition is not limited by the availability of nutrients, and animal bodies tend to decompose much faster than plant material.

When dead organisms or their parts decompose in or on soil, they begin to acquire the C : N ratio of the decomposers. On the whole, if material with a nitrogen content of less than 1.2-1.3% is added to soil, any available ammonium ions are absorbed. If the material has a nitrogen content greater than 1.8%, ammonium ions tend to be released. One consequence is that the C : N ratios of soils tend to be rather constant around values of 10; the decomposer system is in general remarkably homeostatic. However, in extreme situations, where the soil is very acid or waterlogged, the ratio may rise to 17 (an indication that decomposition is slow).

It should not be thought that the only activity of the micro-bial decomposers of dead material is to respire away the carbon and mineralize the remainder. A major consequence of microbial growth is the accumulation of microbial by-products, particularly fungal cellulose and microbial polysaccharides, which may themselves be slow to decompose and contribute to maintaining soil structure.

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