Ecological stoichiometry, defined by Elser and Urabe (1999) as the analysis of constraints and consequences in ecological interactions of the mass balance of multiple chemical elements (particularly the ratios of carbon to nitrogen and of carbon to phosphorus), is an approach that can shed light on the relations between resources and consumers. Many studies have focused on plant-herbivore relations (Hessen, 1997) but the approach is also important when considering decomposers, detritivores and their resources.
There is a great contrast between the chemical composition of dead plant tissue and that of the tissues of the heterotrophic organisms that consume and decompose it. While the major components of plant tissues, particularly cell walls, are structural polysaccharides, these are only of minor significance in the bodies of microorganisms and detritivores. However, being harder to digest than storage carbohydrates and protein, the structural chemicals still form a significant component of detritivore feces. Detritivore feces and plant tissue have much in common chemically, but the protein and lipid contents of detritivores and decomposers are significantly higher than those of plants and feces.
The rate at which dead organic matter decomposes is strongly dependent on its biochemical composition. This is because microbial tissue has very high nitrogen and phosphorus contents, indicative of high requirements for these nutrients. Roughly speaking, the stoichiometric ratios of carbon : nitrogen (C : N) and carbon : phosphorus (C : P) in decomposers are 10 : 1 and 100 : 1, respectively (e.g. Goldman et al., 1987). In other words, a microbial population of 111 g can only develop if there is 10 g of nitrogen and 1 g of phosphorus available. Terrestrial plant material has much higher ratios, ranging from 19 to 315 : 1 for C : N and from 700 to 7000 : 1 for C : P (Enriquez et al., 1993). Consequently, this material can support only a limited biomass of decomposer organisms and the whole pace of the decomposition process will itself be limited by nutrient availability. Marine and freshwater plants and algae tend to have ratios more similar to the decomposers (Duarte, 1992), and their rates of decomposition are correspondingly faster (Figure 11.11a). Figure 11.11b and c illustrate the strong relationships between initial nitrogen and phosphorus concentration in plant tissue and its decomposition rate for a wide range of plant detritus from terrestrial, freshwater and marine species.
The rate at which dead organic matter decomposes is also influenced by inorganic nutrients, especially nitrogen (as ammonium or nitrate), that are available from the environment. Thus, greater microbial biomass can be supported, and decomposition proceeds faster, if nitrogen is absorbed from outside. For example, grass litter decomposes faster in streams running through tussock grassland in New Zealand that has been improved for pasture (where the water is, in consequence, richer in nitrate) than in 'unimproved' settings (Young et al., 1994).
One consequence of the capacity of decomposers to use inorganic nutrients is that after plant material is added to soil, the level of soil nitrogen tends to fall rapidly as it is incorporated into microbial biomass. The effect is particularly evident in agriculture, where the ploughing in of stubble can result in nitrogen deficiency of the subsequent crop. In other words, the decomposers compete with the plants for inorganic nitrogen. This raises a significant and somewhat paradoxical issue. We have noted that plants and decomposers are linked by an indirect mutualism mediated by nutrient recycling - plants provide energy and nutrients in organic form that are used by decomposers, and decomposers mineralize the organic material back to an inorganic form that can again be used by plants. However, stoichiometric constraints on carbon and nutrients also lead to competition between the plants and decomposers (usually for nitrogen in terrestrial communities, often
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Figure 11.11 (a) Box plots showing the recorded decomposition rates of detritus from different sources. The decomposition rate is expressed as k (in log units per day), derived from the equation Wt = W0 e-kt, which describes the loss in plant dry weight (W) with time (t) since the initiation of measurements. Boxes encompass the 25 and 75% quartiles of all data from the literature for each plant type. The central line represents the median and bars extend to the 95% confidence limits. The relationships between decomposition rate and the initial concentrations in the tissues (% dry weight) of (b) nitrogen and (c) phosphorus are also shown. Solid lines represent fitted regression lines and open and closed circles represent detritus decomposing on land and submersed, respectively. (After Enriquez et al., 1993.)
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