Decomposition completes the biogeochemical cycles that photosynthesis initiates. Thus, complete decomposition results in the conversion of the organic (reduced) products of photosynthesis back into the inorganic (generally oxidized) constituents used as the reactants for photosynthesis (see Exercise 14). The major biogeochemical cycles affected by decomposition are those of C, N, P, S, and O, although it is important to realize that all of the minor constituents of biomass (cations, trace metals, etc.) also are released (mineralized) by decomposition. When plants or animals senesce and die, both dissolved (DOM) and particulate (POM) organic matter are available for degradation. Leakage of DOM from dying cells and autolysis of the tissue increase during senescence and reach maximum levels soon after death. The DOM thus produced is leached readily from the tissue into the aquatic environment and can constitute as much as 30% of the total amount of combined particulate and dissolved organic matter in lake water. This detrital DOM is available as an energy source for microflora in the sediments and waters adjacent to particulate detritus. The rate of degradation is dependent on both the enzymatic capabilities of the microflora and the environmental conditions (see Exercise 20). Some compounds of the DOM are more stable than others, but warmer temperatures and increased availability of oxygen reduce their resistence to oxidation (recalcitrance) to some extent (Godshalk and Wetzel, 1978a).
Particulate detritus is colonized by various microflora. The rate of degradation depends on: (1) the chemical composition of specific substrates within the particles, (2) the ability of microbes to get at the tissue (e.g., particle size: surface area), and (3) the rate of microbial metabolism as governed by enzymatic capacities, temperature, and availability of electron acceptors (e.g., oxygen) and mineral nutrients (Alexander, 1965; Godshalk and Wetzel, 1978b; Webster and Benfield, 1986). A succession of types of microflora associated with the detrital particles occurs over time as a result of changes in substrate availability and environmental conditions caused by their metabolism. Total microbial metabolism and biomass often increase initially after colonization of the detritus as a result of increased concentrations of organic nitrogen relative to that of carbon. As the resistance of the residual detritus increases with continued decomposition, the degradabil-ity of the detritus decreases. Organic nitrogen concentration then decreases relative to that of carbon, resulting in high organic C: N ratios.
Measurement of the rates of decomposition of organic matter in situ is difficult. Commonly, changes in the dry weight of a known amount of POM are measured over a period of time. Such parameters as percent weight loss continually change through time and only yield information about the end result of decomposition. Decomposition is a continuous process, but the rate of decomposition varies through time depending on a number of substrate and environmental variables.
For example, k will be low and conditions will not be conducive to rapid decomposition if relative values of T, O, and/or N u are low. Since these factors interact with one another, high values of one variable will offset low values of another only to a limited extent. Mechanical fragmentation by water turbulence or by animals can cause lower values of S p , which, in turn, theoretically will increase decay rates in spite of constant chemical recalcitrance or constant environmental conditions.
The relative rate of decomposition k is the amount of detrital carbon metabolized per unit time (e.g., POM→DOM, POM→microbial cells, DOM→CO2, and so on). Commonly observed rates of decay through time may be separated into three phases (Fig. 21.1). The first phase (A) is a period of increasing weight loss from leaching, autolytic release, or both, of DOM. Much of this DOM is of simple composition and is decomposed readily. The quantity and composition of DOM persisting over time (days) are influenced greatly by temperature and oxygen or alternate electron acceptors. The rapid release and decay of DOM in phase A may follow a logistic S curve (line a′, Fig. 21.1) as, for example, in the release and decay of DOM from a phytoplankton cell. As larger organic particles begin to decompose under natural conditions, decay rates may be slow at first, followed by an increasing rate as more cells senesce and lyse, and then finally slow again as all cells complete senescence (line a).
Following the maximum rate of weight loss in phase A, the decay rate decreases during the phase of decomposition (B) of POM, when the interactions of the factors controlling degradation have their greatest influence. Utilization of the most readily available substrates usually occurs first (phase A), so that the relative chemical recalcitrance of the POM increases over time. Simultaneously, concentrations of dissolved oxygen and mineral nutrients decrease especially under nonturbulent conditions as is commonly the case.
As conditions become more productive, the increased loading of detrital organic matter causes deterioration of conditions conducive to rapid and complete mineralization. As a result, more organic matter persists in the ecosystem in reduced form [cf., Rich and Wetzel (1978) and Wetzel (1979, 1995)]. In the last phase (C of Fig. 21.1), the rate of decay of this increasingly recalcitrant POM approaches closely an asymptotic limit of zero. Decomposition in phase C can be altered or accelerated by changes in physical conditions or by replenishment of mineral nutrients or electron acceptors, as may occur during turbulent circulation. Much of the detritus of this phase is highly recalcitrant and subject to such slow rates of decay that it may be incorporated permanently into the sediments (see Exercise 27).
The following exercises examine the rates of decomposition and mineralization of phytoplankton, macrophytes, and leaf fall. Since rates of decomposition are slow (-1 to 3%/day), it is necessary to use long-term experiments, or to use radioctively labeled organic matter to measure the rates of production of metabolic end products in short-term experiments.
KeywordsParticulate Organic Matter Organic Mass Ergosterol Content Leaf Pack Aquatic Hyphomycetes
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