Decomposition biotic

FIGURE 5.34 Rates of mineralization of lignocellulose in salt and fresh (Okefenokee) water system (Redrawn from Benner et al., 1985). 

The substrate-indnced respiration (SIR) method was developed as a rapid means of estimating microbial activities in soils (Beare et al., 1990 Neely et al., 1991). The use of selective inhibitors such as streptomycin for bacteria and cycloheximide for fungi, in conjunction with substrate additions, has been used to quantify bacterial and fungal contributions to the total metabolism of microbial decomposers. The SIR procedure involves addition of a labile carbon source (e.g., glucose or acetate) to provide a carbon nonlimiting condition. The short-term increase in carbon dioxide production is proportional to the active microbial biomass and activity. The concept of addition of labile carbon to determine the kinetics of substrate utilization by microorganisms has been extensively studied in various ecosystems including wetlands and aquatic systems. 

The k values presented in Tables 5.13a and 5.13b represent examples derived from various studies. The simple exponential decay does not consider the labile pools of carbon, which are either 

Decomposition of Plant Matter as Determined by Mass Loss Using Litterbag Techniques 

---

In addition to being the enzymatic agents of biotic decomposition of SOM, microbial cell by-products are increasingly recognized as major building blocks of SOM. As a result, the microbial community, controlled by climate, vegetation, and soil environment, is a key mediator of organic matter composition and decomposition. The spatial scales of influence, however, have not been well characterized. 

Detenbeck (37) and Detenbeck and Brezonik (38, 39) examined the effect of pH on phosphorus sorption for LRL sediments. Their results suggested that the flux of inorganic P from sediments could be diminished by as much as 90% if the pH of sediments decreased from 6.0 to 4.5. However, there was no observed treatment effect for TP and an apparent increase in SRP summer averages at pH 4.7 (Figure 4). Therefore, chemical sorption-desorption processes probably do not control phosphorus levels in LRL. The direction of response at lower pH implies that the balance between biotic uptake, deposition to sediments, and release from organic detritus by decomposition most likely controls SRP levels in the water column. 

The term biochemical stabilization refers to the biotic or abiotic production of organic substances that are refractory to decomposition by microorganisms and contribute, through condensation and complex formation, to the stabilization of otherwise easily decomposable substrates such as enzymes. This stabilization process coincides with the process of humification. 

A cadaver exposed to the environment is subject to degradation by various types of animals, of which insects are often the most predominant. Insects can affect the breakdown of the corpse by augmenting the internal decomposition process (Campobasso, Di Vella, and Introna 2001). The succession and development of some insects that visit a corpse can be used to estimate PMI. Succession data are useful in providing a minimum and maximum estimate of time since death. However, biotic and abiotic factors are known to influence carrion insect growth and activity and need to be considered when estimating PMI (Wells and Lamotte 2001). 

In this section, we will consider the effects of soil texture and soil nutrient status on decomposition. The initial stages of leaf litter decomposition will be at least partially decoupled from control by edaphic properties of the soil environment. For example, Scott et al. (1996) found that while SOM decomposition varies significantly with soil texture, the CO2 evolution from surface litter does not. However, as partially decomposed litter is incorporated into the soil both through abiotic and biotic means, the physical characteristics of the soil begin to play an important role in the overall degradation and stabilization of the organic inputs. 

Coulson J. C. and Butterfield J. (1978) Investigation of biotic factors determining rates of plant decomposition on blanket bog. J. Ecol. 66(2), 631-650. 

Demethylation in the water column and sediments is receiving increasing attention. Both abiotic (e.g.. Sellers et al., 1996, 2001) and biotic (e.g., Pak and Bartha, 1998 Marvin-Dipasquale and Oremland, 1998 Marvin-Dipasquale et al., 2000 Hintehnann et al., 2000) processes are imphcated. The result is that MMHg accumulation in aquatic systems represents a balance between methylation, bioaccumulation, and the demethylation processes. In sediments, MMHg decomposition is particularly important, and it is possible that some sediments represent net sinks, rather than net sources, for MMHg in the water column. 

The mobility of trace elements during weathering is determined, first by the stability of the host minerals, and second by their electrochemical properties. The association of trace elements with minerals in soils reflects often their origin, and this is an important factor in their distribution and behavior. Several chemical and physical processes are involved in weathering of both biotic (living organisms and their decomposition) and abiotic origins. Basic processes can be characterized as follows ... 

In the absence of oxygen, humic substances are resistant to decomposition and represent a significant carbon and nutrient storage in wetlands. Under drained conditions, humic substances are rapidly degraded, which releases nutrients to the bioavailable pool, thereby affecting downstream water quality. Humus is generally defined as the organic material that has been transformed by abiotic and biotic processes into stable form (see Section 5.4.3) and consists of two major types of compounds (Stevenson, 1986) ... 

---