Biotic factor

In addition to the interactions between plants and microorganisms, a third factor, the soil, also plays a role in determining root exudation and the activity and diversity of rhizosphere microbial populations. In this section, physical and structural aspects of the soil are discussed in relation to their effects on root exudation and microbial populations. Consideration is also given to the role of agricultural management practices on rhizosphere processes. In addition, the role of other biotic factors, such as microfaunal predation, is discussed in relation to nutrient cycling in the rhizosphere. 

Actinomycetes are typically most abundant in well drained, circumneu-tral to alkaline soils having abundant organic matter. Water-logging and low pH may reduce populations (37, 38). The numbers of actinomy-cetous organisms isolated from the various soil samples in our study follow this pattern. No clear trend emerged as to a particular edaphic or biotic factor causing an increase in the proportion of inhibitory isolates in a soil sample. 

McGee, B.L., D.A. Wright, and D.J. Fisher. 1998. Biotic factors modifying acute toxicity of aqueous cadmium to estuarine amphipod Leptocheirus plumulosus. Arch. Environ. Contam. Toxicol. 34 34-40. 

On the other hand, biotic factors include natural interactions (e.g., predation and parasitism) and anthropogenic stress (e.g., the effect of human activity on other organisms). Because of the abiotic and biotic factors, the environment to which an organism is subjected can affect the life functions, growth, and reproductive success of the organism and can determine the local and geographic distribution patterns of the organism. 

CLIMATIC FACTORS EDAPHIC FACTORS BIOTIC FACTORS... 

One aspect of sedimentary 7 8 chlorite formation which is particularly interesting is the fact that these minerals are never found forming at depths greater than 80 meters in recent sediments. Porrenga (1967b) thinks that they are characteristic of tropical sediments and their formation is thus temperature dependent. This appears invalid since they are known to form in recent sediments in a Scottish loch (Rohrlich, e al.. 1969). Nevertheless there does seem to be a bathymetric control on their occurrence. This is probably not a pressure effect but more likely some sort of factor related to organic activity in the sediments which is controlled by the biotic factors of sea depth, temperature, nutrients, etc. 

In addition to proper biotic factors, it appears that the oxidation state of the iron in the sediment is critical to the formation of sedimentary 7 8 chlorites these conditions are more reducing than those which form glauconites (Porrenga, 1967b Leclaire, 1968). As is the case for... 

Table 3.2 Interval of crops due to incompatibilities or biotic factors (Muller 1988)...

In allelopathy studies a central goal is to isolate, identify, and characterize allelochemicals from the soil. However, since it is essentially impossible to simulate exact field conditions, experiments must be designed with conditions resembling those found in natural systems. Indcrjit (1996) argued that allelopathic potential of phenolics can be appreciated only when we have a good understanding of i) species responses to phenolic allelochemicals, ii) methods for extraction and isolation of active phenolic allelochemicals, and iii) how abiotic and biotic factors affect phenolic toxicity. 

Woodhead, S. Environmental and biotic factors affecting the phenolic content of different cultivars of Sorghum bicolor. J Chem Ecol 1981 7 1035-1047. 

The observed pattern of inhibition should not be explained solely by physical factors or other biotic factors, especially competition. 

In Chap. 3 (Sect. 3.6), we discussed limitations of the FREZCHEM model that were broadly grouped under Pitzer-equation parameterization and mathematical modeling. There exists another limitation related to equilibrium principles. The foundations of the FREZCHEM model rest on chemical thermodynamic equilibrium principles (Chap. 2). Thermodynamic equilibrium refers to a state of absolute rest from which a system has no tendency to depart. These stable states are what the FREZCHEM model predicts. But in the real world, unstable (also known as disequilibrium or metastable) states may persist indefinitely. Life depends on disequilibrium processes (Gaidos et al. 1999 Schulze-Makuch and Irwin 2004). As we point out in Chap. 6, if the Universe were ever to reach a state of chemical thermodynamic equilibrium, entropic death would terminate life. These nonequilibrium states are related to reaction kinetics that may be fast or slow or driven by either or both abiotic and biotic factors. Below are four examples of nonequilibrium thermodynamics and how we can cope, in some cases, with these unstable chemistries using existing equilibrium models. 

In summary, detectable population changes of soil and rhizosphere phenolic acid utilizing microorganisms in laboratory systems to phenolic acid enrichment are a function of a variety of soil physicochemical and biotic factors, including phenolic acid enrichment concentrations, presence of other available organic molecules, nutrition, soil type, and initial microbial populations. We have no similar data for field soils systems, something that needs to be determined. 

The growth rate and the onset of maturation are likely to be influenced by a number of abiotic and biotic factors related to the physiology of the host intestine, many of which have been discussed earlier (Chapter 3) or are discussed later (Chapters 10 and 11). 

Lone, E. (1980a). The possible role of the soil fauna in the epizootiology of cysticercosis in cattle. I. Earthworms - the biotic factor in a transmission of Taenia saginata eggs. Angewandte Parasitologie, 21 133-39. 

The structure and biomass of the biocoenoses change over a wide range, which are defined by the combination of biotic and abiotic factors. Among the latter, most important are the salinity, the gas regime, and the properties of the sediment. The biotic factors are the zoobenthos grazing by fish and the competition for the dwelling environment between the benthos representatives. 

Orcutt, D.M., and Nilsen, E.T. 2000. The Physiology of Plants Under Stress. Soil and Biotic Factors. Wiley, New York. 

---