Physical and biotic factors

As in the bracken study, Muller and coworkers carefully eliminated many physical and biotic factors. Differences in soil physical factors and nutrient content were ruled out by soil analyses. Salinity stress was shown not to be a factor, and soil types were the same in all zones (Muller 1966). Soil moisture, however, was found to be slightly lower in the bare zone (W.H. Muller 1965), and field observations indicated that drought intensified the effects of phytotoxins (Muller 1966). The role of herbivorous animals remains a contentious issue to this day. Data are conflicting (Muller 1966 Bartholomew 1970 Muller del Moral 1971). For A. californica, Halligan (1973, 1975) proposed that, similar to what was observed with bracken, allelopathy was important in excluding some species from the bare zone, while animal activity excluded others. 

As a check to eliminate resource competition as a cause, manure was applied to various areas. No stimulation of growth occurred in the bare zones, only slight stimulation occurred in the inhibited zones, while in neighboring grassland, significant growth increase occurred. These results are opposite of what would be expected if nutrients were a limiting factor (Muller 1966). Additionally, the lateral spread of shrub roots reached only to the closest herbs of the zone of inhibition. 

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Lee, S.Y. (1999) Tropical mangrove ecology Physical and biotic factors influencing ecosystem structure and function. Australian Journal of Ecology, 24, 355-366. 

Insects, pathogens, and weeds respond to their physical and biotic environment in predictable ways. For instance, growth of many fungal pathogens varies with temperature in a well-established manner. Growth starts low, increases to a maximum at the optimal temperature and then declines to zero (Fig. 2). In fact, most plant pests will respond to temperature in a similar manner. Fungal pathogens often require free moisture for infection to occur infection increases with increased time of wetness (Fig. 2). The duration of free moisture is dependent on temperature (Fig. 2), as well as other physical factors. 

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. 

The quality of potato tubers depends upon genetic, climatic, biotic, chemical, and edaphic factors varietal characteristics precipitation, temperature and sunshine conditions competition with other plants the use of chemicals and the physical, chemical, and biological properties of the soil, which influence the capacity of the crop to take up the necessary water and nutrients to ensure success. 

Numerous factors, physical, chemical and biotic, can affect the extent of N2 fixation in an ecosystem (Capone, 1988 Howarth et at., 1988a,b Karl et al, 2002). Many factors which bear on nitrogenase activity are interdependent such as light, temperature, oxygen and turbulence. Indeed, a variety of different factors may limit the growth and activity of diazotrophs in various areas of the world s oceans at different times of the year. 

Numerous biotic factors influence N processing within sediments, including the composition of the microbial, macrofaunal, and macrophyte communities, behaviors of the various infaunal communities, and trophic transfer of carbon and nitrogen between communities. Interactions between benthic communities may be physical, chemical, or biological in nature and, in turn, are influenced by the physical nature of the environment, sediment composition, and autotrophic dominance (Fig. 19.3). 

The nature of life is determined by the surroundings in which the life forms must exist. Much of the environment in which organisms live is described by physical factors, including whether or not the surroundings are primarily aquatic or terrestrial. For a terrestrial environment, important physical factors are the nature of accessible soil and availability of water and nutrients. These are abiotic factors. There are also important biotic factors relating to the life forms present, their wastes and decomposition products, their availability as food sources, and their tendencies to be predatory or parasitic. 

The long-term properties and recycling in nature of polymers depend on the synergetic interaction of physical and chemical abiotic factors and also on the biotic effects of the surroundings. 

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 ... 

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