Element cycles

Table 3-5 Elemental cycles and selected important transformations in which organisms play roles...

A TBma,) explicit functions of the available concentrations of the other nutrients. This approach allows for a pronounced interdependence between the fluxes of the different nutrients but it does not ensure that the Redfield ratios are maintained. In the second approach the contents of the nutrients in the biota reservoir are forced to remain close to the Redfield ratios. This method was used by Mackenzie et al. (1993) in their study of the global cycles of C, N, P, and S and their interactions. They were able to demonstrate how a human perturbation in one of these element cycles could influence the cycles of the other elements. 

The following sections summarize only the most prominent interactions between the elemental cycles and the links in the hydrologic cycle. Water also plays a role in many chemical and biological reactions that are beyond the scope of this discussion. The carbon, nitrogen, sulfur, and phosphorus cycles are discussed in detail in Chapters 11, 12, 13, and 14, respectively. 

This brief description leads to Fig. 7-13 which depicts the physical transformations of trace substances that occur in the atmosphere. These physical transformations can be compared to the respective chemical transformations within the context of the individual elemental cycles (e.g., sulfur). This comparison suggests that the overall lifetime of some species in the atmosphere can be governed by the chemical reaction rates, while others are governed by these physical processes. 

Michalopoulos, P. and Aller, R. C. (1995). Rapid clay mineral formation in Amazon Delta sediments reverse weathering and oceanic elemental cycles. Science 270, 614-617. 

Molecular hydrogen plays a major role on the oxidation-reduction processes involved in bacterial energetics, as well as in the degradation and conversion of biomass related with all major elemental cycles. Hydrogenase has a key role on this process and catalyzes the reversible oxidation of dihydrogen, important in bacterial anaerobic metabolism ... 

Blank RR, Young JA (2002) Influence of the exotic invasive crucifer, Lepidium latifolium, on soil properties and elemental cycling. Soil Sci 167 821-829 Bolan NS (1991) A critical review on the role of mycorrhizal fungi in the uptake of phosphorus by plants. Plant Soil 134 189-207. doi http //dx.doi.org/10.1007/BF00012037 Bullock JM, Pywell RF, Burke MJW, Walker KJ (2001) Restoration of biodiversity enhances agricultural production. Ecol Lett 4 185-189... 

In soils of non-agricultural ecosystems, above ground biomass (foliar uptake) and metal cycling is considered important (see Figure 8), due to large impact on the metal distribution in the humus layer and mineral soil profile. Especially in soils of Forest ecosystems, it may affect the accumulation in the humus layer, which is considered a very relevant compartment regarding the calculation of a critical load. In these soils, however, a steady-state element cycle is assumed, which implies that mineralization, Minj, equals litterfall, Mjf. 

Shafer, M. M., and D. E. Armstrong (1990), "Trace Element Cycling in Southern Lake Michigan Role of Water Column Particle Components", Abstract ACS Meeting, 273-277. 

The most noteworthy multistage element cycles in which bacteria play important roles are the nitrogen and sulfur redox cycles. The fixation of nitrogen is a reductive process that provides organisms with nitrogen in a form usable for the synthesis of amino acids, nucleic acids, and other cell constituents. In essence, the overall conversion to the key intermediate, ammonia, can be represented as ... 

It is likely that comparative structural and other studies of hydrogenases and nitrogenases will eventually illuminate events in the early evolution of energy-yielding mechanisms. We are indebted to the anaerobes for their necessary roles as recycling agents in Earth s element cycles. 

In addition to the hydrothermal controls, the elemental cycles of Ca and Mg are also linked to those of CO2 and Si02 because the mobilization of Ca and Mg during terrestrial weathering from silicates, such as plagioclase, involves consumption of CO2. In the case of calcium, the net chemical reaction is... 

The sources of particles in the atmosphere are of much current interest. This is true from the viewpoint of both the geo-atmospheric scientist and the pollution control specialist. For the atmospheric chemist or geochemist, the identities and the relative contributions of sources are important for understanding 1) atmospheric element cycles and budgets, and 2) the chemical content of precipitation. 

Agricultural fertilization with sulphur is not a new concept - at the research level at least, sulphur has long been recognised as an essential plant nutrient. However because the complex role of sulphur - in soils, in plant material and in interaction with other essential element cycles - has never been fully understood, sulphur fertilizers have been used mainly on an empirical basis. As a result, sulphur fertilization has shown somewhat erratic performance Measured sulphur deficiency in soils has not always been correlated with poor crop yield and, as a corollary, sulphur fertilization of sulphur deficient soils has not always improved poor crop yields. Thus it has been difficult to routinely demonstrate an economic benefit to the farmer. 

Naturally occurring stable isotopes of C, N, and S have been used extensively for over a decade as direct tracers of element cycling in marine and terrestrial food webs (34-39). Carbon and sulfur isotopes fractionate very little between food and consumer thus their measurement indicates which primary producers or detrital pools are sources of C and S for consumers. For example, a study of plants and animals in Texas sand dunes showed that insect species had 813C values either like those of C3 plants or like those of C4 plants (-27 and -13%o, respectively). Rodent species had intermediate values near -20%o that indicated mixed diets of both C3 and C4 plants (40). The 13C measurements, used in simple linear mixing models, proved to be quick and reliable indicators of which plant sources provided the carbon assimilated by higher trophic levels. 

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