Living organisms require various kinds of chemical elements such as C, H, O, N, S, P, Fe, etc., for their biosynthetic and metabolic processes. The absorption and utilization of such elements by organisms is compensated by their recycling and regeneration back into the environment.
The more or less cyclical paths of these elements from environment to organisms and into the environment are called biogeochemical cycles, with the phrase nutrient cycles being commonly restricted to those substances (nutrients) which are essential to life.
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The three major categories of cycles usually recognized are:
(a) the hydrologic cycle involving the movement of water;
(b) the biogeochemical cycles involving e.g., phosphorus, calcium and magnesium.
The ways that carbon, nitrogen, water, and other nutrients cycle among land, atmosphere, and oceans is a story of change on scales of millennia to seasons to seconds. Earth has seen some incredible alterations. Perhaps the greatest of which were the origin of life and the evolution of an oxygenated atmosphere.
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Now, because of the exponential growth of one species, Homo sapiens, the global system is changing more rapidly, and more directionally, than it has for millions of years. A clear analysis of the interactions among biological and chemical processes that determine the composition of the atmosphere, oceans, and biosphere, and places these processes in the context of global change is given in Schlesinger, 1991.
Life requires a continuous cycling of water, oxygen and nitrogen. These cycles include a gaseous phase and are characterized by possessing self- regulating feedback mechanisms. Cycles involving a gaseous phase are the most perfect and complete cycles in that the amount in any one phase (e.g., oxygen content of air) tends to remain fairly constant.
The operation of these cycles is elegantly balanced in nature and any reasonable increase in the rate of movement along one path is soon balanced by a suitable adjustment along other paths. However, industrial activities of mankind can seriously upset this balance, as has become evident for the nitrogen cycle which can be upset by the introduction of pesticides and other substances into the biosphere.
Id the sedimentary cycles, on the other hand, the element concerned is continually lost from biological systems through erosion, and ultimately deposited in the oceans. The rate of recycling of the element from ocean to land is rather slow and depends on such processes as biological transfer (e.g., through the excreta of marine birds), weathering of rocks, and additions from volcanic eruptions. Sedimentary cycles are much less perfect than gaseous in that some of the element may get stuck in a certain phase of the cycle.
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The three primary factors affecting terrestrial ecosystems are:
(a) the rate of release of nutrients from minerals by weathering,
(b) the addition of nutriepts by erosion, and
(c) the loss of nutrients by soil-erosion, etc. Unfortunately, however, the significance of these three factors has not been adequately appreciated by many ecologists so far; these cologists have mostly confined their studies to the rates of uptake, retention and release of a particular nutrient in a certain ecosystem.
Bormann and Likens (1967) emphasized that for a total and meaningful understanding of the dynamics of ecosystems, it is necessary also to take into account the effects of erosion and weathering, in addition to the uptake and cycling of nutrients.
They have successfully measured both weathering and erosion in small watersheds located in New Hampshire (US A). This was achieved by a careful study of the interaction that exists between the nutrient cycle and the hydrologic cycle. They found that it was possible to study the interrelations between the biota and the hydrologic cycle, various nutrient cycles and energy flow, in the small watershed studied.
Some of man’s recent activities such as nuclear explosions and nuclear reactor reactions tend to add radioactive isotopes into some material cycles. Thus, strontium cycles with calcium in the sedimentary cycle and flows through biological systems. Sr is known to be involved in this cycle.
The cycling of a number of different elements amongst the lily plant (Nymphaea odorata), its aphid Rhopalosiphum nymhpaea, pond water and sediment, and the rocks and soils of the basin has been experimentally followed by Cowgill (1973). All these different components form parts of an integrated pond ecosystem.
The aphid concentrates Na, Li, Cs, Ba, Zn, Al, Ga,Si, Ge, Ph, Ti, Hf, P, Bi, S, Se, Cr, Mo, I, Fe, Co, Ni, Mn, Y, La, Ce, Pr, and Sm to levels higher than those found in the lily leaves. On the other hand, the latter contain higher amounts of Ag, Ca, Mg, Cd, Hg, B, Sn, Zr, Th, CI, Br, Nd and Sc than those in the aphids.
Lily plants concentrate Be, Y, La, Ce, Pr, Sm, Gd, Dy, Er, Na, K, Ag, Mg, Cd, Hg, B, Sn, P, As, Bi, Nb, Se, F, CI and Mn over that of the bottom sediment. The sediment is richer in respect of Li, Cu, Ca, Sr, Ba, Zn, Al, Ga,Si, Ge, Pb, Ti, Zr, Hf, Th, V, S, Cr, Mo, Br, Fe, Co, Ni, Sc, Nd and Yb as compared to lilies.
The relative amounts of the following elements were essentially identical in both aphids and lily leaves: Be, Gd, Dy, Er, Yb, K, Rb, Cu, Sr, As, V and F. Likewise the amounts of the following were fairly similar in bottom sediments and lilies: Rb, Cs and I.
Lily leaves and flowers concentrated Co, Li, As, Bi, V, Ag, Mg, Hg, B, Ga, Ge, Th, P, Y and Sm. The stems concentrated Na, Silk, Mo, Be, CI; and leaves concentrated Ca, Sc, and S. The highest contents of La and K occurred in the stems whereas Nb was mostly accumulated in the flowers as well as stems.
The transformations of manganese in soils and waters have attracted the attention of many workers (see Proc. Sympos. on Environmental Biogeo- chemistry, Utah State Univ., Logan, 1973). In nature, Mn is reduced (dissolved) or oxidized (precipitated) by transport of cation particles from one place to another or through physico-chemical or biological changes at a particular place. Some of these processes seem to be facilitated by bacterial action.
Ferromanganese nodules occur on ocean bottoms and represent a major source of manganese oxides; the average Mn content of seawater is about 2 ppb whereas that of ocean sediments may be as high as 0.5 per cent. Certain microorganisms and bacteria have been found associated with the nodules. The stability of ferromanganese compounds depends mostly on redox potential and pH. Cation exchange reactions are thought to regulate the equilibria between water soluble and exchangeable manganese.
The metals in manganese nodules do not accumulate from a single source but from several sources including manganese plus the associated elements found on the deep ocean floor.
Manganese nodules vary enormously in shape, size, and form. They may be spherical, discoidal, oblate, saucer-, potato-, or grape-shaped. Ni, Cu and Mn tend to concentrate on the lower sides of the nodules whereas Fe, Co, and Pb tend to be more abundant on the upper sides. Most nodules grow very slowly, perhaps only a few mm per million years, but in certain circumstances they can grow either more rapidly or not at all.
The economically more important nodules, which are rich in Ni and Cu, but poor in Co, are mainly deposited in the abyssal sea floor in certain areas of the Indian Ocean and the south Pacific. The greatest abundance of commercial ore-grade nodules, however, occurs in the tropical north Pacific (Cronan, 1978).