Global Biogeochemical Cycling

Biosphere-wide biogeochemical cycling is formed by matter transfer between the land and the ocean. There are several ways of this transfer (Figure 2). Particularly, water steam and other substances in the atmosphere are spread over the Earth by winds (flow 17). Tectonic movement leads to interchange by matter of different parts of the lithosphere; or some rocks can become a land from an ocean floor, and vice versa, as a result of sea regression or transgression (flow 18). Water drain (rivers and ground-waters, flow 19) transports solute mineral matter from the land to the ocean. The matter is to get to the drain from the lithosphere as a result of erosion and ablation processes (flow 20).

A slow cycling of biogenic elements is connected with the big geological turnover. In the course of lowering lithospheric layers, some rocks transform to magma (flow 21) and return to the biosphere as a result of volcanic eruptions (flow 22).

Structure

When one talks about global biogeochemical cycling, it does not mean that there is only one cycle. As is evident from Figure 2, the real structure is closer to a web with many cyclic cells. A global property of the web, which allows considering it as a cycle, is the necessity ofbalance between inputs and outputs for every acting subject or subsystem. For example, in accordance with Figure 2, for terrestrial biogeocoenose the input of each chemical element (the total intensity of flows 2, 6, 7, 9, 22) should be equal to its output (the total intensity of flows 1, 3, 4, 5, 8, 19). Although, as ecosystems are never in equilibrium, all these balances do not absolutely hold true.

The other important peculiarity of biological cycles is their essential spatial heterogeneity. In different natural conditions, biosphere is represented by different biogeo-coenoses (forests, steppes, deserts, etc.), which are characterized by special types of biogeochemical cycling. Data on net primary production of main kinds of ecosystems are represented in Table 1. The primary production reflects the amount of energy (and, indirectly, matter) put into cycling every year. Finally, this amount estimates the intensity of biogeochemical cycling in corresponding ecosystems.

In Figure 2, flows connecting biogeochemical cycling with external environment are not shown. For example, the biosphere loses hydrogen and helium, dissipating to the outer space, and, probably, permanently needs carbonic gas from the mantle.

General Characteristics

The biosphere as a whole can be interpreted as a peculiar heat engine, which uses, for its operation, solar (and, unessentially, radioactive geothermal) energy. In addition to artificial mechanical engines, the global biological machine is based on periodical processes and has its 'work cycle': the global biogeochemical cycling. Contrary to human-made engines, the biogeochemical one has no external destination. Finally, nearly all accepted energy is converted into the thermal energy of the environment, although only a small part is spent for biosphere self-organization.

An important characteristic of the biological turnover is intensity of the matter or energy flow in the cycle. It is necessary to note that in a stable state this intensity (measured in kgs-1 or, for the energy flow in watts) should be the same in all sections of the cycle (except very long term effects of biosphere development). Particularly, the amount of producing organic matter

Table 1 Indices of biogeochemical cycles' intensity (the net primary production) of the main types of ecosystems

Types of ecosystems 12 3 4 5 6 7 8 9 10 11 12 13 14

Net primary production (106 Jm~2yr~1) 2.5 15 25 11 9 0.6 13.5 37.5 37 9 37 11.5 7 1.7 Total world net primary production 25 200 440 75 85 40 210 750 60 25 60 175 190 800

(1012Jyr1)

1, Tundra; 2, coniferous forests; 3, deciduous forests; 4, forest steppes; 5, steppes; 6, deserts; 7, savanna; 8, tropic forest; 9, bogs and marshes;

10, lakes and rivers; 11, reaches; 12, agricultural lands; 13, continental shelf; 14, open ocean.

must be equal to the amount of the reducing one. Another important fact is the relative independence of two values: the flow intensity through a cycle's component and its saturation by mass or energy. The concept of the trophic pyramid is based on the idea about a direct relation between these values, but it is known that the concept does not work in some cases. For example, in aquatic ecosystems, autotrophic algae communities can have relatively small biomass (less than the accompanied herbivores), but they 'pump' through themselves much more matter than the herbivorous block.

Other global turnover characteristics are its period, 'length', 'width', and 'amplitude' (amount of involved matter). In general, the amplitude equals to the product of the intensity and period or, otherwise, the length and width. It is important to take into account that the real turnover is not a circle, but a network with a lot of paths and loops; and for any portion of matter, each element has its own way in the cycle with unique values of period, length, etc. The general characteristics are the average values, corresponding to an abstract average cycle of the pure circle form.

Average terms, needed for general renewal of biosphere components, can be used as integral indices of the matter turnover. Thus, the total living matter is renewable on average during 8 years; the marine one circulates much quicker, during 33 days. Corresponding values for plant biomass are even more contrasting: 14 years and 1 day. In the mean, all water of the hydrosphere does its cycling during 2800 years, and it passes through photosynthetic decomposition during 5 millions years only.

Energy Aspects

The intensity of cycling is determined by two main factors. The first key factor is the amount of available biogenic elements, which will be considered below. The second and the main one is the power of the producers block. An amount of energy that can be put into the cycle per unit time by producers determines both the cycle intensity and length. The energetic concept of A. Lotka and H. Odum, based on the consideration of energy as a peculiar ecological 'currency' is very useful for understanding functioning of biogeo-chemical cycling.

In contrast to the element cycling, it is not correct to consider energy cycling. Energy does not circulate; it is fixed by producers from the solar irradiation and then step-by-step degrades in food webs, transforming to thermal energy and dissipating in the outer space. The role of energy in biogeochemical cycling is compared sometimes with the role of water in water-mill functioning. A mill-wheel, as well as the elements in the course of biogeo-chemical cycling, really circulates; all its components permanently repeat their positions. At the same time water, rotating the wheel, does not return back; each of its portions leaves the mill forever. It is reasonable to talk about energy flow, but not cycling.

Similarly to energy flows through ecosystems, it is possible to consider information flows. If we associate information with negentropy, as it is often done, we can talk about the creation of information by producers and its gradual destruction by consumers. Although two consumers can use the same amount of energy, their production of entropy can be different. Some consumers can keep information better and transmit it to the next level. More careful use of energy gives essential evolution advantages to species. It is especially important for consumers of high levels, which deal with a relatively poor flow of energy; but energetic effectiveness also demands careful use of information. These considerations explain partly why intelligence of species usually increases in the direction of the top of trophic pyramid.

Cycles of the Main Biogenic Elements

All chemical elements of the Earth crust are involved in the biogeochemical cycling, but the intensity of their turnover and their importance for biosphere are quite different things. Each element is characterized by its own paths in the biosphere; importance of different matter flows (1-22 in Figure 2) for different elements is quite changeable. Some characteristics of the most important elements cycling are presented in Table 2.

The elements can be divided by different criteria. By their importance for living organism functioning (and representative in their bodies) they are classified for obligatory biogenic elements (the first six rows in Table 2: oxygen, hydrogen, carbon, nitrogen, sulfur, and phosphorus). Because water is the main part of biomass (60% of terrestrial organisms, 80% of water ones), the latter is formed essentially by oxygen and hydrogen. Both of these elements are important components of organic substances, together with carbon (the master biogenic element). Nitrogen, sulfur, and phosphorus are not represented in biomass in such big amounts, but they are absolutely necessary for forming such key organic substances as proteins, DNA, etc. Hydrogen is the only important element (the other one is helium), which has principally unclosed cycling, since its molecules permanently leave the Earth because of their low molecular weight.

Elements of the next group (potassium, calcium, magnesium, sodium, chlorine, silicon) together with the obligatory elements form the group of macroelements. Their concentrations in the total biomass are more than 0.01%. Two other elements from Table 2 (iron and manganese) represent the group of microelements,

Table 2 Participation of the main biogenic elements in the global biogeochemical cycling

Part in

Part in

Element

terrestrial

marine

Mass, involved

Mass, involved

migration from

biomass

biomass

in the terrestrial

in the marine

land to ocean

Flows of migration (in accordance

Element

(%)

(%)

cycling (109 t)

cycling (109 t)

(1091)

with Figure 2)

O

69

74.1

170

125

6

1, 3, 5, 6, 8, 12, 14, 16, 17, 19-22

H

10.2

12.4

20

15

1-3,5-10,12,14,16,17,19,21,22

C

18

9.4

70

50

0.8

1-5, 7-9, 11, 13-15, 18-22

N

0.75

1.6

3.4

6

0.05

4-6, 9, 11, 13-16, 18-22

S

0.19

0.38

0.6

1.32

0.16

6-11, 13-16, 18-22

P

0.08

0.1

0.35

1.2

0.025

7-9, 13-15, 18-20

K

0.45

0.6

1.8

1.2

0.34

5-11, 13, 14, 18-20

Ca

0.6

0.2

2.3

1.1

0.9

5, 7-9, 13, 14, 17-20

Mg

0.13

0.1

0.5

0.8

0.7

7-9, 13, 14, 18-20

Na

0.05

0.4

0.2

2.8

1.3

7-9, 11, 13, 14, 17-20

Cl

0.08

0.09

0.3

4.4

2.3

7-9, 11, 13, 14, 17-22

Si

0.2

0.45

0.86

5.5

0.2

5, 7, 9, 10, 14, 18-20

Fe

0.01

0.034

0.047

0.9

7-9, 13, 14, 18-20

Mn

0.01

0.035

0.001

0.02

7-9, 13, 14, 18-20

which embraces practically all stable elements from the periodical table.

Based on how elements involve in biogeochemical cycling they can be divided into products of Earth mantle degasification (O, H, C, N, S, Cl) and Earth crust lixiviation (P, K, Ca, Mg, Na, Si). These processes took place during Earth formation in earlier geological epochs and still are typical for the present big geological cycling of corresponding elements (flows 7, 13 or 21, 22).

Water (and, correspondingly, oxygen and hydrogen) follows the standard ways (see Figure 1), for example, by flows 1, 3, 6. Carbon, as a component of carbonic gas, is also involved in the circulation through the atmosphere (flows 1-5). Some other biogenic elements (such as nitrogen and phosphorus) are characterized by relative stable circulation in biological cycles 1-3.

Chemical Elements' Presentation in Cycling

The general terrestrial (flows 1-9) and marine (flows 10-16) cycles are more or less close. Amounts of different elements participating in these cycling are represented in Table 2. The numbers are very rough approximations of real values, because the current state of ecology as a science does not allow integrating data within all type of ecosystems and, additionally, the situation can essentially change from year to year.

The element representations in the cycles are determined by both presenting of elements in environment and their necessity for organisms of ecosystems. The last two factors are under mutual influence: organisms adapt to deficiency or excess of some elements and, conversely, can change the environment in desirable direction (e.g., soil bacteria radically increase the amount of available nitrogen in environment).

Rates between different elements, involved in cycling, are more or less proportional to their presentation in biomass. There are the so-called stoichiometric rates. For example, the rates of Redfield describe relations between amplitudes of cycles of carbon, nitrogen, and phosphorus.

In general, the amplitude of the cycling is determined by the total mass of available carbon, but in separate links it can be limited by the amount of other bioelements, playing an important role in internal organization of these special links. This fact is reflected in the well-known law of Liebig.

Usually, the limiting elements in terrestrial and especially aquatic ecosystems are nitrogen and phosphorus. As in accordance with the rates of Redfield the amount of carbon is strictly connected with amounts of these limiting elements, the amplitude of carbon cycle can be determined by their availability. Carbon buffer in soil humus, peat, and 'ocean humus' (dissolved organic substances in ocean water) includes the most part of cycling carbon and can be used very quickly for stabilization of global biogeochemical cycling.

The close character of the cycles determines the relative stability of ecosystems, although the permanent migration of many elements from the terrestrial cycle to the marine one (flows 19, 20) takes place. Corresponding estimations are also represented in Table 2. In marine ecosystems the surplus of elements is compensated by the process of sedimentation (flow 14); in other words, the elements are directed to the big biogeochemical cycling. Correspondingly, terrestrial ecosystems mainly compensate the losses of elements from the big cycles (flows 7, 18, 21). The unclosed character of some biogeochemical cycles by some elements, however, produces long-term processes of ecosystems' development, adaptation, and self-organization. In many cases, the biological evolution is a result of instability of biogeochemical cycling and, in its turn, produces such instability. One of the brightest manifestations of life on Earth is its embedding in the geochemical cycling and radical transformation of the latter.

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