Biogeochemistry and biogeochemical cycles

We can conceive of pools of chemical elements existing in compartments. Some compartments occur in the atmosphere (carbon in CO2, nitrogen as gaseous nitrogen, etc.), some in the rocks of the lithosphere (calcium as a constituent of calcium carbonate, potassium in feldspar) and others in the hydrosphere -the water in soil, streams, lakes or oceans (nitrogen in dissolved nitrate, phosphorus in phosphate, carbon in carbonic acid, etc.). In all these cases the elements exist in an inorganic form. In contrast, living organisms (the biota) and dead and decaying bodies

Figure 18.1 Diagram to show the relationship between energy flow (pale arrows) and nutrient cycling. Nutrients locked in organic matter (dark arrows) are distinguished from the free inorganic state (white arrow). DOM, dead organic matter; NPP, net primary production.

can be viewed as compartments containing elements in an organic form (carbon in cellulose or fat, nitrogen in protein, phosphorus in adenosine triphosphate, etc.). Studies of the chemical processes occurring within these compartments and, more particularly, of the fluxes of elements between them, comprise the science of biogeochemistry.

Many geochemical fluxes would occur in the absence of life, if only because all geological formations above sea level are eroding and degrading. Volcanoes release sulfur into the atmosphere whether there are organisms present or not. On the other hand, organisms alter the rate of flux and the differential flux of the elements by extracting and recycling some chemicals from the underlying geochemical flow (Waring & Schlesinger, 1985). The term biogeochemistry is apt.

The flux of matter can be investigated at a variety of spatial and temporal scales. Ecologists interested in the gains, uses and losses of nutrients by the community of a small pond or a hectare of grassland can focus on local pools of chemicals. They need not concern themselves with the contribution to the nutrient budget made by volcanoes or the possible fate of nutrients leached from land to eventually be deposited on the ocean floor. At a larger scale, we find that the chemistry of streamwater is profoundly influenced by the biota of the area of land it drains (its catchment area; see Section 18.2.4) and, in turn, influences the chemistry and biota of the lake, estuary or sea into which it flows. We deal with the details of nutrient fluxes through terrestrial and aquatic ecosystems in Sections 18.2 and 18.3. Other investigators are interested in the global scale. With their broad brush they paint a picture of the contents and fluxes of the largest conceivable compartments - the entire

... but nutrient cycling is never perfect the 'bio' in biogeochemistry biogeochemistry can be studied at different scales

Wetfall Dryfall

Wetfall Dryfall

Nutrient Components

Figure 18.2 Components of the nutrient budgets of a terrestrial and an aquatic system. Note how the two communities are linked by stream flow, which is a major output from the terrestrial system and a major input to the aquatic one. Inputs are shown in color and outputs in black.

atmosphere, the oceans as a whole, and so on. Global biogeo-chemical cycles will be discussed in Section 18.4.

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