Biogeochemical Cycles

Biosphere and Biogeochemical Cycles

During the past 3.5 billion years of Earth's history, the biosphere has come to play an increasingly dominant role in shaping the chemistry of Earth's surface, making the study of Earth surface chemistry inherently the study of biogeochemistry. Indeed, as noted by James Lovelock and Lynn Margulis in their Gaia hypothesis, it was the biosphere that shaped the biogeochemistry of the earth surface into a chemical composition most favorable for the continued presence of life on earth. Most important, the biosphere changed the earth's atmosphere from an inhospitable, carbon-dioxide-rich, hot atmosphere into the hospitable, oxygen-rich, temperate atmosphere of the present. [Page 205]This dominant role of the biosphere has been further solidified with the rise of one biotic species—humans. In particular, the advent of the agricultural and industrial revolutions has caused human actions to vastly increase many elemental fluxes and pools within the most important biogeochemical cycles.

Earth's Intersecting Biogeochemical Cycles

Analysis of a biogeochemical cycle may focus on any given chemical element found on Earth, but it is most commonly applied to the abundant elements that play an integral role in the functioning of the Earth system. The elements with a lighter atomic weight are most abundant in the solar system, and those light elements, which were soluble in the early seas, formed the basis of modern biogeochemistry. The key effect of the biosphere on the biogeochemistry of Earth's surface can be traced to the evolution of the photosynthetic metabolic pathway. Photosynthesis represents the critical link between the biogeochemical cycles most important for humans and the functioning of the Earth system. The simplified form of this crucial reaction of photosynthesis is

It is vital to note that photosynthesis must be sustained by a consistent supply of mineral nutrients. Thus, in addition to linking biogeochemical cycles of carbon (C), oxygen (O), and hydrogen (H), photosynthesis causes the biogeochemical cycling of mineral nutrients to also play key roles in the functioning of the Earth system. Of these mineral nutrients, nitrogen (N) is the most important. After nitrogen, photosynthesis requires an abundant supply of several other macronutrients. All these remaining macronutrients are weathered from rocks, thereby forming an important biogeochemical link between the biosphere and lithosphere.

In short, photosynthesis is the nexus of the major biogeochemical cycles. Given this role of photosynthesis, a plant's stoichiometry, or ratio of the concentration of its chemical elements, provides a useful means for roughly assessing the relative importance of the various biogeochemical cycles within the earth system. By dry mass, the average plant is 45% carbon, 45% oxygen, 6% hydrogen, 1.5% nitrogen, 1.0% potassium, 0.5% calcium, 0.2% magnesium, 0.2% phosphorous, 0.1% sulfur, and 0.1% silicon. Biogeochemical cycles of other “micronutrients” such as iron (Fe) or manganese (Mn) can also be important to examine in instances where a shortage of one of these rarer elements may adversely affect the growth or health of a plant, animal, or human. This entry proceeds by concentrating on the major biogeochemical cycles organized here as (a) the hydrogen and oxygen cycles, (b) the carbon cycle, (c) the nitrogen cycle, and (d) cycling of rock-derived nutrients.

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