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11 1 The Emergence of a Biogeochemical Cycle How did the nitrogen cycle come to be the nitrogen cycle? That is, how has this biogeochemical cycle evolved over time? Strictly speaking, the nitrogen cycle is not an entity capable of evolving. It is not even an entity. After all, when we use the term nitrogen cycle, we are imposing a pattern on nature that suits our own interests. What we choose to incorporate into or leave out of the cycle is somewhat arbitrary. That said, what we call the nitrogen cycle depends on the activity of a wide variety of organisms, each carrying evolutionary-acquired knowledge embedded in its genetic material, and these organisms have evolved over time. If nothing else, asking about the emergence of a biogeochemical cycle reminds us that, when we alter such cycles, we are altering something that took several billion years to reach a dynamic balance. Over the expanse of geological time, as countless species of organisms co-evolved in changing environments, refining their ability to exploit the material available to them, they became part of the cycle through which that material flowed. In the case of nitrogen, different types of bacteria eventually acquired (through evolutionary processes of variation and selection) the ability to fix atmospheric nitrogen, to decompose organic material into ammonia, to convert ammonia into nitrite and nitrate, and to transform those compounds back into gases. The ability of plants to use nitrate as a raw material also involved the evolutionary acquisition of knowledge, as did the ability of certain plants to work symbiotically with nitrogen-fixing bacteria. The notion of sustainability surely has something to do with humans learning to integrate their activities with Earth systems that have been refined by several billions years of evolutionary change. 12 T HE KNOW LEDGE OF NAT UR E The Knowledge of Nature Cosmologists tell us that our entire universe burst into existence approximately 15 billion years ago. In the opening moments of the universe, however , the element we call nitrogen did not exist. No elements existed until the universe had cooled enough to allow various configurations of mass and energy to form stable structures. The lightest elements—hydrogen and helium—formed first. Heavier elements, such as carbon, nitrogen, and oxygen , did not form until much later, when great clusters of matter coalesced as stars. There, in the center of stars, nuclear fusion converted lighter elements into heavier ones, a process that continued until a star exploded and dispersed the newly created elements.1 To emerge, more complex structures, including molecules that serve as the basis of life, needed conditions favorable to their formation. One such set of conditions emerged, cosmologists tell us, after interstellar matter thrown from the furnace of an exploding star consolidated into a new star and a handful of orbiting bodies. On one of those bodies, the planet Earth, something unusual happened: several billion years of evolutionary change eventually gave rise to a complex web of organisms, ecological relationships, and geophysical processes. As it happened, nitrogen atoms turned out to be important in the system of nature that emerged on Earth. Unlike most elements, nitrogen can bond in eight different ways—by donating one, two, three, four, or five electrons or accepting one, two, or three electrons—which makes it exceptionally versatile in joining with other atoms. It is not surprising, then, that nitrogen came to occupy pivotal positions in two of nature’s most crucial building blocks: amino acids (which are the basic components of all proteins) and nucleotides (sequences of which serve as the basis of all genetic material). For the first several hundred million years of Earth’s existence, few complex molecular structures had a chance to form. The young planet was a hellish place, unrecognizable as the Earth that exists today. Chunks of debris traveling in the same solar orbit frequently collided with the new planet, vaporizing on impact and generating tremendous amounts of heat and gas. Even after the number of collisions dwindled and the surface of the new planet cooled, countless volcanoes continued to spew forth gases. The planet, as it turned out, was massive enough to keep most gases from escaping its gravitational grasp and just far enough from the sun to allow for condensation and the formation of seas without everything freezing solid. The consensus among most geologists (but not all) is that this post-cooling atmosphere consisted primarily of carbon dioxide (CO2 ), water (H2 O), and molecular...

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