Abstract

Human activities are substantially modifying the global carbon and nitrogen cycles. The global carbon cycle is being modified principally by the burning of fossil fuels, and also by deforestation; these activities are increasing the carbon dioxide concentration of the atmosphere and changing global climate. The nitrogen cycle is being modified principally by the production of nitrogen fertilizer, and also by the planting of legumes and the combustion of fossil fuels; these activities are more than doubling the rate of fixation of nitrogen and contributing to the unbalanced productivity and acidification of ecosystems. With the aim of quantifying these disruptions, the principal flows among reservoirs in preindustrial times and today are estimated in the framework of simplified models. The methane subcycle of the carbon cycle and the nitrous oxide subcycle of the nitrogen cycle are also discussed from this viewpoint.

The Grand Cycles

Carbon (C), nitrogen (N), sulfur (S), and phosphorus (P), the important biochemical building blocks of life, find their way to plants and animals, thanks to the interplay of biological and geochemical processes. Each of the four elements moves from one chemical state to another and from one physical location to another on the earth's surface in a closed loop, or “cycle.” In view of their central role in life on this planet, the four cycles are here termed the “grand nutrient cycles.”

The cycles are powered by solar energy, in conjunction with the earth's gravity and geothermal energy. The nutrients flow among “reservoirs.” The reservoirs of interest are life forms (living and dead plants and animals), the soil, the oceans and other water bodies, the atmosphere, and rocks. The quantity of nutrient stored in a reservoir (the reservoir's “stock” of nutrient) changes whenever the total nutrient flows in and out of the reservoir are not equal.