Biogeochemical Cycles

Nutrients are elements that plants require for growth. In most terrestrial ecosystems, a lack of essential nutrients may limit plant primary productivity. Net primary productivity (NPP), other ecosystem processes, and ecosystem structure and function may be best understood by examining and studying the cycles of these nutrients. To trace the movement of a nutrient—such as nitrogen (N), phosphorus (P), sulfur (S), or carbon (C)—as it travels between the living (biotic) and nonliving (abiotic) components of an ecosystem is to trace its biogeochemical cycle.

The Phosphorus Cycle

A generalized biogeochemical cycle of phosphorus, for example, starts with its release from apatite (phosphorus-containing rock). Inorganic, plant-available forms of phosphorus in most soils derive from apatite. Mechanical and chemical weathering reactions release phosphorus from apatite into the soil solution. Plants take up available forms of phosphorus, such as orthophosphate (H₂ PO₄^3-), from the soil solution into their roots. After uptake from the soil, phosphorus travels as the phosphate anion HPO₄^2- through the plant before accumulating in leaves and other living tissues. Phosphate is present in plant cells and circulating fluids at a concentration of about 10^-3m. Plants also incorporate inorganic phosphate into organic forms that may be used for various metabolic processes. Because there are no gaseous forms of phosphorus and soil reserves are small, phosphorus is difficult for plants to acquire. As a result, plants hoard phosphorus. Rather than releasing it back into the environment, plants send the phosphorus to the roots for storage before dropping their leaves. This process, called translocation, ensures that the plant will have a sufficient supply of phosphorus for the next growing season. Any phosphorus remaining in the dead leaves falls to the ground in leaf litter. This phosphorus gradually returns to the soil organic matter after microorganisms, such as fungi and bacteria, break down the litter through decomposition, and the cycle commences again.

The time that it takes for the phosphorus to move from apatite into the soil solution is called the flux rate. This represents the amount of time it takes a given amount of a certain element to move between the pools, or reservoirs. Flux rates can be very slow or very rapid. It may take hundreds of years for the phosphorus in the apatite to move into the soil solution. In contrast, once plant-available phosphorus is in the soil solution it is rapidly taken up by the plant roots. The mean residence time (MRT) is the length of time that elements remain in a pool. The MRT for phosphorus in apatite may be thousands of years, but within a plant, the MRT may be only one year. Nutrients move between pools through meteorological, geological, hydrological, biological, or anthropogenic mechanisms.

Transport Mechanisms

Meteorological mechanisms of nutrient transport are generally related to precipitation in the form of rain, fog, snow, or ice. For example, nitrogen has various gaseous phases including ammonia (NH₃) and nitrous oxides (NO_x). The nitrogen-containing gas may dissolve in precipitation, whereupon the nitrogen is subsequently deposited on plant and soil surfaces. In contrast, phosphorus, which has no gaseous phase, is not incorporated in rain but may be transported as dust by wind currents through the atmosphere. Nutrients also move slowly over the long term (hundreds of thousands of years) and over long distances via geological mechanisms, such as sedimentation, uplift, and volcanism. For example, carbon (C) may be stored in combination with calcium (Ca) as calcium carbonate (CaCO₃) in seashells. The shells fall to the ocean floor and through sedimentation processes become calcite, or calcium carbonate rock. Over thousands or millions of years, this carbon may be released slowly to the atmosphere from near-shore sedimentary rocks. Eventually, the nitrogen deposited on the land by precipitation and the carbon released from the calcite become incorporated into organic matter, the biological component of ecosystems.

Biological mechanisms generally refer to the microbial transformations of elements that are stored in organic matter into inorganic forms of nutrients that may be used by plants. For example, soil bacteria and fungi release acids that break down leaf litter and release the phosphorus and nitrogen that are bound in it. The phosphorus and nitrogen then combine with oxygen or hydrogen to form plant-available compounds.

Biological mechanisms of nutrient distribution can also include movement of nitrogen or phosphorus from one area to another via mammals or birds. Studies of bison movement in Oklahoma's tallgrass prairie ecosystem show that, when nitrogen-containing bison fecal pats decompose due to fire or chemical breakdown, they create a spatially patchy distribution of soil nitrogen; this patchiness of nitrogen may influence plant distributions. Likewise, a trip to the Caribbean island chain Los Roques, off the coast of Venezuela, provides a striking example of phosphorus distribution by seabirds. Guano, the white bird droppings that coat the island's rock out-crops, contains some of the highest phosphorus concentrations in the world.

Humans are probably the most important biological vectors for nutrient transport on Earth, particularly for carbon, nitrogen, sulfur, and phosphorus. Anthropogenic combustion of fossil fuels releases carbon dioxide (CO₂) to the atmosphere in quantities that exceed the combined releases of CO₂ from plant, animal, and microbial respiration, natural forest and grassland fires, and volcanic emissions. This has contributed to the build-up of CO₂ in Earth's atmosphere and may alter the biogeochemical cycles of other elements. Fossil fuel combustion also releases nitrogen and sulfur, which ultimately contributes to the formation and deposition of acid rain. Mining of phosphorus, such as in Los Roques, has altered the long-term storage of phosphorus, increased the flux rate of the global phosphorus cycle, and contributed to the phosphorus pollution of freshwater ecosystems worldwide. These and other human activities are altering the biogeochemical cycles of nitrogen, phosphorus, sulfur, and carbon at the global scale, with largely unknown consequences for Earth's inhabitants and ecosystems.

Anne Fernald Cross

Bibliography

Berner, E. K., and R. A. Berner. Global Environment: Water, Air, and Geochemical Cycles. Upper Saddle River, NJ: Prentice-Hall, 1996.

Mackenzie, F. T., and Judith A. Mackenzie. Our Changing Planet: An Introduction to Earth System Science and Global Environmental Change. Upper Saddle River, NJ: Prentice-Hall, 1995.

Schlesinger, W. H. Biogeochemistry: An Analysis of Global Change. San Diego, CA: Academic Press, 1997.