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 (NOx). 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 outcrops, 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. see also Biogeography; Carbon Cycle; Nitrogen Fixation; Nutrients.

Anne Fernald Cross

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