key concept 57.4 Nutrient Cycling in Ecosystems Involves Chemical and Biological Transformations

All nutrients that make up the tissues of organisms originate from either soil, water, or the atmosphere, and then cycle between organisms and their environment through (1) primary and secondary production and (2) decomposition (Figure 57.11). We considered production in the last two sections, but not decomposition. Decomposition occurs when detritus, or dead organisms and their waste products, are broken down by bacteria and fungi into energy and nutrients. Decomposition releases nutrients in a soluble form that can then be taken up by primary producers and microorganisms. Detritus can come in many forms, including plants and animals, but is mostly made up of plant or algal matter. Detritus must first be broken down into smaller fragments by detritivores such as worms and arthropods before it can undergo mineralization (see Figure 57.11). Mineralization is the process by which animals, bacteria, and fungi convert organic compounds into inorganic soluble nutrients. Along the way, some of the nutrients may be lost to the system as gas to the atmosphere or minerals to ground water. The nutrient cycle is complete when the inorganic soluble nutrients produced through mineralization are used once again by primary producers or bacteria.

image
Figure 57.11 Nutrient Cycles A generalized nutrient cycle, showing the movement of nutrients among biotic and abiotic compartments of an ecosystem and the potential pathways for inputs and losses.

1237

focus your learning

  • Decomposition and mineralization are necessary to complete nutrient cycling in ecosystems.

  • The average residence time of water molecules in the various pools of the hydrologic cycle ranges from one week to thousands of years.

  • Fossil-fuel burning by humans is affecting Earth's carbon cycle, changing its climate and oceans.

  • Nitrogen, an abundant gas in Earth’s atmosphere, is cycled primarily by microbial activity.

  • Most phosphorus is stored in rocks and deep-sea sediments, but it cycles rapidly through organisms and often limits plant growth.

  • Most sulfur is pooled in rocks and ocean sediments, but cycling produces different compounds on land and in oceans.

The movement of nutrients through food webs from uptake to decomposition—that is, through the biotic components of ecosystems—occurs primarily on a local scale. In contrast, abiotic processes can move nutrients far beyond the boundaries of the local ecosystem. Each of the chemical elements that organisms use in large quantities cycles in a distinctive way through the globe. Because geological, chemical, and biological processes are all important in moving materials around the planet, the pattern of movement of an element is called its biogeochemical cycle. The cycles are considered in terms of pools—the amount of elements within certain parts of the abiotic and biotic environment—and fluxes, or the rates of movement between pools. We will consider the biogeochemical cycling of water, carbon, nitrogen, phosphorus, and sulfur at the global scale because of their functional role in biological systems and their roles as pollutants in the environment.