Nutrient Cycling in Ecosystems PDF

# The Living World Depends on a Flow of Energy and the Cycling of Nutrients ## All Nutrients Follow Biogeochemical Cycles - All nutrients flow in a more or less cyclic path known as a biogeochemical cycle. - The important players in all nutrient cycles are green plants, decomposers, and air and...

# The Living World Depends on a Flow of Energy and the Cycling of Nutrients ## All Nutrients Follow Biogeochemical Cycles - All nutrients flow in a more or less cyclic path known as a biogeochemical cycle. - The important players in all nutrient cycles are green plants, decomposers, and air and water. - There are two basic types of biogeochemical cycles: - Gaseous cycles: The main reservoirs are the atmosphere and the oceans. - Sedimentary cycles: The main reservoir is the soil, rocks, and minerals. - Both gaseous and sedimentary cycles involve biological and nonbiological agents. - Both are driven by the flow of energy through the ecosystem and tied to the water cycle. - All biogeochemical cycles have three basic components: inputs, internal cycling and outputs. ## Nutrients Enter the Ecosystem Via Inputs - The input of nutrients to the ecosystem depends on the type of biogeochemical cycle. - Nutrients with a gaseous cycle enter via the atmosphere. - Nutrients with sedimentary cycles depend on the weathering of rocks and minerals. - Soil formation and the resulting soil characteristics have a major influence on processes involved in nutrient release and retention. - Supplementing nutrients in the soil are nutrients carried by rain, snow, air currents, and animals. ## The Gaia Hypothesis - Earth's atmosphere is extremely different from that predicted for a nonliving Earth and from that of other planets in the solar system. - The Gaia hypothesis says that Earth's biosphere, atmosphere, oceans, and soil together make up a feedback system that maintains an optimal physical and chemical environment for life on Earth. - It's a theory that explains how the Earth's atmosphere, oceans, and soil all interact to create a stable environment for life. ## Nutrients Are Recycled Within the Ecosystem - Primary productivity in ecosystems depends on the uptake of essential mineral (inorganic) nutrients by plants and their incorporation into living tissues. - As these living tissues senesce, the nutrients are returned to the soil or sediments in the form of dead organic matter. - Various microbial decomposers transform the organic nutrients into a mineral form, a process called mineralization. - This process is called internal cycling and it represents a recycling of nutrients within the ecosystem. ## Key Ecosystem Processes Influence the Rate of Nutrient Cycling - The internal cycling of nutrients through the ecosystem depends on the processes of primary production and decomposition (Figure 21.1). - Primary productivity determines the rate of nutrient transfer from inorganic to organic form and decomposition determines the rate of transformation of organic nutrients into inorganic form. - Nutrient availability depends heavily on the turnover of nutrients in phytoplankton and zooplankton. - Retention of nutrients in flowing-water ecosystems is difficult, but helped by logs and rocks that hold detritus in place, by algal uptake of nutrients, and by aquatic invertebrates. ## Hot Ecology - Ecologists have developed a good understanding of the processes controlling the cycling of nutrients within ecosystems. - It's difficult to quantify the rates of exchange between the various components of the ecosystem. - Radiation ecology emerged with the development of nuclear weapons in the Manhattan Project. - The use of radioactive compounds that could be used as tracers to examine the movement of nutrients through ecosystems. ## Both Climate and Plant Characteristics Influence the Rate of Nutrient Cycling - Climate directly affects the rate of nutrient cycling in ecosystems by influencing rates of primary production and decomposition. - Nutrient cycling in an ecosystem is also influenced by the nature of its organisms. - Organisms such as phytoplankton and zooplankton in aquatic systems are short-lived, grow rapidly, absorb nutrients, and quickly return their nutrients to the ecosystem through decomposition (Figure 21.2). ## Focus on Ecology: Hot Ecology - Ecologists have developed a good understanding of the processes controlling the cycling of nutrients within ecosystems. - They can quantify the amount of nutrients in various components of the ecosystem by sampling the soil, plants, and other organisms and determining their nutrient concentrations. - Quantifying the rates of exchange between the various components of the ecosystem is much more difficult. - J. P. Witherspoon of Oak Ridge National Laboratory conducted a pioneering study using radioisotopes of elements to quantify the cycling of nutrients through an ecosystem. - The goal of the experiment was to follow the pathway of a radiolabeled trace element (micronutrient) through a forest ecosystem. - The element was a radioactive isotope of cesium (134Cs). - Witherspoon inoculated the trunks (boles) of 12 white oak (Quercus alba) trees with 20 microcuries (µC) of this isotope. - He followed the gains, losses, and transfers of the isotope in the trees and soil. - This research quantified and provided insights into the process of internal cycling and retention of elements in forest trees. ## Feedback That Occurs Between Nutrient Availability, Net Primary Productivity, and Nutrient Release in Decomposition - High nutrient availability encourages high plant tissue concentrations and high net primary productivity. - This results in a high quantity and quality of dead organic matter, which encourages high rates of net mineralization and nutrient supply in the soil. - Conversely, low nutrient availability encourages a low net primary productivity, resulting in low leaf nutrient concentrations and low nutrient return to the soil in dead organic matter. - This leads to a low net mineralization rate. **Figure 21.1:** A generalized model of nutrient cycling in a terrestrial ecosystem. The three common components of inputs, internal cycling, and outputs are shown in bold. The key ecosystem processes of net primary productivity and decomposition are italicized. **Figure 21.2:** Feedback that occurs between nutrient availability, the net primary productivity, and nutrient release in decomposition for initial conditions of low and high nutrient availability.