# 2.4: Nutrient Cycling in Ecosystems


In drawing your food web, you depicted a combination of the flow of energy and the cycling of nutrients through an ecosystem. Where as energy has an input and an output, nutrients are continually recycled. These elements often cycle between the biotic and abiotic components in an ecosystem.

For example, water is composed of hydrogen and oxygen atoms. Water molecules precipitate from the atmosphere and fall as rain, saturating the soil. A plant absorbs those molecules through its roots, transporting them up to its leaves. Some of these water molecules will be broken apart during the process of photosynthesis, the oxygen exiting the plant as $$\ce{O2}$$ gas that you will breathe in while the hydrogen might be used to form molecules of glucose that you will eat. The rest of the water exits the plant through its stomata, evaporating back into the atmosphere in a process called transpiration, the evaporation of water from plant tissues. The oxygen atoms that you breathed in will be used for cellular respiration and be joined back to hydrogen molecules to once again form water. This water might exit your body as vapor on your breath or as perspiration and return directly to the atmosphere. It might also soak into your clothes as sweat or exit as urine, be processed in a wastewater facility, then sent out to the ocean where it will evaporate back into the atmosphere.

The atoms of hydrogen and oxygen are broken apart and reassembled into other molecules multiple times in this process, but the overall outcome is a cycle, with atoms traveling between the atmosphere, hydrosphere, lithosphere, and biosphere, but rarely ever exiting our Earth system.

In the space below, diagram the flow of water through the ecosystem around you.

What is the role of plants in the global cycling of water?