|MadSci Network: Earth Sciences|
Philip- All life depends upon the constant cycling of nutrients in ecosystems. Matter cycles allow nutrients to move from one part of an ecosystem to another and change forms, ensuring that nutrients continue to be available. In this way, nature is the ultimate recycler. To envision why nutrients need to be cycled, let's think about what would happen if they didn't cycle. Imagine a deciduous forest. The trees in the forest take up carbon from the air and nitrogen, water and phosphorus from the soil and use it to grow leaves and wood. Each Autumn the leaves fall to the ground where they form leaf litter. Now without a nutrient cycle, nitrogen, phosphorus and carbon would get taken up by the trees and end up as organic matter in leaf litter. Pretty soon, trees would use up all the nitrogen and phosphorus in the soil and wouldn't be able to grow any more. If there were tons of nitrogen and phosphorus in the soil, plants could eventually use up all the CO2 in the atmosphere! Either way, resources run out because all of the carbon, nitrogen and phosphorus would get "locked up" in the leaf litter. Luckily, this doesn't happen: a wide range of organisms break down the leaf litter which releases some of the carbon back into the air and the nitrogen and phosphorus back into the soil. So, nutrients cycle between the soil, the tree, and the leaf litter or between the atmosphere, the tree and the leaf litter. A great example of this is tropical forests where leaves are recycled so quickly that very little leaf litter builds up on the forest floor. In this way, huge forests can grow on soils that don't have very many nutrients in them. These processes act at a wide range of scales. At small scales, nutrients are transformed from form to form within the soil of a forest or water of a lake. At a much larger spatial scale and over longer time periods, phosphorus washes from the land into the sea where some of it ends up in sediment, which forms rock, which is later uplifted to be weathered and wash into the ocean again. Likewise water rains down, flows through rivers or groundwater, evaporates, forms clouds, and rains down again. I hope this helps. Alex
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