MadSci Network: Earth Sciences

Re: Why are matter cycles essential in an ecosystem?

Date: Wed Mar 14 21:49:40 2001
Posted By: Alex Barron, Graduate Student, Ecology(Biogeochemistry)
Area of science: Earth Sciences
ID: 983331612.Es

	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.


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