Nutrient cycling is the cyclical movement of organic and inorganic matter through an ecosystem. Nutrients are said to be “recycled” in ecosystems because they are broken down from organic matter and then contribute to the growth of new organic matter. Examples of nutrients that are cycled through ecosystems include nitrogen, carbon, potassium, phosphorus, and magnesium.
Nutrient cycles are natural resources because they are ecosystem services that sustain life on Earth. Effectively protecting them requires conservation of entire ecosystems rather than particular species, as there are many interdependent actors in nutrient cycling.
Sources: See http://www.ipni.net/article/IPNI-3326 for examples of nutrient cycles or http://citeseerx.ist.psu.edu/viewdoc/download doi=10.1.1.175.1762&rep=rep1&type=pdf for the role of soil microbes in nutrient cycling
Picture sources:
http://en.wikipedia.org/wiki/Nitrogen_cycle
http://rmsiblog.files.wordpress.com/2011/11/nutrient_cycle.jpg