Unit 1.2 DB: Human Effect on Nutrient Cycles

The end of Chapter 2 in the 4th edition (Chapter 3 in the 3rd edition) describes the importance of each of the Carbon, Phosphorus, Nitrogen nutrient cycles, as well as the Hydrologic cycle, to the ecosystem in which it occurs and to humans.  Choose one of the cycles and fully explain one way in which human activity altered these ecosystems.  What is the activity?  What effect does the activity have on the nutrient cycle you chose?  What environmental problems have arisen (or could arise) because of the activity's affect on the nutrient cycle?  Read through the answers that have already been given before participating in this discussion board, and try to come up with a unique response.  Then be sure to discuss each others answers and comments.  This may seem like a mundane discussion board, but it can yield some interesting conversation!