In the water cycle, water changes between its various phases (solid, liquid and gas), but always stays water. In the case of the carbon and nitrogen cycles, various chemical forms of carbon (carbon dioxide, carbonates, sugars, etc.) and nitrogen (ammonia, nitrite and nitrate) are cycled through regularly.
The temperature of tropical areas relative to places more distant to the equator. Another reason why is higher nutrient cycling, which is also an effect of higher temperature.
