ABSTRACT
Nutrients are chemical elements that organisms require for life. Biogeochemistry is the study of the cycling of nutrients through the biosphere, hydrosphere, lithosphere and atmosphere. All living organisms require specific combinations of nutrients to grow, survive and reproduce. Plant leaves, for example, have an average carbon-to-nitrogen-to-phosphorus ratio of 800:30:1 (Elser et al. 2000), whereas microbes have an average ratio of 60:7:1 (Cleveland and Liptzin 2007). The specific ratios of elements in living tissue couple the cycling of these nutrients (Sterner and Elser 2002). If the availability of one element declines, it can become a limiting nutrient on overall productivity. Similarly, if one element becomes too abundant, it can cause toxicity. Six elements—Hydrogen (H), Carbon (C), Nitrogen (N), Oxygen (O), Phosphorus (P) and Sulfur (S)—constitute 95 percent of the mass of the biosphere (Schlesinger and Bernhardt 2013). Of these key elements, the C, N and P cycles have been most dramatically modified by humans and urbanization.
