Figure 1.

Schematic illustration of the coupling of water, nitrogen cycling, and carbon in ecosystems. Principle features of these coupled controls are that (i) water controls the inputs and outputs of nitrogen, (ii) increasing net primary productivity (NPP) allows more of the N flux through the system to be captured into organic matter, (iii) increasing organic N stocks allow for more N mineralization, supporting more NPP, and (iv) increasing precipitation both allows more NPP and more N cycling, thus water and nutrient limitation of NPP tend to become correlated.