Within-System Element Cycles, Input-Output Budgets, and Nutrient Limitation

Summary

Widely used conceptual models for controls on nutrient cycling and input-outputs budgets of forest ecosystems suggest that: (1) nutrient losses from ecosystems originate in the available nutrient pool in soil; (2) nutrients that limit plant production are retained tightly within those systems; (3) this retention leads to accumulation of the limiting nutrient(s), eventually to the point at which it no longer limits production; and (4) losses of nutrient(s) thereafter should reflect rates of nutrient input, rather than biotic demand In this chapter, we explore mechanisms that could constrain the accumulation of a limiting nutrient, and therefore could allow nutrient limitation to persist indefinitely. Possible mechanisms include episodic disturbance-related nutrient losses, closed element cycles, and losses of nutrients from sources other than the available inorganic pool of nutrients in soil. For the last mechanism, both a simple and a more complex model are used to show that losses of dissolved organic forms of a nutrient could constrain nutrient accumulation and permit nutrient limitation to persist indefintely. Emissions of nitrogen (N) trace gases produced during nitrification could have a similar effect. To the extent that losses of nutrients by these and related pathways are important, anthropogenic inputs of nutrients (particularly N) could alter forest ecosystems substantially, to an extent greater than standard conceptual models would allow.