Document: EDW-3-59-32

A theoretical examination of symbiotic N fixation.

RASTETTER, E.B.* ¹, P.VITOUSEK ², C.FIELD ³, G.SHAVER ¹ and D.HERBERT ¹

The Ecosystems Center, Woods Hole, MA 02543 USA ¹
Stanford University, Stanford, CA 94305 USA ²
Carnegie Institute of Washington, Stanford, CA 04305 USA ³

Abstract:
We examine symbiotic N fixation from the perspective of the resource optimization paradigm. Two predictions derived from this paradigm are (1) that vegetation should tend to redistribute internal resources such that all external resources are equally limiting and (2) that if an external resource can be acquired from two sources, the less costly source will be exploited. We assume a fixed C cost of N fixation and assess the cost of N uptake as the decrease in photosynthesis per unit increase in N uptake as a result of reallocating internal resources below ground. Using the Multiple-Element-Limitation (MEL) model, we examine these tradeoffs and conclude that N fixation will tend to be favored when: 1.) CO₂ concentration is high so there is a high return on resource allocation to the canopy; 2.) available N concentration is low so there is a low return on resource allocation below ground; 3.) the canopy is open so there is a high return on resource allocation to the canopy; and 4.) the soil is well exploited by roots so there is a low return on resource allocation below ground. Evolution may not have had time to act upon the first of these criteria, which would result in sub-optimal resource allocations under elevated CO2. We derive an equation that automatically scales the relative contributions of these four criteria and predicts the conditions under which symbiotic fixation should occur (e.g., early succession).

Keywords: N fixation, optimal resource allocation,resource limitation

This abstract is being presented at: 3:15 PM in session:
Oral Session #13: N Fixation and Biochemical Patterns.