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Abstract

Using experimental data from two different types of CO2 enrichment experiments, we show that there is empirical evidence to support an approximation for the behavior of multiple plant and microbial players as they compete for common nutrient resources, known as the Relative Demand (RD) approach. The RD approach is founded on the hypothesis that various plant and microbial components of an ecosystem have efficient mechanisms for obtaining nutrient resources, which can be abstracted as varying nutrient sink strengths. To the extent that multiple competitors share space in a common rhizosphere, the relative magnitude of expressed sink strength should correlate with nutrient demands for persistence and growth, given a current ecosystem and organism state, and these relative demand quantities should correlate as well with actual nutrient uptake by each competitor. To take the simple example of two plant species growing growing in a mixed plot, one with a well-developed canopy exhibiting rapid biomass accumulation, and another with a poorly developed canopy and low growth rate: The nutrient demand to maintain growth of the larger, more vigorous species is obviously higher than the nutrient demand to maintain growth of its smaller and less vigorous neighbor. The RD hypothesis is that, through a multitude of biological, physiological, and ecological mechanisms, the more vigorous plant species will be able to compete more effectively than its less vigorous neighbor for the available nutrient resources in the soil, in approximate proportion to the ratio of its own demand to the total demand. 

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