#L18 Experimental evidence supports Relative Demand hypothesis

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. 

We use experimental and modeling data from a recent intercomparison study based on the long-term Free Air CO2 Enrichment (FACE) experiments carried out at Oak Ridge National Laboratory and at Duke University to demonstrate that observed total plant nitrogen uptake under conditions of increasing nitrogen demand driven by higher rates of photosynthesis is qualitatively and quantitatively consistent with an operational implementation of the RD approach as used in the ACME Land Model. We also show that independent short-term CO2 enrichment experiments under conditions of varying nitrogen availability offer important guidance for how storage pools for carbon and nutrients in plants are likely to regulate short-term carbon and nutrient uptake dynamics, and how those storage pools can influence long-term plant-microbe interactions through competition.