Interactions between the carbon and nitrogen cycles under rising atmospheric CO2. By Cesar Terrer Moreno

Elevated CO2 (eCO2) triggers a cascade of effects on plants from above to below-ground that may alter nitrogen (N) availability, with important consequences for plant growth and carbon (C) storage. From Free-Air CO2 Enrichment (FACE) experiments we know that eCO2 can potentially increase both photosynthesis and net primary production (NPP). However, in some experiments the eCO2 effect on NPP disappeared after a few years due to a progressive N depletion in the soil, whereas in other experiments NPP continues enhanced by eCO2 after a decade.

How can some plants manage to avoid N limitations under eCO2?

Plants may allocate below-ground part of the extra C fixed under eCO2, potentially increasing N availability throughout three main mechanisms: root growth, root exudation and C allocation to symbionts. In this presentation I reviewed these mechanisms and propose a conceptual model of the interactions between the C and N cycles under eCO2 that could explain current experimental observations in two main scenarios: with and without N limitations. From this analysis I concluded that the allocation of C to ectomycorrhizal symbionts may play a very important role to increase N availability to sustain plant growth under eCO2. On the other hand, eCO2 in plants of which roots are colonised just by arbuscular mycorrhizae might lead to a progressive N-depletion in the long-term. In the second scenario, the allocation of C below-ground is unimportant, potentially increasing aboveground biomass. Furthermore, when enhanced, high-quality biomass eventually turns over, N mineralization and availability may increase, preventing progressive N-depletion in the long-term. Current terrestrial ecosystem models should include herein described dynamic interactions between the C and N cycles in order to predict plant carbon sink activity in the future.

Cesar 1

Ectomycorrhizal association between Pinus resinosa (red pine) and an unknown fungal species (Lambers et al., 2008)

Cesar 2

DukeFACE experiment (C. Hildreth/Duke University Photography)

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