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.
Ectomycorrhizal association between Pinus resinosa (red pine) and an unknown fungal species (Lambers et al., 2008)
DukeFACE experiment (C. Hildreth/Duke University Photography)