Terrestrial ecosystems sequester annually about a quarter of anthropogenic CO2 emissions, slowing climate change appreciably. The terrestrial carbon sink is generally attributed to the effect of increasing atmospheric CO2 concentrations via the CO2 fertilization effect” (CFE) on plant biomass. However, results from CO2 enrichment (eCO2) experiments range from large and persistent to transient or even non-existent CFE (Norby & Zak, 2011), complicating projections of Earth’s future climate. Plant nitrogen (N) availability is thought to be one of the most important constraints for the CFE (Hungate et al. 2003), however, N-availability alone cannot explain why some ecosystems seem to stay productive under eCO2 under N-limitations (e.g. Duke FACE, McCarthy et al. 2010). I hypothesize that the association of plants with certain species of mycorrhizal fungi might play an important role in increasing plant N-availability to take advantage of the CFE. Among the most common mycorrhizal associations, ectomycorrhizal (ECM) fungi have been associated with a larger transfer of N to the host plant than arbuscular mycorrhizal (AM) fungi (Phillips et al. 2013), and their different nutrient economies might explain the magnitude of the CFE. In this presentation I show, by synthesizing data from 88 eCO2 experiments in a meta-analysis, that the enhancement of biomass by CO2 depends largely on N-availability and the type of mycorrhizae, whereas climate, age and functional plant type are far less important predictors. In N-limited ecosystems eCO2 (~640 μmol mol−1) enhanced total plant growth by 30±5% in species dominated by ECM fungi, and non-significantly in species dominated by AM fungi. In N-fertilized environments total biomass was enhanced by ~35% in both ECM and AM-dominated species. These results provide a consistent framework to explain the range of observations in eCO2 experiments and conclude that the projected CFE (IPCC 2013, Chapter 6) is not valid for N-limited AM-dominated ecosystems, unless other longer-term processes (such as potentially enhanced N fixation) come into play.
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Hungate, B. A., Dukes, J. S., Shaw, M. R., Luo, Y. & Field, C. B. Atmospheric science. Nitrogen and climate change. Science 302, 1512–1513 (2003).
McCarthy, H. R. et al. Re‐assessment of plant carbon dynamics at the Duke free‐air CO2 enrichment site: interactions of atmospheric [CO2] with nitrogen and water availability over stand development. New Phytol 185, 514–528 (2010).
Phillips, R. P., Brzostek, E. & Midgley, M. G. The mycorrhizal‐associated nutrient economy: a new framework for predicting carbon–nutrient couplings in temperate forests. New Phytol 199, 41–51 (2013).