Elevated Atmospheric CO2 Increases Root Exudation of Carbon in Wetlands: Results from the First Free-Air CO2 Enrichment Facility (FACE) in a Marshland

Abstract

Experiments employing free-air CO2 enrichment (FACE) facilities have indicated that elevated atmospheric carbon dioxide (eCO2) stimulates growth in diverse terrestrial ecosystems. Studies of the effects of eCO2 on wetland plants have indicated a similar response, but these studies were mostly performed in growth chambers. We conducted a 2-year FACE experiment [CO2 ≈ 582 µmol mol−1] in a marsh in Spain to test whether the common reed (Phragmites australis) responds to carbon enrichment, as previously reported in other macrophytes. More specifically, we tested the effect of eCO2 on P. australis growth, photosynthesis, transpiration, and biomass, its effect on modifying plant and soil ratios of carbon, nitrogen, and phosphorus, and whether the strong environmental variability of this wetland modulates these responses. Our findings show that effects of eCO2 in this wetland environment are more complex than previously believed, probably due to hydrological effects. The effects of eCO2 on reed plants were cumulative and manifested at the end of the growing season as increased 38–44% instantaneous transpiration efficiency (ratio of net photosynthesis to transpiration), which was dependent on plant age. However, this increase did not result in a significant increase in biomass, because of excessive root exudation of carbon. These observations contrast with previous observations of wetland plants to increased atmospheric CO2 in growth chambers and shed new light on the role of wetland plants as a carbon sink in the face of global climate change. The combined effects of water stress, eCO2, and soil carbon processes must be considered when assessing the function of wetlands as a carbon sink under global change scenarios.

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Acknowledgements

This study was supported by the DECAMERON Project (2008/001) which was funded by the Agency for National Parks of the Spanish Ministry of Agriculture, Food and Environment. Carburos Metálicos SA (Air Products and Chemicals, Inc.) provided additional support for which we are very grateful. The TDNP staff, especially Carlos Ruiz de la Hermosa, and TRAGSA team provided important logistical and technical support. We also thank José Luis Ayala, Carlos Menor, Jesus Iglesias, Adrian Carrero, María José Ortíz, Ana Meco, and Juan Carlos Rodríguez-Murillo for their valuable support during FACE operation and field sampling. LSG and RSA were supported by JAE-PreDoc and JAE-Doc contracts, respectively, which were also funded by the European Social Funds.

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Correspondence to Salvador Sánchez-Carrillo.

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SSC, MAC, and SC conceived and designed the study. SSC, LSG, RSA, and TS conducted field research. SSC, DGA, MAC, SC, and RS analyzed data. SSC wrote the paper with inputs from all authors.

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Sánchez-Carrillo, S., Álvarez-Cobelas, M., Angeler, D.G. et al. Elevated Atmospheric CO2 Increases Root Exudation of Carbon in Wetlands: Results from the First Free-Air CO2 Enrichment Facility (FACE) in a Marshland. Ecosystems 21, 852–867 (2018). https://doi.org/10.1007/s10021-017-0189-x

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Keywords

  • elevated carbon dioxide
  • Phragmites australis
  • above-ground biomass
  • photosynthesis
  • transpiration
  • root exudation
  • wetland
  • free-air CO2 enrichment (FACE)