Forest soil CO2 flux: uncovering the contribution and environmental responses of ectomycorrhizas

Authors

  • ANDREAS HEINEMEYER,

    1. Stockholm Environment Institute (SEI-York centre), Department of Biology, University of York, York YO10 5DD, UK,
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    • 1Members of the Centre for Terrestrial Carbon Dynamics (CTCD), a centre of excellence of the Natural Environment Research Council (NERC), UK.

  • IAIN P. HARTLEY,

    1. Department of Biology, University of York, York YO10 5YW, UK,
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    • 2Present address: School of Biological & Environmental Sciences, University of Stirling, Stirling FK9 4LA, UK.

  • SAM P. EVANS,

    1. Biometrics, Surveys & Statistics Division, Forest Research, Alice Holt Lodge, Farnham, Surrey GU10 4LH, UK,
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    • 1Members of the Centre for Terrestrial Carbon Dynamics (CTCD), a centre of excellence of the Natural Environment Research Council (NERC), UK.

  • JOSÉ A. CARREIRA DE LA FUENTE,

    1. Departamento de Biología Animal, Biología Vegetal y Ecología, Universidad de Jaén, 23071 Jaén, Spain
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  • PHIL INESON

    1. Stockholm Environment Institute (SEI-York centre), Department of Biology, University of York, York YO10 5DD, UK,
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    • 1Members of the Centre for Terrestrial Carbon Dynamics (CTCD), a centre of excellence of the Natural Environment Research Council (NERC), UK.


Andreas Heinemeyer, tel. +44 1904 432991, fax +44 1904 432898, e-mail: ah126@york.ac.uk

Abstract

Forests play a critical role in the global carbon cycle, being considered an important and continuing carbon sink. However, the response of carbon sequestration in forests to global climate change remains a major uncertainty, with a particularly poor understanding of the origins and environmental responses of soil CO2 efflux. For example, despite their large biomass, the contribution of ectomycorrhizal (EM) fungi to forest soil CO2 efflux and responses to changes in environmental drivers has, to date, not been quantified in the field. Their activity is often simplistically included in the ‘autotrophic’ root respiration term. We set up a multiplexed continuous soil respiration measurement system in a young Lodgepole pine forest, using a mycorrhizal mesh collar design, to monitor the three main soil CO2 efflux components: root, extraradical mycorrhizal hyphal, and soil heterotrophic respiration.

Mycorrhizal hyphal respiration increased during the first month after collar insertion and thereafter remained remarkably stable. During autumn the soil CO2 flux components could be divided into ∼60% soil heterotrophic, ∼25% EM hyphal, and ∼15% root fluxes. Thus the extraradical EM mycelium can contribute substantially more to soil CO2 flux than do roots. While EM hyphal respiration responded strongly to reductions in soil moisture and appeared to be highly dependent on assimilate supply, it did not responded directly to changes in soil temperature. It was mainly the soil heterotrophic flux component that caused the commonly observed exponential relationship with temperature. Our results strongly suggest that accurate modelling of soil respiration, particularly in forest ecosystems, needs to explicitly consider the mycorrhizal mycelium and its dynamic response to specific environmental factors. Moreover, we propose that in forest ecosystems the mycorrhizal CO2 flux component represents an overflow ‘CO2 tap’ through which surplus plant carbon may be returned directly to the atmosphere, thus limiting expected carbon sequestration from trees under elevated CO2.

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