Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
Biogeosciences, 9, 79-95, 2012
© Author(s) 2012. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
06 Jan 2012
Exploring the "overflow tap" theory: linking forest soil CO2 fluxes and individual mycorrhizosphere components to photosynthesis
A. Heinemeyer1, M. Wilkinson2, R. Vargas3, J.-A. Subke4, E. Casella2, J. I. L. Morison2, and P. Ineson1 1Stockholm Environment Institute (SEI-York centre) and Centre for Terrestrial Carbon Dynamics (CTCD-York centre) at the Environment Department, University of York, York, YO10 5DD, UK
2Centre for Forestry {&} Climate Change, Forest Research, Alice Holt Lodge, Farnham, Surrey, GU10 4LH, UK
3Departamento de Biología de la Conservación, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Ensenada, BC, Mexico
4School of Natural Sciences, Biological and Environmental Sciences, University of Stirling, Stirling, FK9 4LA, UK
Abstract. Quantifying soil organic carbon stocks (SOC) and their dynamics accurately is crucial for better predictions of climate change feedbacks within the atmosphere-vegetation-soil system. However, the components, environmental responses and controls of the soil CO2 efflux (Rs) are still unclear and limited by field data availability. The objectives of this study were (1) to quantify the contribution of the various Rs components, specifically its mycorrhizal component, (2) to determine their temporal variability, and (3) to establish their environmental responses and dependence on gross primary productivity (GPP). In a temperate deciduous oak forest in south east England hourly soil and ecosystem CO2 fluxes over four years were measured using automated soil chambers and eddy covariance techniques. Mesh-bag and steel collar soil chamber treatments prevented root or both root and mycorrhizal hyphal in-growth, respectively, to allow separation of heterotrophic (Rh) and autotrophic (Ra) soil CO2 fluxes and the Ra components, roots (Rr) and mycorrhizal hyphae (Rm).

Annual cumulative Rs values were very similar between years (740 ± 43 g C m−2 yr−1) with an average flux of 2.0 ± 0.3 μmol CO2 m−2 s−1, but Rs components varied. On average, annual Rr, Rm and Rh fluxes contributed 38, 18 and 44%, respectively, showing a large Ra contribution (56%) with a considerable Rm component varying seasonally. Soil temperature largely explained the daily variation of Rs (R2 = 0.81), mostly because of strong responses by Rh (R2 = 0.65) and less so for Rr (R2 = 0.41) and Rm (R2 = 0.18). Time series analysis revealed strong daily periodicities for Rs and Rr, whilst Rm was dominated by seasonal (~150 days), and Rh by annual periodicities. Wavelet coherence analysis revealed that Rr and Rm were related to short-term (daily) GPP changes, but for Rm there was a strong relationship with GPP over much longer (weekly to monthly) periods and notably during periods of low Rr. The need to include individual Rs components in C flux models is discussed, in particular, the need to represent the linkage between GPP and Ra components, in addition to temperature responses for each component. The potential consequences of these findings for understanding the limitations for long-term forest C sequestration are highlighted, as GPP via root-derived C including Rm seems to function as a C "overflow tap", with implications on the turnover of SOC.

Citation: Heinemeyer, A., Wilkinson, M., Vargas, R., Subke, J.-A., Casella, E., Morison, J. I. L., and Ineson, P.: Exploring the "overflow tap" theory: linking forest soil CO2 fluxes and individual mycorrhizosphere components to photosynthesis, Biogeosciences, 9, 79-95,, 2012.
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