Exploring the "overflow tap" theory: linking forest soil CO2 fluxes and individual mycorrhizosphere components to photosynthesis

A. Heinemeyer, M. Wilkinson, R. Vargas, J. -A. Subke, E. Casella, J. I. L. Morison, P. Ineson

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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 (R-s) are still unclear and limited by field data availability. The objectives of this study were (1) to quantify the contribution of the various R-s 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 (R-h) and autotrophic (R-a) soil CO2 fluxes and the R-a components, roots (R-r) and mycorrhizal hyphae (R-m).

Annual cumulative R-s values were very similar between years (740 +/- 43 g Cm-2 yr(-1)) with an average flux of 2.0 +/- 0.3 mu mol CO2 m(-2) s(-1), but R-s components varied. On average, annual R-r, R-m and R-h fluxes contributed 38, 18 and 44 %, respectively, showing a large R-a contribution (56 %) with a considerable R-m component varying seasonally. Soil temperature largely explained the daily variation of R-s (R-2 = 0.81), mostly because of strong responses by R-h (R-2 = 0.65) and less so for R-r (R-2 = 0.41) and R-m (R-2 = 0.18). Time series analysis revealed strong daily periodicities for R-s and R-r, whilst R-m was dominated by seasonal (similar to 150 days), and R-h by annual periodicities. Wavelet coherence analysis revealed that R-r and R-m were related to short-term (daily) GPP changes, but for R-m there was a strong relationship with GPP over much longer (weekly to monthly) periods and notably during periods of low R-r. The need to include individual R-s components in C flux models is discussed, in particular, the need to represent the linkage between GPP and R-a 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 R-m seems to function as a C "overflow tap", with implications on the turnover of SOC.

Original languageEnglish
Pages (from-to)79-95
Number of pages17
Issue number1
Publication statusPublished - 2012

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