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

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 (<i>R</i>_s) are still unclear and limited by field data availability. The objectives of this study were (1) to quantify the contribution of the various <i>R</i>_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 (<i>R</i>_h) and autotrophic (<i>R</i>_a) soil CO₂ fluxes and the <i>R</i>_a components, roots (<i>R</i>_r) and mycorrhizal hyphae (<i>R</i>_m). <br><br> Annual cumulative <i>R</i>_s values were very similar between years (740 &plusmn; 43 g C m⁻² yr^&minus;1) with an average flux of 2.0 &plusmn; 0.3 μmol CO₂ m⁻² s⁻¹, but <i>R</i>_s components varied. On average, annual <i>R</i>_r, <i>R</i>_m and <i>R</i>_h fluxes contributed 38, 18 and 44%, respectively, showing a large <i>R</i>_a contribution (56%) with a considerable <i>R</i>_m component varying seasonally. Soil temperature largely explained the daily variation of <i>R</i>_s (<i>R</i>² = 0.81), mostly because of strong responses by <i>R</i>_h (<i>R</i>² = 0.65) and less so for <i>R</i>_r (<i>R</i>² = 0.41) and <i>R</i>_m (<i>R</i>² = 0.18). Time series analysis revealed strong daily periodicities for <i>R</i>_s and <i>R</i>_r, whilst <i>R</i>_m was dominated by seasonal (~150 days), and <i>R</i>_h by annual periodicities. Wavelet coherence analysis revealed that <i>R</i>_r and <i>R</i>_m were related to short-term (daily) GPP changes, but for <i>R</i>_m there was a strong relationship with GPP over much longer (weekly to monthly) periods and notably during periods of low <i>R</i>_r. The need to include individual <i>R</i>_s components in C flux models is discussed, in particular, the need to represent the linkage between GPP and <i>R</i>_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 <i>R</i>_m seems to function as a C "overflow tap", with implications on the turnover of SOC.