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 (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 C m⁻² yr⁻¹) with an average flux of 2.0 ± 0.3 μmol 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 (R² = 0.81), mostly because of strong responses by R_h (R² = 0.65) and less so for R_r (R² = 0.41) and R_m (R² = 0.18). Time series analysis revealed strong daily periodicities for R_s and R_r, whilst R_m was dominated by seasonal (~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.