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 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.