Abstract
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.
Highlights
Soils contain the largest terrestrial organic carbon (C) stock (Bolin et al, 2000), representing two-thirds or more of terrestrial C (Schimel et al, 1994; Tarnocai et al, 2009)
Quantifying soil organic carbon (SOC) dynamics accurately is crucial for better predictions of climate change feedbacks within the atmosphere-vegetation-soil system (Cox et al, 2000; Smith and Fang, 2010)
This study addressed four related research questions: (1) How much of the measured total Rs derives from heterotrophic Rh versus autotrophic Ra components (Rr and Rm) and how constant are the proportional contributions over temporal scales from hours to years? (2) Do these Rs components respond to weather variability and key environmental factors? (3) To what extent do these component fluxes depend on gross primary productivity (GPP)? (4) Is there continued evidence to support the concept of mycorrhizal activity to depend on a plant regulated “overflow tap” for labile C in plants (Heinemeyer et al, 2007)?
Summary
Soils contain the largest terrestrial organic carbon (C) stock (Bolin et al, 2000), representing two-thirds or more of terrestrial C (Schimel et al, 1994; Tarnocai et al, 2009). Heinemeyer et al (2007) proposed that the mycorrhizal component (Rm) might function as an “overflow tap” in forest C allocation, allowing the plant to allocate C to the mycorrhizal partner under either C surplus or nutrient limiting conditions with consequent impacts on ecosystem C turnover and storage. This hypothesis was based on only a short-term seasonal flux separation, and only indirectly indicated the Rm dependence on assimilate supply. Long-term partitioning studies of Rs components into soil Ra and Rh and their temperature responses (Fitter et al, 2004; Kirschbaum, 2006; Heinemeyer et al, 2007) are increasingly becoming a research focus in order to better understand the measured Rs responses to key environmental factors and model forest C cycling (Hanson et al, 2000; Bond-Lamberty et al, 2004; Ekblad et al, 2005)
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