Importance of root uptake of 14CO2 on 14C transfer to plants impacted by below-ground 14CH4 release

Publication date: May 2019

Source: Journal of Environmental Radioactivity, Volume 201

Author(s): Masakazu Ota, Taku Tanaka

Abstract

¹⁴C-labelled methane (¹⁴CH₄) released from deep underground radioactive waste disposal facilities can be a below-ground source of ¹⁴CO₂ owing to microbial oxidation of ¹⁴CH₄ to ¹⁴CO₂ in soils. Environmental ¹⁴C models assume that the transfer of ¹⁴CO₂ from soil to plant occurs via foliar uptake of ¹⁴CO₂. Nevertheless, the importance of ¹⁴CO₂ root uptake is not well understood. In the present study, below-ground transport and oxidation of ¹⁴CH₄ were modeled and incorporated into an existing land-surface ¹⁴CO₂ model (SOLVEG-II) to assess the relative importance of root uptake and foliar uptake on ¹⁴CO₂ transfer from soil to plants. Performance of the model in calculating the below-ground dynamics of ¹⁴CH₄ was validated by simulating a field experiment of ¹³CH₄ (as a substitute for ¹⁴CH₄) injection into subsoil in a wheat field in the UK. The proposed model simulation was then applied to ¹⁴C transfer in a hypothetical ecosystem impacted by continuous ¹⁴CH₄ input from the water table (bottom of 1-m thick soil), which simulated continuous release of ¹⁴CH₄ from a deep underground radioactive waste disposal facility. The contrast between the results obtained from the model calculation that assumed different distributions of roots (rooting depths of 11 cm, or 97 cm) and methane oxidation (characterized by e-folding depths of 5 cm, 20 cm, or 80 cm) in the soil provided insight into the relative importance of root uptake and foliar uptake pathways. In the shallowly rooted ecosystem with rooting depth of 11 cm, foliar uptake of ¹⁴CO₂ was significant, accounting for 80% of the ¹⁴C accumulation (as organic ¹⁴C) in the plant (leaf compartment). By contrast, in a deeply rooted ecosystem (rooting depth of 97 cm), where the root penetrated to depths close to the water-table, more than half (63%) the ¹⁴C accumulated in the plant was transferred via the root uptake pathway. We found that ¹⁴CO₂ root uptake (thus ¹⁴C accumulation in the plant) in this ecosystem depended on the distribution of methane oxidation in the soil; all ¹⁴C accumulated in the plant was transferred by the root uptake pathway when methane oxidation occurred at considerable depths (e-folding depths of 20 cm, or 80 cm) in the soil. The high level of ¹⁴CO₂ root uptake was ascribed to the oxidation of added ¹⁴CH₄ (i.e., production of ¹⁴CO₂) in the deep part of the soil and the subsequent high level of root uptake of the deep soil-water containing ¹⁴CO₂. These results indicate that ¹⁴CO₂ root uptake contributes significantly to ¹⁴CO₂ transfer to plants if ¹⁴CH₄ oxidation occurs at great depths and roots penetrate deeply into the soil. It is recommended that current environmental ¹⁴C models must be refined to consider the importance of the root uptake pathway to ensure that dose estimates of ¹⁴CH₄ release from deep underground waste disposal facilities are accurate.

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