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Importance of root uptake of $$^{14}$$CO$$_{2}$$ on $$^{14}$$C transfer to plants impacted by below-ground $$^{14}$$CH$$_{4}$$ release

$$^{14}$$CH$$_{4}$$の地下放出影響下での植生への$$^{14}$$C移行における$$^{14}$$CO$$_{2}$$経根吸収の重要性

太田 雅和; 田中 拓*

Ota, Masakazu; Tanaka, Taku*

放射性廃棄物地下処分施設から漏洩する$$^{14}$$CH$$_{4}$$は、土壌中で微生物による酸化を受けて$$^{14}$$CO$$_{2}$$となる。既存の$$^{14}$$C移行モデルでは、土壌中$$^{14}$$CO$$_{2}$$の植生への移行が主に葉面吸収によって起こることが仮定されている。一方、$$^{14}$$CO$$_{2}$$の経根吸収の影響は把握されていない。本研究は、$$^{14}$$CO$$_{2}$$の経根吸収が植生への$$^{14}$$C移行に及ぼす影響を評価するため、土壌中の$$^{14}$$CH$$_{4}$$の輸送と酸化をモデル化し、これを陸面$$^{14}$$CO$$_{2}$$移行モデル(SOLVEG-II)に組み込んだ。モデルによる土壌中$$^{14}$$CH$$_{4}$$移行の計算性能は、深部土壌への$$^{13}$$CH$$_{4}$$注入の野外実験データを用いて検証した。次に、モデルを地下水面(深度1m)からの$$^{14}$$CH$$_{4}$$の連続放出時の陸面$$^{14}$$C移行に適用した。土壌中で根が浅く分布(深度11cm)する状況では、植生への$$^{14}$$C移行では$$^{14}$$CO$$_{2}$$の葉面吸収の影響が支配的となり、葉への$$^{14}$$C蓄積の80%に寄与した。一方、根が地下水面近くまで分布(深度97cm)する状況では、葉への$$^{14}$$C蓄積の半分以上(63%)が経根吸収によってもたらされた。更に、メタン酸化が土壌深部(深度20cmあるいは80cmまで分布)で起きた場合には、葉に蓄積した$$^{14}$$Cの全量が経根吸収によってもたらされた。これらの結果から、根が地下水面近くまで分布し、$$^{14}$$CH$$_{4}$$の酸化が土壌深部で起きる場合は、$$^{14}$$CO$$_{2}$$の経根吸収が植生への$$^{14}$$C移行において支配的となることが明らかとなった。

$$^{14}$$CH$$_{4}$$ released from deep underground radioactive waste disposal facilities can be a belowground source of $$^{14}$$CO$$_{2}$$ owing to microbial oxidation of $$^{14}$$CH$$_{4}$$ to $$^{14}$$CO$$_{2}$$ in soils. Environmental $$^{14}$$C models assume that the transfer of $$^{14}$$CO$$_{2}$$ from soil to plant occurs via foliar uptake of $$^{14}$$CO$$_{2}$$. Nevertheless, the importance of $$^{14}$$CO$$_{2}$$ root uptake is not well understood. In the present study, belowground transport and oxidation of $$^{14}$$CH$$_{4}$$ were modeled and incorporated into an existing land surface $$^{14}$$CO$$_{2}$$ model (SOLVEG-II) to assess the importance of root uptake on $$^{14}$$CO$$_{2}$$ transfer to plants. Performance of the model in calculating the belowground dynamics of $$^{14}$$CH$$_{4}$$ was validated by simulating a field experiment of $$^{13}$$CH$$_{4}$$ injection into subsoil. The model was then applied to $$^{14}$$C transfer in a hypothetical ecosystem impacted by continuous $$^{14}$$CH$$_{4}$$ input from the water table (bottom of one-meter thick soil). In a shallowly rooted ecosystem with rooting depth of 11 cm, foliar uptake of $$^{14}$$CO$$_{2}$$ was significant, accounting for 80% of the $$^{14}$$C accumulation in the leaves. 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 $$^{14}$$C accumulated in the leaves was transferred by the root uptake. We found that $$^{14}$$CO$$_{2}$$ root uptake in this ecosystem depended on the distribution of methane oxidation in the soil; all $$^{14}$$C accumulated in the leaves was transferred by the root uptake when methane oxidation occurred at considerable depths (e-folding depths of 20 cm, or 80 cm). These results indicate that $$^{14}$$CO$$_{2}$$ root uptake contributes significantly to $$^{14}$$CO$$_{2}$$ transfer to plants if $$^{14}$$CH$$_{4}$$ oxidation occurs at great depths and roots penetrate deeply into the soil.

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パーセンタイル:100

分野:Environmental Sciences

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