A plant-soil-atmosphere microcosm for tracing radiocarbon from photosynthesis through methanogenesis

We designed a CO₂-controlled cuvette and stripping system to trace a ¹⁴CO₂ pulse-label from photosynthetic assimilation by wetland plants (in this study Orontium aquaticum L.) to its release as ¹⁴CH₄ by microbial respiration. The system maintained cuvette CO₂ concentrations to within ± 5 Pa of the set-point, and it allowed continuous recovery of ¹⁴CO₂ and ¹⁴CH₄ for 17 d without damage to the enclosed plant. The first emissions of ¹⁴CH₄ were detected <12 h after photosynthetic assimilation of the label. The ¹⁴CH₄ flux increased linearly from 0.12 Bq min^-1 at 12 h to 3.0 Bq min^-1 at 5 d, then plateaued at ≃2 Bq min^-1. We could not distinquish between ¹⁴CH₄ produced by aceticlastic methanogenesis vs. that produced by CO₂ reduction. Radiocarbon activity in the soil dissolved inorganic C pool peaked on the first day then declined slowly. We did not detect radiocarbon activity in soil solution pools of several low molecular weight organic acids (acetate, formate, lactate, and propionate), but the label was detected in the bulk dissolved organic C pool. We argue that radiocarbon will be useful for investigating the contribution of root exudates to methanogenic metabolism, but data interpretation will require separation of the relative contributions of CO₂ reduction and aceticlastic methanogenesis to overall ¹⁴CH₄ emissions. Processes such as CH₄ oxidation and acetogenesis must also be considered in quantitative estimates of photosynthetic support of methanogenesis.