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

We designed a CO2-controlled cuvette and stripping system to trace a (CO2)- C-14 pulse-label from photosynthetic assimilation by wetland plants tin thi s study Orontium aquaticum L.) to its release as (CH4)-C-14 by microbial re spiration. The system maintained cuvette CO2 concentrations to within +/-5 Pa of the set-point, and it allowed continuous recovery of (CO2)-C-14 and ( CH4)-C-14 for 17 d without damage to the enclosed plant. The first emission s of (CH4)-C-14 mere detected <12 h after photosynthetic assimilation of th e label. The (CH4)-C-14 flux increased linearly from 0.12 Bq min(-1) at 12 h to 3.0 Bq min(-1) at 5 d, then plateaued at approximate to 2 Bq min(-1). We could not distinguish between (CH4)-C-14 produced by aceticlastic methan ogenesis vs, that produced by CO2 reduction. Radiocarbon activity in the so il dissolved inorganic C pool peaked on the first day then declined slowly. We did not detect radiocarbon activity in soil solution pools of several l ow molecular weight organic acids (acetate, formate, lactate, and propionat e), but the label was detected in the bulk dissolved organic C pool. We arg ue that radiocarbon will be useful for investigating the contribution of ro ot exudates to methanogenic metabolism, but data interpretation will requir e separation of the relative contributions of CO2 reduction and aceticlasti c methanogenesis to overall (CH4)-C-14 emissions. Processes such as CH4 oxi dation and acetogenesis must also be considered in quantitative estimates o f photosynthetic support of methanogenesis.