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

Megonigal, J.P., Whalen, S.C., Tissue, D.T., Bovard, B.D., Albert, D.B., Allen, A.S.
Soil Science Society of America journal 1999 v.63 no.3 pp. 665-671
biogeochemical cycles, wetlands, Orontium, photosynthesis, carbon dioxide, chemical reactions, wetland soils, microbial activity, biological activity in soil, methane, labeling techniques, radiolabeling, methane production, photosynthates
We designed a CO(2)-controlled cuvette and stripping system to trace a (14)CO(2) pulse-label from photosynthetic assimilation by wetland plants (in this study Orontium aquaticum L.) to its release as (14)CH(4) by microbial respiration. The system maintained cuvette CO(2) concentrations to within +/- 5 Pa of the set-point, and it allowed continuous recovery of (14)CO(2) and (14)CH(4) for 17 d without damage to the enclosed plant. The first emissions of (14)CH(4) were detected < 12 h after photosynthetic assimilation of the label. The (14)CH(4) flux increased linearly from 0.12 Bq min(-1) at 12 h to 3.0 Bq min(-1) at 5 d, then plateaued at approximately 2 Bq min(-1). We could not distinquish between (14)CH(4) produced by aceticlastic methanogenesis vs. that produced by CO(2) 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 CO(2) reduction and aceticlastic methanogenesis to overall (14)CH(4) emissions. Processes such as CH(4) oxidation and acetogenesis must also be considered in quantitative estimates of photosynthetic support of methanogenesis.