A Plant-Soil-Atmosphere Microcosm for Tracing Radiocarbon from Photosynthesis through Methanogenesis

Abstract

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.