Large impacts of small methane fluxes on carbon isotope values of soil respiration

Carbon dioxide isotope (delta C-13 of CO2) analysis is increasingly used to address a broad range of questions involving soil C dynamics and respiration sources. However, attaining delta C-13 mass balance is critical for robust interpretation. Many ecosystems exhibit methane (CH4) fluxes that are small in the context of total C budgets, yet may significantly impact delta C-13 values of CO2 due to large kinetic fractionations during CH4 production. Thus, the delta C-13 values of CO2 do not directly reflect respiration C sources when co-occurring with CH4, but few studies of terrestrial soils have considered this phenomenon. To assess how CH4 altered the interpretation of delta C-13 values of CO2, we incubated a Mollisol and Oxisol amended with C-4-derived plant litter for 90 days under two head-space treatments: a fluctuating anaerobic/aerobic treatment (four days of anaerobic conditions alternating with four days of aerobic conditions), and a static aerobic treatment (control). We measured delta C-13 values of CO2 and CH4 with a tunable diode laser absorption spectrometer, using a novel in-line combustion method for CH4. Cumulative delta C-13 of CO2 differed significantly between treatments in both soils. The delta C-13 values of CO2 were affected by relatively small CH4 fluxes in the fluctuating anaerobic/aerobic treatment. Effects of CH4 on delta C-13 values of CO2 were greater in the Oxisol due to its higher percent contribution of CH4 to total C mineralization (18%) than in the Mollisol (3%) during periods of elevated CH4 production. When CH4 accounted for just 2% of total C mineralization, the delta C-13 values of CO2 differed from total C mineralization by 0.3-1 parts per thousand, and by 1.4-4.8 parts per thousand when CH4 was 10% of C mineralization. These differences are highly significant when interpreting natural abundance delta C-13 data. Small CH4 fluxes may strongly alter the delta C-13 values of CO2 relative to total mineralized C. A broad range of mineral and peatland soils can experience temporary oxygen deficits. In these dynamic redox environments, the delta C-13 values of CO2 should be interpreted with caution and ideally combined with delta C-13 of CH4 when partitioning sources and mechanisms of soil respiration.