Pulse-labeling studies of carbon cycling in Arctic tundra ecosystems: The contribution of photosynthates to methane emission

We investigated a possible mechanism underlying observed correlations between net ecosystem carbon dioxide exchange and methane emission. Using the technique of C-14 pulse-labeling, we traced the movement of carbon fixed by photosynthesis as it moved through wet sedge and moist tussock tundra plant-soil mesocosms and was emitted as methane to the atmosphere. The C-14 tracer provided a definitive way of quantifying the fate of recently fixed carbon. Carbon fixed by photosynthesis was measured as emitted methane from both moist tussock and wet sedge tundra mesocosms within 2 hours after labeling. Integration of time series measurements of methane emission showed that recent photosynthates are an important source of carbon for methane production. Approximately 2% of carbon fixed by photosynthesis at peak growing season was subsequently emitted as methane from moist tussock tundra, and approximately 3% was emitted as methane from wet sedge tundra. Measurements of soil pore water carbon pools demonstrate rapid transfer of C-14 from plant carbon to dissolved forms and subsequently to the atmosphere as carbon dioxide or methane.