The Use of Carbon Mass Budgets and Stable Carbon Isotopes to Examine Processes Affecting CO2 and CH4 Production in the Experimental FLUDEX Reservoirs

Abstract

The FLooded Uplands Dynamics EXperiment (FLUDEX) was initiated to quantify carbon dioxide (CO2) and methane (CH4) production in boreal reservoirs, and to better understand underlying biogeochemical processes (dissolved inorganic carbon [DIC] production, net primary production [NPP], methanogenesis, and CH4 oxidation) governing CO2 and CH4 production in flooded boreal landscapes. The study experimentally flooded three upland boreal forest sites with different organic carbon (OC) storage in soils and vegetation over three seasons (June to September 1999–2001). Mass budgets of all reservoir inorganic carbon (inorganic C) and CH4 inputs and outputs were calculated to quantify net reservoir CO2 and CH4 production, and isotopic ratio mass budgets were calculated to quantify biogeochemical processes controlling net reservoir CO2 and CH4 production.The three reservoirs produced both CO2 and CH4 during each of the three flooding seasons, but neither CO2 nor CH4 production was related to overall mass of flooded OC. Net reservoir CO2 production in the second and third flooding seasons (408 to 479 kg C ha−1) was lower than in the first flooding season (703 to 797 kg C ha−1), while reservoir CH4 production steadily increased with each successive flooding season (from 3.2 to 4.6 kg C ha−1 in 1999 to 29.7 to 35.2 kg C ha−1 in 2001). Over three flooding seasons, NPP ranged from 77 to 273 kg C ha–1 and consumed 15 to 40% of gross reservoir CO2 production. CH4 oxidation was negligible during the first flooding season, but reduced gross reservoir CH4 production by 50% during the second flooding season, and by 70 to 88% during the third flooding season. However, despite decreases in net reservoir CO2 production and increases in CH4 oxidation over the study period, the overall total global warming potential (GWP) of the FLUDEX reservoirs remained constant due to successive increases in net CH4 production.