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.