Soil CO₂ Emission Largely Dominates the Total Ecosystem CO₂ Emission at Canadian Boreal Forest

The natural carbon dioxide (CO₂) emission from the ecosystem, also termed as the ecosystem respiration (R_eco), is the primary natural source of atmospheric CO₂. The contemporary models rely on empirical functions to represent decomposition of litter with multiple soil carbon pools decaying at different rates in estimating R_eco variations and its partitioning into autotrophic (R_a) (originating from plants) and heterotrophic (originating mostly from microorganisms) respiration (R_h) in relation to variation in temperature and soil water content. Microbially-mediated litter decomposition scheme representation are not very popular yet. However, microbial enzymatic processes play integral role in litter as well as soil organic matter (SOM) decomposition. Here we developed a mechanistic model comprising of multiple hydro-biogeochemical modules in the soil and water assessment tool (SWAT) code to explicitly incorporate microbial-enzymatic litter decomposition and decomposition of SOM for separately estimating regional-scale R_a, R_h and R_eco. Modeled annual mean R_eco values are found varying from 1,600 to 8,200 kg C ha⁻¹ yr⁻¹ in 2000–2013 within the boreal forest covered sub-basins of the Athabasca River Basin (ARB), Canada. While, for the 2000–2013 period, the annual mean R_a, R_h and soil CO₂ emission (R_s) are varying within 800–6,000 kg C ha⁻¹ yr⁻¹, 700–4,200 kg C ha⁻¹ yr⁻¹ and 1,200–5,000 kg C ha⁻¹ yr⁻¹, respectively. R_s generally dominates R_eco with nearly 60–90% contribution in most of the sub-basins in ARB. The model estimates corroborate well with the site-scale and satellite-based estimates reported at similar land use and climatic regions. Mechanistic modeling of R_eco and its components are critical to understanding future climate change feedbacks and to help reduce uncertainties particularly in the boreal and subarctic regions that has huge soil carbon store.