Forest soil decomposition and its contribution to heterotrophic respiration: A case study based on Canada

Soil carbon (C) stocks are large C reservoirs that are characterized by turnover times of decades to centuries. The ability to predict how long C remains in soils requires an understanding of soil decomposition and the influence of climate change on destabilization processes. This study examined forest soil decomposition and quantified the influence of the choice of decomposition parameters on national-scale heterotrophic respiration (Rh) estimates for the managed forest of Canada. arbon (C) stock estimates from 597 ground plots were used to optimize five decomposition-related parameters of the two slowly decaying C pools of the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3). An exhaustive grid search method found multiple optimal solutions, i.e. combinations of base decay rates and temperature quotients for the slowly decaying C pools. Impacts on the Rh for the 2.3 × 106 km2 of Canadaʹs managed forest were estimated using national inventory and disturbance data for ten optimal combinations of base decay rates and temperature quotients. The Rh was estimated in contemporary (1990) and potential future (2099) temperature conditions for which an increase of 4 °C in mean annual air temperature was assumed. The selection of optimal parameter combination had little impact on the resultant estimates of Rh, with a maximum difference of 1.7 g C m−2 yr−1 or 4.0 Tg C yr−1 (0.54% of total Rh) in 1990 and a maximum difference of 8.2 g C m−2 yr−1 or 18.8 Tg C yr−1 (2.3% of total Rh) in 2099. ow pool within the mineral soil accounted for 54% of the total dead organic matter C stock. It had a disproportionately small contribution of 8.7% to the total contemporary national-scale Rh estimate and was relatively insensitive to temperature changes. If we accept the space-for-time substitution of temperature sensitivities used in the model parameterization, then these results suggest that the mineral soil decomposition modeled by the CBM-CFS3 will result in a weaker positive feedback in response to an increase in global temperatures than is currently anticipated in other models. However, the majority of the Rh comes from relatively small, but quickly decaying pools. The overall temperature quotient was estimated to be 2.15, which would result in an increase in Rh in response to an increase in global temperatures.