Carbon Sequestration in Fine Roots and Foliage Biomass Offsets Soil CO2 Effluxes along a 19-year Chronosequence of Shrub Willow (Salix x dasyclados) Biomass Crops

Previous greenhouse gas (GHG) assessments for the shrub willow biomass crops (SWBC) production system lacked quantitative data on the soil CO2 efflux (F-c). This study quantifies the mean annual cumulative F-c, the C sequestration in the above- and belowground biomass, and the carbon balance of the production system. We utilized four SWBC fields, which have been in production for 5, 12, 14, and 19 years. Two treatments were applied: continuous production (CP)-shrub willows were harvested, and stools were allowed to regrow, and tear-out (TO) (crop removal)-shrub willows were harvested, and stools were sprayed with herbicide following spring, crushed, and mixed into the soil. Mean annual cumulative F-c were measured using dynamic closed chambers (LI-8100A and LI-8150). Across different age classes, the mean cumulative F-c ranged from 27.2 to 35.5 Mg CO2 ha(-1) year(-1) for CP and 26.5 to 29.3 Mg CO2 ha(-1) year(-1) for TO. The combined carbon (C) sequestration of the standing above- and belowground biomass, excluding stems, ranged from 50.6 to 94.8 Mg CO2 eqv. ha(-1). In the CP treatment, the annual C sequestration in the fine roots and foliage offsets the annual cumulative F-c. Across different age classes, C balances ranged from -21.5 to -59.3 Mg CO2 ha(-1) for CP and 26.5 to 29.3 Mg CO2 ha(-1) for TO. The GHG potential of SWBC is about -36.3 Mg CO2 eqv. ha(-1) at the end of 19 years, suggesting that the SWBC system sequesters C until termination of the crop.