Organic Carbon Accumulation in Topsoil Following Afforestation with Willow: Emphasis on Leaf Litter Decomposition and Soil Organic Matter Quality

Short-rotation intensive cultures (SRICs) of willows can potentially sequester carbon (C) in soil. However, there is limited information regarding the factors governing soil organic C (C-org) accumulation following afforestation. The objectives of this study were to: (i) determine whether willow leads to C-org accumulation in the topsoil (0-10 cm) two to six years after establishment in five SRICs located along a large climatic/productivity gradient in southern Quebec, and (ii) assess the influence of leaf litter decomposition and soil organic matter (OM) quality on C-org accumulation in the topsoil. Topsoil C-org concentrations and pools under SRICs were, on average, 25% greater than reference fields, and alkyls concentrations were higher under SRICs. On an annualized basis, C-org accumulation rates in the topsoil varied between 0.4 and 4.5 Mg ha(-1) yr(-1). Estimated annual litterfall C fluxes were in the same order of magnitude, suggesting that SRICs can accumulate C-org in the topsoil during early years due to high growth rates. Leaf litter decomposition was also related to C-org accumulation rates in the topsoil. It was positively correlated to growing season length, degree-days, and growing season average air and topsoil temperature (r > 0.70), and negatively correlated to topsoil volumetric water content (r = -0.55). Leaf litter decomposition likely occurred more quickly than that of plants in reference fields, and as it progressed, OM became more decay resistant, more stable and accumulated as C-org in the topsoil.