Assessing Changes in Soil Carbon Quantity and Chemistry in Short-Rotation Hybrid Poplar Plantations

There is increasing interest in using short-rotation woody biomass plantations as a source of fiber and as a carbon neutral energy supply. Willow, poplar, and alder are currently used in plantations in areas ranging from the Lake States to the Northwest. As with any cropping system, maintaining soil productivity through succeeding rotations is a key management goal. Where plantations are used to provide carbon sequestration benefits (i.e. bioenergy production), building and maintaining soil carbon stocks is of particular concern. We sampled three hybrid poplar farms in the Northwestern United States; all three farms are in the rain shadow of the Cascades and are on sandy soils. The farms share a similar land use history; originally sagebrush, the land was in annual crops such as peas, onions, and alfalfa, before conversion to poplar. At each farm, soil cores were taken from a field in annual crops, a first rotation poplar stand, and a second rotation poplar stand. Although results varied by farm, soil carbon concentrations were generally higher in the first and second rotation poplar stands than in the row-cropped fields; this was more pronounced in the 0-5 cm and 5-10 cm depths. There were no apparent declines in soil carbon concentration between the first and second rotations. Soil carbon concentrations under poplar were also higher than those in soils from native sagebrush, the original land cover. Analysis of the chemical composition of the carbon using pyrolysis molecular beam mass spectrometry indicates that by the second rotation, the chemical signature of the carbon resembled that found in materials taken from the poplar trees.