Soil Carbon Dynamics of Tree Plantings for Woody Biomass Feedstock.

Agroforestry practices are being considered for their bioenergy potential as the wood could be harvested for direct combustion, cellulose to ethanol conversion, or pyrolysis to bio-oils. The objective of this project was to use spatially-distributed soil sampling and soil profile descriptions to determine the soil carbon sequestration potential of tree plantings across climatic gradients in the Central Russian Uplands and the U.S. Great Plains. Three Russian sampling locations spanned a gradient of mean annual temperature from 5.3 to 5.8 °C and precipitation from 480 to 580 mm. Three U.S. sampling locations had ranges of annual temperature from 3.9 to 9.2°C and precipitation from 450 to 690 mm. Tree species at the sampling sites included black poplar (Populus nigra), silver birch (Betula pendula), box elder (Acer negundo), English oak (Quercus robur) and balsam poplar (Populus balsamifera) in Russia and green ash (Fraxinus pennsylvanica), redcedar (Juniperus virginiana), Siberian elm (Ulmus pumila, L.), mulberry (Morus rubra, L.), and cottonwood (Populus deltoides) in the U.S. Trees ranged in age from 19 to 70 years.  Soil core samples (30 cm depth) were taken in a grid across each windbreak and subdivided into 0-15 and 15-30 cm sections. Soil pit and auger samples to 1.2 m or greater were collected in each land cover (tree, crop, and undisturbed grassland as reference). Samples were analyzed for organic (SOC), inorganic (SIC), and permanganate oxidizable (POXC) carbon as well as for stable carbon isotope signature (del 13C, natural abundance).  Soil pH and total nitrogen (TN) content were also determined. Averaged across all sampling sites, SOC content in the surface 30 cm beneath the trees averaged 22.4% greater than the adjacent cropped fields. Stable carbon isotope analyses were used to determine the source of the SOC and SOC mean residence times and turnover rates. Results from two U.S. sites indicate that a significant proportion of the SOC accumulation could be attributed to tree sources. The relationship between SOC accumulation and climate as characterized by the hydrothermal coefficient of Selyaninov (1928) enables the estimation of soil carbon stocks in existing windbreaks and the potential to predict carbon sequestration in future plantings.