Profile Soil Organic Matter Content Beneath Tree Windbreaks in the U.S. Great Plains.

Tree windbreaks for wind erosion control and crop microclimate modification became a common practice in the U.S. Great Plans following a large tree planting program during an extended drought in the 1930’s. The objective of this study is to quantify soil organic carbon (SOC) stocks beneath tree windbreaks over a range of tree species, soils, previous land use, and climate in 4 Great Plains states (ND, SD, NE, and KS). Two sites were chosen in each state to represent common windbreak plantings and soils of the area. Soil samples were collected from a pit and two adjacent auger holes at 0-10, 10-20, 20-30, 30-50, 50-75, 75-100, 100-125, and 125+ depth increments within the windbreak and in a neighboring field within the same soil map unit. Full profile descriptions were prepared for each pit and soil samples were analyzed for SOC, inorganic carbon (SIC), total nitrogen (TN), particulate organic matter (POM), pH (in water and KCl), particle size, bulk density, and water stable aggregates. Soils beneath tree windbreaks averaged 1.75 kg m^-2 greater SOC stock than adjacent pastures, hay, and row crop fields. Differences ranged from +7.2 kg m^-2 for a 50+ year-old elm windbreak in North Dakota adjacent to a row cropped field to a -3.3 kg m^-2 for a 15 year-old honeylocust planting in South Dakota with an adjacent grass hay field. Differences in SOC stocks were observed over all depth increments with the greatest differences in the 0-75 cm layers. Stable carbon isotope analysis will be used to determine the source of SOC in the horizons and estimate the stability of the SOC and POM. Large differences in bulk density and pH were also observed with potential implications for SOC stabilization and soil health.