Minnesota Soil Survey Sites Revisited: Long-Term Trends In Soil Carbon.

Sites in different ecoregions of Minnesota were resampled and analyzed in 2009 and 2010 to compare C concentrations to those from archived soil survey samples stored for 30-40 years. Overall findings of the project show that for 102 sites, mean C concentration had increased significantly for forest soils (+38%) and decreased in cropped soils (-13%) over mean depths of 60-75 cm.  Grassland soil C increases were not significant due to small sample numbers (n=9).  When averaged for each horizon, carbon increased significantly in the top two horizons (0-10 cm and 10-33 cm) at forest sites, while grassland sites had significant increases only in the second horizon (20-41 cm).  Cropland sites had mean decreases of 24% in the surface horizon. Where sites had undergone changes from cropland to grassland, C concentration increased 53% (from 12.2 to 18.8 g C kg^-1 soil for 12 sites), but site numbers were too small to see C differences for other management conversions. Regional differences in C changes reflected the predominant managements sampled in each area.