Carbon, organic matter, and bulk density relationships in a forested spodosol

Forest soil on a 23-ha watershed in New Hampshire was sampled quantitatively by excavating 60 0.5-m²pits by horizon in the forest floor and depth strata in the mineral soil. Carbon (C), organic matter (OM), and bulk density (BD) were measured to determine relationships among these properties by soil depth. Because BD and soil depth are frequently used to estimate soil C reserves, we wished to compare actual measured values with values predicted by regression analyses. Bulk density was calculated based on measured soil mass and soil volume exclusive of the > 2-mm coarse fragments. Standard methods were used to determine soil carbon and organic matter. Carbon concentration decreased more rapidly with depth than OM concentration, resulting in a decrease in C/OM ratio from 0.56 in the Oi + Oe of the forest floor to 0.44 in the Bs2 horizon. The accuracy of estimates of soil C reserves can be improved by employing C by OM regressions developed by horizon or depth strata. The relationship between bulk density and organic matter in the mineral soil is described by a regression equation similar to equations developed in other New England forest soils. The similarity between OM and BD regressions observed in this study and those reported for other New England forest soils using other methods supports the use of OM measurements to obtain estimates of bulk density for use in the calculation of soil C pools.