The Importance of Rock Fragment Density for the Calculation of Soil Bulk Density and Soil Organic Carbon Stocks

To determine the bulk density of the fine fraction (BDFF) and soil organic carbon (SOC) stocks from core samples of a known volume, the volume of discarded rock fragments (RFs) has to be known. Measuring the RF density (rho(RF)) is labor intensive and time consuming, so the volume of RFs is often estimated based on measured RF mass and constant values for rho(RF). In this study, we determined the rho(RF) of 87 soils in Germany and showed how different rho(RF) of limestone affected the calculation of BDFF and SOC stocks. For limestone, the measured rho(RF) in our soils was lower (2.49 +/- 0.14 g cm(-3)) than a constant value of 2.70 g cm(-3). The average difference in BDFF and SOC was 2.30 and 2.35%, respectively, which was lower than errors that arose from sampling or analytical techniques. However, differences in the BDFF and SOC stocks can be as high as 37% when rho(RF) is below 2.20 g cm(-3) and there is an increase in the subsoil RF content of up to 70%. Therefore, accurately determining the rho(RF), at least in the subsoil, is recommended when RFs dominate the total volume of the sample to reduce potential measurement errors. The necessity of using the actual rho(RF) instead of using a constant value depends on the variation in rho(RF), the RF content, and the intended accuracy of the BDFF and SOC stock estimation.