Varied Carbon Content of Organic Matter in Histosols, Spodosols, and Carbonatic Soils

Soil organic carbon (OC) content is often estimated by determining organic matter (OM) content via mass loss on ignition (LOI) and assuming that there are 580 g OC kg(-1) OM. We utilized thermogravimetry (TG), oven-determined mass LOI, and the Walkley-Black (WB) wet oxidation method to evaluate the relationship between OC and OM contents of carbonatic and organic soils from Florida and Puerto Rico and Spodosols from Florida. These soils have contrasting components and formed under different ecological settings. The OC content of OM, based on TG-determined OM mass LOI (TG OM) and WB OC, was higher for Histosols and Spodosols than for carbonatic soils. Regression coefficients between WB OC and TG OM were high for all soils. The factor to convert OM to OC, based on TG OM and WB OC, was 0.56 +/- 0.04 (+/- 0.04 is the 95% confidence interval) (R-2 = 0.93) for carbonatic soils, 0.67 +/- 0.06 (R-2 = 0.94) for Spodosols, and 0.69 +/- 0.04 (R-2 = 0.99) for Histosols. Results show that accuracy in estimating OC from OM content may be improved if the conversion equation is developed from soils within limited compositional and/or biogeochemical ranges. As such, there is no universal conversion factor between OM and OC for all soils. Loss on ignition is not a reliable OM measure if there are mass losses below 200 degrees C that are mistakenly attributed to OM combustion, as in the case of reversible dehydration for Histosols of this study. However, TG enables detection of and accounting for such mass losses. Isothermal TG also confirmed that reversible mass loss between 105 and 200 degrees C was energy-rather than time dependent.