Changes in the Carbon Concentration of Organic Matter in Different Soil Types and Horizons in Forested Spodosols.

Knowing the organic matter content of soil provides a tremendous amount of information regarding a soil’s chemical and physical properties, and is relatively easily determined through combustion methods, commonly referred to as loss on ignition.  A conversion factor of approximately 0.5 to 0.58 is commonly used to estimate soil carbon (C) amounts from soil organic matter (SOM) content, and when only percent C data are available, the inverse of the conversion factor can be used to estimate SOM.  However, there is a lot of variation in ratios of C to OM, particularly with depth in a soil profile.  The causes for variation are not well understood, and despite the likelihood for error a single C:OM conversion factor is often used in estimating C stocks at all depths.  Here, we examine C:OM ratio profiles in Spodosols of northern hardwood and coniferous forests in northern Michigan.  Across all samples C:OM ranged from approximately 0.55-0.25, and generally declined with depth.  However, results are not straightforward, and in some cases spodic horizons exhibited lower C:OM ratios than underlying B or C horizons.  Here, we present these patterns and nominate some hypotheses for changes in the stoichiometry of SOM with depth.  We place results in the context of other literature demonstrating declines in C:N ratios with depth in a profile, and explore common mechanisms.