Changes in Estimates of Carbon Stocks in a High Shrink/Swell Soil.

Soil profile carbon storage is generally expressed as a mass of carbon per unit area measured to a specific depth.  The use of a fixed depth requires that the surface be used as a reference.  However, this practice is not ideal because the surface can fluctuate for a variety of reasons including compaction, tillage, and shrink/swell of clays.  These changes in surface location manifest as changes in bulk density.  The impact of shrink/swell is of particular importance to Oklahoma soils as the majority (greater than 60%) contain more than 35% montmorillonite clay (OSU Tech. Bull.). This suggests that the bulk density of Oklahoma soils can change as a function of soil moisture.  This change in bulk density may in turn cause the calculated carbon stocks in a soil to change, regardless of soil management.  This could present severe limitations to efforts to monitor changes in carbon stocks in high shrink/swell soils.  Recently, researchers have proposed an alternative to the spatial coordinate method for assessment of soil carbon stocks.  This cumulative mass method calculates carbon stocks found in a constant mass of soil instead of a constant depth and therefore may reduce errors associated with changes in bulk density. However, no efforts have yet been made to test the ability of this new method to improve soil carbon stock measurements.  Therefore, an experiment was initiated in a long-lived Bermudagrass lawn located at the Oklahoma State University, Agronomy Farm in Stillwater, OK on a Norge Loam (Fine-silty, mixed, active, thermic Udic Paleustoll).   Soil samples were collected at near field capacity and near permanent wilting point to evaluate changes in bulk density and calculated carbon stocks as a function of soil moisture.  This data will determine if the cumulative mass method improves the consistency soil carbon stock measurement.