Bulk Density and Carbon Concentration Variance Influence on Soil Carbon Stock Measurements

Soil carbon (C) storage is generally expressed as a mass of C per unit area measured to a specific depth. This practice is not ideal as the soil surface can fluctuate for several reasons including compaction, tillage, and shrink/swell of clays. This change in bulk density may cause the calculated C stocks in a soil to change from one sampling period to the next. In soils with high shrink/swell capacities, the bulk density has the potential to change regardless of soil management. This could present limitations to efforts to monitor changes in C stocks in high shrink/swell soils. Therefore, a study was conducted in two soils with shrink/swell capacity, in which they were sampled to a depth of 90cm at two or three different moisture conditions and analyzed for bulk density and organic C at 10-cm increments. Profile C stocks were analyzed using the conventional fixed depth method and also with a fixed mass method. The analysis showed that under moist conditions the swelling of clays did not decrease bulk density. On the contrary, moist conditions generally resulted in elevated bulk density values. This resulted in significant differences in C stocks within depth increments where the variation in C concentration was low. Differences in profile C stocks, as determined using the fixed depth method, caused by differences in bulk density were removed by using the fixed mass method of calculating C stocks. The fixed mass method provided measurements of C stocks, which vary only as a result of variation in C concentrations.