Physical Fractionation of Soil Organic Matter Using Sodium Hexametaphosphate Requires Caution.
Kelly K. Moran1, Julie Jastrow1, and Sarah O'Brien2. (1) Argonne National Laboratory, Argonne, IL 60439, (2) University of Illinois at Chicago, 845 W Taylor St M/c 066, Chicago, IL 60607
Soil’s importance as a major sink in global C cycling has led to a myriad of methods to divide this heterogenous pool into meaningful fractions to characterize soil C dynamics. Turnover rates of soil organic matter (OM)-C have been found to decrease with decreasing particle-size, therefore methods that physically fractionate soil may be used to determine C pools of a range of stabilities. Invariably, these schemes require dispersing aggregated soil by agitation in solution. Sodium hexametaphosphate (HMP) solution is a favored chemical dispersant in many studies. Deionized water (DI) may also be used, sometimes with glass beads. Shaking in HMP or DI causes soil C and N to be solubilized and lost. Whether this is a significant amount of C or N is unknown unless recoveries are calculated, and studies sometimes ignore soluble C loss when determining an organic matter fraction by difference. The amount of C loss probably varies with soil properties such as texture, clay mineralogy, and percent organic carbon. This study compared the effects of using HMP vs. DI on recovery of C and N using a high OM, highly aggregated soil from a tallgrass prairie site to compare to a low OM, loosely aggregated soil from a sandy forest site. It was found that soil properties must be considered when choosing an appropriate dispersing agent to minimize C loss. In addition, HMP can also cause difficulty in completely recovering clay-sized particles. Our results suggest that when choosing an appropriate dispersant, the trade-offs between adequately dispersing soils and the dispersant’s effect on the recovery/loss of soil C should be evaluated with regards to the questions being addressed by each study.