Exploring Methods for Reducing Uncertainty in Determining the Bulk Density of Soils

In response to ongoing anthropogenic climate change, scientists have examined carbon sequestration in soils as a climate mitigation strategy that could lower greenhouse gas concentrations in the atmosphere. This approach involves quantifying rates of carbon removed from the atmosphere by plants, and rates of plant carbon transferred and stored in soil. In order to improve global carbon accounting, our study seeks to reduce the error in calculating soil bulk density, a key parameter used to determine soil carbon stocks. Depth-based approaches to calculate soil carbon stocks are often used, but do not account for the effect of soil bulk density changes on the vertical distribution of soil mass. A theoretically more accurate approach is to calculate soil carbon stocks on an equivalent soil mass (ESM) basis. The theoretical considerations of this approach have been previously reported, but rarely has it been tested against depth-based approaches on a large scale. Here we compare depth-based to ESM-based approaches to calculate soil carbon stocks on Californian rangeland ecosystems that have received organic matter amendments (compost) as part of climate change mitigation research. We evaluate whether the ESM-based approach reduces error and improves ability to detect carbon changes in soil. We also aim to build on the existing methodologies for bulk density calculation by finding ways to improve the ESM method through a combination of field work and laboratory testing, as well as simple mathematical modeling. The soil samples for lab work were obtained from the Sierra Foothill Research and Extension Center run by the University of California. Sampling was performed on soils at 10-centimeter layers and different soil treatment types; treated plots received 57.4 Mg compost ha^-1. The soil samples were sieved to remove plant fragments and rocks, and then ground for a carbon and nitrogen analysis. The results highlight the advantages and disadvantages of using the ESM versus traditional bulk density measurements in systems that are receiving organic matter amendments.