Soil analysis

Soils from flask resp measurements

These were run for %C and δ^13^C at SIRFER.

Soil extracts

Soil samples were collected at all forest plots. Soils were homogenized, a subsample was fumigated with chloroform, and fumigated and unfumigated samples were then extracted with a salt solution. The extracts must be analyzed for total organic carbon (TOC), total nitrogen (TN), and δ^13^C. There is currently no method for δ^13^C analysis of soil extracts at SIRFER.

Preliminary info

  • 5g soil extracted with 25ml of 0.5M K~2~SO~4~
  • Nicole Trahan's data from similar Niwot Ridge soil extracts show between 200 and 2500 µg C/g of dry soil.
  • If ours are similar, we should expect between 40 and 500 µgC/ml of extract.

TOC/TN Analysis

We are sending subsamples of our extracts (3-4ml) to the Kiowa lab in Colorado. These will be run on a Shimadzu TOC instrument using a combustion with catalyst method.

δ13C Analysis

This analysis will be done at the SIRFER lab. There is currently no protocol for δ^13^C measurements of soil extracts at SIRFER. Originally, we proposed to run the freeze dried extracts on an EA-IRMS unit there. Unfortunately, these extracts contain too much salt for the combustion system. To run 0.4mg of C through the system, a sample containg around 70mg of salt (or more) would need to to be loaded into the instrument. The C in the extracts must be run without this salt (some possible methods for this are in the references below). We will use a form of liquid oxidation to produce CO~2~ from the C in the extracts and then run the CO~2~ to the mass spec from the Gasbench. Here is a proposed outline:

  • Prepare sugar standard extracts (for testing and as reference material during analysis)
  • See sugar standards section
  • Place a small amount of extract in an Exetainer.
  • Scrub out all CO~2~ in the headspace of the vial (vacuum?).
  • Add an oxidant to convert C in extracts to CO~2~. There are multiple possible oxidants.
  • Load exetainers into the Gasbench and run through the MS from there.
  • It may be necessary to purify the CO~2~ before it enters the MS using some type of cryotrapping.

See the full procedure


  • Vance et al, An extraction method for measuring soil microbial biomass C. SBB, 1987
  • Murrage et al,Modification of the original chloroform fumigation extraction technique to allow measurement of δ13C of soil microbial biomass carbon. SBB, 2007
  • Potthoff et al, The determination of d13C in soil microbial biomass using fumigation-extraction
  • Werth and Kuzyakov, 13C fractionation at the root-microorganisms-soil interface: A review and outlook for partitioning studies SBB, 2010