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Soil analysis

Soils from flask resp measurements

These were run for %C and δ13C 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 δ13C. There is currently no method for δ13C analysis of soil extracts at SIRFER.

Preliminary info

  • 5g soil extracted with 25ml of 0.5M K2SO4
  • 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 δ13C 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 CO2 from the C in the extracts and then run the CO2 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 CO2 in the headspace of the vial (vacuum?).
  • Add an oxidant to convert C in extracts to CO2. There are multiple possible oxidants.
  • Load exetainers into the Gasbench and run through the MS from there.
  • It may be necessary to purify the CO2 before it enters the MS using some type of cryotrapping.

See the full procedure

References

  • 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