The assessment of microbial biomass C in subsoil samples using fumigation-extraction is negligibly affected by residual chloroform

Abstract

The potential overestimation of soil microbial biolmass C, using the fumigation-extraction method, arising from the contribution of residual (i.e., non-evacuated) chloroform (CHCl₃) to the fumigation flush of C, was recently shown to be minor for agricultural topsoil samples. However, we hypothesized larger flush proportions of residual CHCl₃–C, and hence greater biomass overestimations in low-biomass, subsoil samples due to the characteristically lower fumigation flushes of C in subsoils. The depth stratification (0–100 cm) of fumigation-extractable C and CHCl₃–C was determined in six soil profiles (three loesses ranging from 6 to 20% clay and three vertisols ranging from 46 to 69% clay). Residual chloroform levels correlated positively with soil organic matter content but not with clay content and decreased down the profiles. Residual chloroform contributed ∼2.0% to biomass-C overestimation, independent of soil type or depth, thus refuting the study hypothesis and supporting the safe use of the fumigation-extraction method also with subsoil samples.

Keywords:

• Chloroform fumigation-extraction
• clay content
• extractable chloroform
• microbial biomass overestimation
• organic matter content
• soil depth

Acknowledgments

The authors express their gratitude to Dr. Guy Levy (ARO), Prof. Yosef Steinberger (Bar-Ilan University, Israel) and Ms. Damilola Adeyanju (Hebrew University of Jerusalem, Israel) for assistance in soil analyses.

Additional information

Funding

This work was financially supported by the Chief Scientist, Ministry of Agriculture and Rural Development, State of Israel (Research project 20-03-0001).