Soil microbial carbon
Microbial carbon is the carbon contained in the living microorganisms of the soil. It is also referred to as “microbial biomass” or “Living Organic Matter” (LOM). It is an indicator of the abundance of microorganisms (bacteria and fungi) in the soil.
Results are expressed both in absolute values (LOM in mg of organic carbon/kg of dry soil) and in percentage of total organic carbon in the soil (LOM as % of TOC).
Standard
The measurement method is the chloroform fumigation-extraction method (Jenkinson & Powlson, 1976; Chaussod et al., 1988; Wu et al., 1990).
NF EN ISO 14240-2: Soil quality - Determination of soil microbial biomass - Part 2: fumigation-extraction method.
Sampling and logistics
Analysis on fresh soil.
Description of the measurement method
The method consists of measuring the carbon released from the cell lysis of microorganisms by chloroform fumigation.
The result corresponds to the difference in carbon content between a fumigated and a non-fumigated sample. This carbon difference is directly proportional to the amount of microorganisms killed by the chloroform.
Example ranges of variation
| Situation | Min | Max | Median |
| Vineyard soil / IFV-Sicarex Beaujolais, 2021
Nb obs = 61 Depth: 0-20 cm |
24 mg C/kg soil | 540 mg C/kg soil | 81 mg C/kg soil |
| 0.4 % of total C | 7.2 % of total C | 1.4 % of total C | |
| Arable and and polyculture-livestock soil / France
Nb obs: 183 Depth: 0-20 cm |
114 mg C/kg soil | 690 mg C/kg soil | 309 mg C/kg soil |
| 0.9 % of total C | 4.8 % of total C | 2.3 % of total C |
Interpretation
The higher the microbial carbon content, the greater the quantity of microorganisms. This indicator must be interpreted according to soil type (for example texture) because it strongly influences the measured level. There is a positive correlation between microbial biomass and organic matter mineralization (the more microorganisms, the more organic matter is mineralized).
A positive link with structural stability has also been observed. Microbial biomass is a sensitive and early indicator of changes in soil organic status; thus, if the soil is disturbed, microbial biomass increases or decreases rapidly. Values expressed as % of Corg (i.e. Cmicrobial/Corg x 100) allow assessment of changes in microorganism quantities independently of changes in carbon content. If the Cmicrobial/Corg ratio increases, the proportion of LOM increases and conversely if the ratio decreases, the proportion of LOM also decreases.
Advantages and limitations
Advantages
- Indicator of microorganism quantity fairly well established
- Usable in advisory situations
Limitations
- Must be performed on a fresh soil sample
- This indicator must be interpreted according to certain physico-chemical soil parameters, notably texture, pH, and organic carbon content
- Cost: €60-80
Source
- RMT Bouclage. 2025. Biological functioning indicators of agricultural soils. [03/11/25]. https://www.rmt-fertilisationetenvironnement.org/moodle/pluginfile.php/5041/mod_resource/content/3/Recueil%20indicateurs%20de%20fonctionnement%20biologique%20des%20sols%20agricoles_vf251009.pdf
- Chaussod R, Houot S. 1993. Soil microbial biomass: perspectives for using this measurement to estimate nitrogen supply by soils. 5th Forum of Fertilization Management, Blois, November 16-18, 1993, pp. 17-26.
- Chaussod R, Houot S, Guiraud G, et al. 1988. Size and turnover of the microbial biomass in agricultural soils: laboratory and field measurements. Deschamps T, et al. (2022). Guide to interpreting bioindicator analysis. https://www.arvalis.fr/infos-techniques/%20douze-indicateurs-pour-evaluer-la-fertilite-biologique-du-sol
- Jenkinson D S, Powlson D S. 1976. The effects of biocidal treatments on metabolism in soil—V: A method for measuring soil biomass. Soil Biology and Biochemistry, vol. 8, no 3, pp. 209-213.
- Wu J, Pommerening B, Chaussod R, et al. 1990. Measurement of soil microbial biomass C by fumigation-extraction – an automated procedure. Soil Biology and Biochemistry, vol. 22, no 8, pp. 1167-1169.
