Measurement of Soil Microbial Biomass Carbon and Nitrogen

Importance of the Topic

The quantification of soil microbial biomass carbon (MBC) and nitrogen (MBN) is essential for understanding soil health, carbon sequestration potential, and nutrient cycling. Soil microorganisms drive decomposition of organic matter, influencing greenhouse gas emissions and fertility. Reliable measurement of microbial biomass informs ecological research, agricultural management, and environmental monitoring.

Objectives and Study Overview

This study demonstrates simultaneous measurement of MBC and MBN in soil extracts using a Shimadzu TOC-L total organic carbon analyzer paired with a TNM-L total nitrogen unit. The goal was to apply the chloroform fumigation–extraction method with automated analysis to enhance throughput, reproducibility, and data quality across diverse soil depths at two locations.

Methodology

Soil samples from topsoil and subsoil at two sites underwent chloroform fumigation for 24 hours to lyse microbial cells. Both fumigated and non-fumigated subsamples were extracted with 0.5 M potassium sulfate. Extracts were diluted fivefold to protect catalysts and combustion tubes from high salt content. The non-purgeable organic carbon (NPOC) method, involving acidification and sparging, removed inorganic carbon before measuring total carbon. Total nitrogen was determined in parallel. Two-point calibration curves were constructed for carbon (0 and 50 mgC/L) and nitrogen (0 and 10 mgN/L), with blank corrections for water background. MBC and MBN were calculated from the difference between fumigated and non-fumigated measurements divided by literature conversion factors (kEC = 0.45, kEN = 0.54).

Used Instrumentation

• Shimadzu TOC-L total organic carbon analyzer
• Shimadzu TNM-L total nitrogen unit
• Shimadzu ASI-L autosampler
• TOC/TN catalyst and high-salt combustion tube kit

Main Results and Discussion

Results showed consistently higher MBC and MBN in topsoil compared to subsoil at both locations, reflecting elevated microbial activity near the surface. MBC concentrations in fumigated topsoil ranged over 172–221 mgC/L, versus 54–71 mgC/L in subsoil. Corresponding MBN values were 19.9–27.5 mgN/L versus 3.8–4.5 mgN/L. Repeatability was high, with data from multiple injections showing minimal variance. The combined TOC-L/TNM-L system proved effective for concurrent C and N analysis without manual intervention.

Benefits and Practical Applications

The described approach offers:

• Simultaneous TOC and TN measurement, reducing analysis time.
• Automated dilution to protect analytical components from salt damage.
• High sample throughput via autosampler integration.
• Robust reproducibility suitable for large-scale soil surveys, quality control, and research projects.

Future Trends and Application Opportunities

Advances may include integration with hyphenated techniques, miniaturized field-deployable analyzers, and machine-learning-driven data processing. Expanding application to other environmental matrices, such as sediments, wastewater sludge, and compost, can broaden understanding of microbial processes. Enhanced automation and real-time monitoring will further streamline soil health assessment.

Conclusion

The Shimadzu TOC-L and TNM-L system, combined with the chloroform fumigation–extraction method and ASI-L autosampler, provides a rapid, reliable, and high-throughput solution for quantifying soil microbial biomass C and N. Its automated dilution and specialized combustion tube kit ensure instrument longevity and consistent performance across diverse sample types.

References

1. Vance ED, Brookes PC, Jenkinson DS. An extraction method for measuring soil microbial biomass C. Soil Biology and Biochemistry. 1987;19(6):703-707.
2. Joergensen RG. The fumigation-extraction method to estimate soil microbial biomass: Calibration of the kEC value. Soil Biology and Biochemistry. 1996;28(1):25-31.
3. Joergensen RG, Mueller T. The fumigation-extraction method to estimate soil microbial biomass: Calibration of the kEN value. Soil Biology and Biochemistry. 1996;28(1):33-37.