Measuring microbial carbon: A comparison of four methods for Western Australian soils

Within the soil system, biological properties, including microbial biomass, are typically more sensitive to environmental stresses than physical and chemical properties. Microbial carbon in is an important component of the soil carbon pool, essential to ecosystem functioning and soil health. This fraction is highly sensitive to environmental changes, particularly those associated with land use alterations, due to its rapid cycling and short residence time in the soil. Traditional methods for quantifying microbial carbon include chloroform fumigation, gamma-ray and, less so, microwave irradiation.  The chloroform fumigation and gamma-ray methods are somewhat complex, time-consuming or expensive and there is no clear consensus on the most suitable technique, as their effectiveness depends on factors such as clay content, pH, and water-holding capacity. Each method presents advantages and challenges, influencing their precision, sensitivity, and applicability across different soil types. In this research we analysed soil microbial carbon from twelve soils with different pH, clay content, carbon concentration and land use, using four methods: the chloroform fumigation, direct application of chloroform, gamma ray irradiation with radiation doses of 5, 10, 20, 30, and 40 KGy and microwave irradiation. After applying each treatment, we incubated the soils using the MicroResp method. Our results demonstrated a strong correlation between the methodologies, however, soils with higher clay and carbon content showed more variability between methodologies. The microbial carbon measured by the microwave method was consistent with those determined by the 20 and 30 KGy gamma irradiation method. These findings highlight the importance of incorporating new, cost-effective, and time-efficient methods for measuring sensitive carbon fractions. Such approaches can enhance the accuracy of microbial carbon assessments, particularly in less studied ecosystems which are essential for advancing our understanding of microbial carbon in a global scale.