Evaluation of the microwave irradiation method for measuring soil microbial biomass

Soil microorganisms are an important and labile component of soil organic matter. We investigated the feasibility of measuring soil microbial biomass using microwave irradiation as a rapid and nontoxic alternative to chloroform fumigation. The efficacy of microwave irradiation was affected by the distribution pattern and location of soil samples in the oven, the amount of soil, soil moisture content, irradiation time, and soil properties. To ensure uniform and consistent microwave application to soil samples, we modified the microwave treatment procedure (i) by using a round sample rack to improve the uniformity of microwave delivery; (ii) by adjusting the moisture of different soils to the same percentage of their weight; (iii) by using a constant sample weight and irradiation time; and (iv) by grouping soil samples according to their clay contents. The results using 30 Australian soils, with a wide range in soil properties, showed that organic C released by microwave irradiation had weak or no correlation with the microbial biomass C measured by chloroform-fumigation extraction, chloroform-fumigation incubation, and substrate-induced respiration methods; while the correlations between the measurements of the three conventional methods were generally close. The increase in extractable C after microwave treatment was often accompanied by an increase in the optical density of the soil extract at 410 nm, which was indicative of the liberation of humified substances by microwave irradiation. We conclude that the organic C liberated from soil by microwave irradiation may contain an appreciable portion of nonbiomass compounds