Microwave irradiation of soil for routine measurement of microbial biomass carbon

Abstract

 Microwave irradiation was evaluated as a non-toxic alternate to chloroform fumigation for routine measurement of soil microbial biomass C. Microwave energy was applied to moist soil to disrupt microbial cells. The flush of C released was then measured after extraction or incubation. Microwave irradiation at 800 J g^–1 soil was optimal because this level resulted in an almost instantaneous rise in soil temperature (≥80  °C), an abrupt reduction in microbial activity, maximal release of biomass C, and minimal solubilization of humic substances. Both incubation-CO₂ titration and extraction-colorimetry methods were used on separate 20-g subsamples to compare the labile C in the microwave-treated and untreated soil samples. The incubation-titration method was also used to measure C in chloroform-fumigated soil samples. Averaged across soils, the chloroform fumigation yielded 123.3±5.1 mg CO₂-C kg^–1. Microwave irradiation yielded 93.6±3.9 mg CO₂-C kg^–1 soil determined by incubation and 52.4±2.4 mg C kg^–1 soil determined by extraction, accounting for 76% and 42% of the net flush of C measured by the chloroform fumigation. Microwave-stimulated net flushes of C were correlated closely (r ²=0.974 for incubation or 0.908 for extraction) with microbial biomass C measured by the chloroform fumigation. Little correlation was found with the total soil organic C (r ²=0.241 for incubation or for 0.166 extraction). Mean efficiency factors for incubation (K _MI) or extraction (K _ME) were used to calculate microbial biomass C from net flushes of C between microwaved and unmicrowaved soils. Values of K _MI and K _ME were not affected by soil pH, bulk density or clay contents. Extraction of microwaved soil by 0.5M K₂SO₄ proved to be a simple, fast, precise, reliable, and safe method to measure soil microbial biomass C.