Microwave irradiation of soil for routine measurement of microbial biomass carbon

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 (> or = to 80 degrees 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 (r2 = 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 (r2 = 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.