MICROWAVE IRRADIATION OF SOIL FOR ROUTINE MEASUREMENT OF MICROBIAL BIOMASS CARBON

Microwave irradiation was evaluated as a non-toxic alternate to chloro form fumigation for routine measurement of soil microbial biomass C. M icrowave energy was applied to moist soil to disrupt microbial cells. The flush of C released was then measured after extraction or incubati on. Microwave irradiation at 800 J g(-1) soil was optimal because this level resulted in an almost instantaneous rise in soil temperature (g reater than or equal to 80 degrees C), an abrupt reduction in microbia l activity, maximal release of biomass C, and minimal solubiliza tion of humic substances. Both incubation-CO2 titration and extraction-colo rimetry methods were used on separate 20-g subsamples to compare the l abile C in the microwave-treated and untreated soil samples. The incub ation-titration method was also used to measure C in chloroform-fumiga ted soil samples. Averaged across soils, the chloroform fumigation yie lded 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 t he net flush of C measured by the chloroform fumigation. Microwave-sti mulated net flushes of C were correlated closely (r(2) = 0.974 for inc ubation 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) we re 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 a ffected by soil pH, bulk density or clay contents. Extraction of micro waved soil by 0.5 M K2SO4 proved to be a simple, fast, precise, reliab le. and safe method to measure soil microbial biomass C.