Soil microbial biomass estimates using 2450 MHz microwave irradiation or chloroform fumigation followed by direct extraction

Microwave irradiation and chloroform fumigation followed by direct extraction were compared as potential methods to estimate microbial biomass carbon and nitrogen in grassland soils. Soil amended with 700 ppm C and 70 ppm N and unamended soil were incubated at 25°C for 3 days, irradiated for various time intervals and extracted with 0.5 M K2SO4. An increase in microwave time resulted in an increase in extractable C in both amended and unamended soils. A 2-min irradiation time was selected as the optimal treatment because it resulted in a difference between amended and unamended soils close to what was expected and avoided excessive heating of the sample. CHCl3 fumigation for 24 h yielded (mean±SD) 138.2±18.5 ppm extractable C and 11.6±6.7 ppm extractable N in the unamended soil and 315.5±48.3 ppm extractable C and 17.9±3.0 ppm extractable N in the amended soil. Compared to the CHCl3 treatment, the 2-min microwave treatment resulted in 70.6% less extractable C and 79.3% less extractable N in the unamended soil and 52.2% less extractable C and 58.3% less extractable N in the amended soil. Sequential fumigation followed by irradiation produced extractable C and N values similar to fumigation alone, while fumigating twice for a total of 48 h resulted in higher extractable C in unamended soil and higher extractable N in amended soil. These results indicate that although trends of treatments are similar, microwave irradiation is not as effective as CHCl3 fumigation for microbial biomass estimates in the soils examined