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

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Agriculture, Ecosystems and Environment

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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.

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