ARS | Publication request: Soil microbial biomass carbon measurement using microwave irradiation: effects of soil water content, texture and temperature on microbial cell kill and C release

Submitted to: Soil Ecology Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: February 10, 2007
Publication Date: N/A

Technical Abstract: Fumigation-based methods of soil microbial biomass carbon (C) have been replaced in many labs by microwave (MW) irradiation-based methods to reduce hazardous chemical use. Sine the introduction of the MW method concerns have been raised about the use of water filled porosity (WFP) for water content and effects of the total MW energy on the measurements of biomass carbon. The reported strong correlation between chloroform fumigation and MW irradiation, has failed to be reproduced. Possible reasons include inaccuracy of temperature readings, uneven distribution of power output within the microwave oven, water content variations, clay content variations, size of the soil sample, release of humic C compounds and heat absorption by the soil containers. Three soils ranging in clay content from 6 to 20 percent were treated with different amounts of MW energy at different water contents determined by various methods. Within a microwave field, heat is produced by the oscillation and friction of polar molecules. Pure free water oscillates less than water bound near a biological membrane, making water matric potential perhaps a better measurement of water content in a MW heated soil. In addition to measuring carbon flux after microwave irradiation, samples were tested for live microbial cells using a BacLight live/dead assay (Invitrogen). The kit quantifies the ratio of live cells to dead cells, producing a flourometric picture of microbial cell kill for each experimental parameter. Optimizing the MW irradiation method will provide a quick, less toxic, more accurate method for analyzing soil microbial biomass carbon.