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ARS Home » Pacific West Area » Pullman, Washington » Northwest Sustainable Agroecosystems Research » Research » Publications at this Location » Publication #58037


item Smith, Jeffrey
item Bolton, Harvey
item Halvorson, Jonathan

Submitted to: Biology and Fertility of Soils
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/20/1994
Publication Date: N/A
Citation: N/A

Interpretive Summary: The amount of microorganisms in the soil system is important since they contribute to soil health and plant growth. Determining the mass of organisms is not easy due to the vast numbers and different species. A generalized procedure was developed by fumigating the soil and measuring the decomposition of the biomass through gaseous products released. This procedure is dependent upon a non-fumigated control sample for comparison. The problem exists that gas evolving from the control sample was not also evolving form the fumigated sample at the same rate. Thus estimates of microbial biomass were low and sometimes negative. We developed a procedure to correct this discrepancy which would take the gas evolution difference into consideration.

Technical Abstract: Estimates of soil microbial biomass are important for both comparative system analysis and mechanistic models. The method for measuring microbial biomass that dominates the literature is the chloroform fumigation incubation method (CFIM), developed on the premise that killed microorganisms are readily mineralized to CO2 which is a measure of the initial population. Factors that affect the CFIM have been thoroughly investigated over the last 15 years. A question that still remains after countless experiments is the use of an appropriate non-fumigated control for accounting for native soil organic matter (SOM) mineralization during incubation. Our approach was to add hot water leached 14C labeled straw to both fumigated and non-fumigated samples assuming the straw would mimic a recalcitrant C substrate fraction of SOM. The ratio of the 14C evolved from the fumigated sample over the 14C evolved from the control sample would provide a corrected control value to be used in calculating microbial biomass. This experiment was conducted on soils from forest, agricultural, grassland and shrub-steppe ecosystems. The results clearly indicate that equal recalcitrant C mineralization during incubation is not a valid assumption. The results with these soils indicate that on the average only 20% of the control CO2 should be subtracted from the fumigated CO2 for the biomass calculation. The correction value ranged from 18% for agricultural soils to 25% for shrub-steppe soil, with the average correction value being 20%. Our experiments show that corrected biomass values will be 1.5 to 2 times greater than uncorrected biomass values.