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Title: Molar concentration on k2so4 and soil ph estimation of extractable c with chloroform fumigation-extraction

Author
item HANEY, R - TEXAS A&M UNIVERSITY
item Franzluebbers, Alan
item HONS, F - TEXAS A&M UNIVERSITY
item HOSSNER, L - TEXAS A&M UNIVERSITY
item ZUBERER, D - TEXAS A&M UNIVERSITY

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/2/2001
Publication Date: 9/1/2001
Citation: Haney, R.L., Franzluebbers, A.J., Hons, F.M., Hossner, L.R., Zuberer, D.A. 2001. Molar concentration on k2so4 and soil ph estimation of extractable c with chloroform fumigation-extraction. Soil Biology and Biochemistry.

Interpretive Summary: Microbial biomass is an important biological component of soil that mineralizes nutrients from organic residues, aids in the physical stabilization of soils, and mediates trace gas emissions from soils. Quantifying this important pool of organic matter has been the subject of many investigations using a wide variety of techniques. Unfortunately, not all techniques are adequately suitable under a wide diversity of conditions. We tested whether extractant salt concentration affected estimates of microbial biomass with the chloroform fumigation-extraction method, the most commonly used technique today. Salt concentration did affect estimates of microbial biomass. There appeared to be a bias in microbial biomass estimates that depended upon soil pH. Microbial biomass estimated with chloroform fumigation-incubation without subtraction of a control was better related to basal soil respiration and organic matter content of soil than was microbial biomass estimated with chloroform fumigation-extraction with different extractant soil concentrations. The commonly used chloroform fumigation-extraction method may not be reliable in a wide range of soils and therefore, should be employed with caution.

Technical Abstract: Methods of determining soil microbial biomass need to be reliable and produce consistent results across soils with a wide range of properties. We investigated the effect of extractant molarity (distilled water and 0.001, 0.01, 0.1, and 0.5 M K2SO4) on the flush of C (i.e., the difference between fumigated and unfumigated subsamples) with the chloroform fumigation-extraction (CFE) method in soils of different pH. The ratio of extractable C in water to that in 0.5 M K2SO4 for five acidic soils was 1.5 +/- 0.3 in unfumigated controls, 1.4 +/- 0.2 in fumigated samples, and 1.8 +/- 0.7 in fumigated minus control flushes, respectively. Ratios in six alkaline soils were 1.0 +/- 0.2, 0.9 +/- 0.2, and 0.8 +/- 0.2, respectively. Flocculation or dispersion of organic colloids and changes in the diffuse double layer surrounding clay particles are possible reasons for differences in extractable C with changes in extractant molarity and soil pH. CFE with any of the extractants was less related to soil organic C and potential C and N mineralization than was chloroform fumigation-incubation without subtraction of a control. Changes in microbial biomass estimates with changes in extractant molarity and soil pH suggest that CFE may not be reliable in a wide range of soils.