Microbial biomass in soil: Effects of some experimental variables on biochemical estimations

The effects of experimental variables on estimates of biomass C and mineral-N (Min-N) flush by the chloroform fumigation technique were determined in near neutral to slightly alkaline topsoil samples of a Typic Haplaquoll taken at three different times under grazed grass-clover pastures. The variables were soil mesh size ( < 3.3 and < 2 mm), water content 50 and 60% of water-holding capacity (WHC)], the use of samples that were “fresh” or that had been previously incubated (7 days at 50% of WHC at 25°C), and, for the biomass C estimates, various incubation periods for measuring CO₂C production.Estimates of biomass C were most strongly influenced by the incubation period selected for CO₂C production by unfumigated soil. The effects of soil mesh size and water content were significant for some samples, but were not consistent. Prior incubation lowered all biomass C estimates significantly, except for some samples where a 0–10 day period was used for measuring CO₂C production by unfumigated soil; (the presence or absence of soda-lime during incubation had no influence on subsequent rates of CO₂ production by the unfumigated samples).Min-N flush was not consistently influenced by these variables, although some significant treatment effects occurred. Biomass C-to-Min-N flush ratios were predictably dependent upon the biomass C estimates used. They averaged 9.0 in “fresh” samples and 6.0 in incubated samples, when the incubation periods for CO₂C production by unfumigated samples were 10–20 and 0–10 days respectively.Results indicate that care should be taken in interpreting and comparing biochemical indices of microbial biomass, and their ratios, obtained by different or modified experimental procedures.