The effects of biocidal treatments on metabolism in soil—I. Fumigation with chloroform

This series of five papers is a study of how biocidal treatments influence metabolism in soil, directed particularly towards the flush of decomposition caused by fumigation, and designed to see if the size of this flush can be used as a measure of the soil biomass. Chloroform fumigation caused an immediate increase in the amounts of ammonium and organic C extracted from a soil by 1 N K2SO4. When the CHCl3-treated soil was then inoculated with fresh soil and incubated for 10 days. it consumed 2·8 times more O2, evolved 2·2 times more CO2 and mineralised 7·3 times more N than an unfumigated soil. Extractable organic C decreased by about 40% when the fumigated soil was incubated for 10 days. A second fumigation given immediately after the first produced no further increase in the flush, but some recovery occurred if the soil was incubated between fumigations. However, this recovery was slow and incomplete; a second fumigation given 53 days after the first gave a flush only one-seventh the size of the first. Glucose (or ryegrass) added to the soil and allowed to decompose before fumigation increased the size of the flush. After a 52-day incubation, 29% of the C originally added as 14C labelled glucose remained in the soil; fumigation on the 52nd day increased the evolution of labelled CO2 during the subsequent 10-day period by a factor of 8. Fumigation of a soil that had already been sterilized by 2·5 Mrads of gamma radiation increased the flush slightly; the amount of O2 consumed in 10 days increased from 123 to 137 mg/100 g soil. It is proposed that the flush of decomposition following CHCl3 fumigation is caused by the decomposition of killed organisms by the survivors (or by organisms added in the inoculum) and that organisms are more rapidly and completely attacked after exposure to CHCl3 than after irradiation. On this hypothesis. 10% of the glucose C originally added to the soil was located in the soil biomass after 52 days.