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Influence of Simulated Fall and Spring Conditions on the Soil System. I. Effect on Soil Microflora¹

ABSTRACT

Fall and early spring conditions were simulated by incubating unamended and N-amended Wood Mountain loam (i) at diurnal temperatures of 14 to 3C, and (ii) by slow freezing to –23C followed by incubation at 14 to 3C. Numbers of viable soil organisms were determined by the plate-count method after incubation at 28C. Dilution plates from the freeze-thaw treatments were also incubated at 14, 4.5 and 14 to 3C. Plates incubated at 4.5 and 14 to 3C were reincubated at 28C.

Freezing caused a small but significant reduction in microbial numbers. Thawing at 14 to 3C was extremely lethal to the microflora. The active microflora from a recently cropped soil proved much more susceptible to this type of injury than the resting population of a stored soil. After one freeze-thaw cycle bacteria in recently cropped soil were reduced by 92%, fungi by 55%, and actinomycetes by 33%. Addition of mineral N to stored soil reduced the detrimental effect. Simulated fall conditions caused a significant but smaller reduction of microbial numbers than simulated spring conditions. Both conditions effected qualitative changes in the soil microflora.

When soil dilution plates were incubated at 14 to 3C damage to the microflora was even more pronounced than when soil per se was exposed to simulated spring or fall conditions. It is suggested that the biocidal effect exerted by fluctuating low temperatures as occur in fall and spring can result in rates of soil nutrient cycling which are considerably lower than expected on the basis of mean temperatures.

¹ Contribution from the Research Station, Research Branch, Canada Department of Agriculture, Swift Current, Saskatchewan.

² Soil Microbiologist and Soil Chemist, respectively.

Received for publication July 21, 1970. Accepted for publication February 1, 1971.