SOIL BACTERIAL CHANGES AS AFFECTED BY GROWING SEASON WEATHER CONDITIONS: A FIELD AND LABORATORY STUDY

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Abstract
During the 1972 and 1974 growing season, soil bacteria, actinomycetes, moisture and temperature in the top 15 cm of a Wood Mountain loam were monitored regularly under sheltered and unsheltered summer-fallow and cropped conditions. The 1972 temperatures and rainfall were later simulated in the laboratory and their effects on the bacterial population in the Wood Mountain loam, a Sceptre heavy clay, and a Melfort silty clay loam determined. Multiple regression was used to quantify the relationships between the different microbial populations and several selected environmental variables. Microbial change was generally directly proportional to moisture change, the latter being the main factor influencing response. The effect of moisture change was usually dependent on the temperature and/or the initial moisture content prior to the change. Soil microbial populations increased in response to both increases and decreases in temperature. The inverse relationship with temperature appeared to be the result of the dry soil surface being wetted by dew which accompanied low night temperatures. Microbial responses were greater in the top 2.5 cm than in the 2.5- to 15-cm segment. Bacteria and actinomycetes responded to environmental stimuli in a similar manner but the response of bacteria was usually greater than that of actinomycetes. The bacteria: actinomycetes ratio was 3:1 in the top 2.5 cm but only 1.3:1 in the 2.5- to 15-cm segment. Cropping the soil had no effect on microbial numbers. In the laboratory study, denitrifier response was more closely related to rainfall than were bacterial or actinomycetal response. Denitrifier response was greatest in the clay and least in the loam.

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