SOIL MICROBIAL ACTIVITY AS INFLUENCED BY TEMPERATURE TRENDS AND FLUCTUATIONS

Abstract

Unamended and NH4-N-amended soils were incubated for 14 days at three different diurnally fluctuating temperatures (T13/2, T18/7, T27/16), and at combinations of these for a further 14 days. Replicate samples were incubated simultaneously at the corresponding constant mean temperatures. Microbial growth was considerably greater at constant mean than at the corresponding diurnally fluctuating temperatures. The pattern of population changes in response to the temperature treatments was similar for bacteria, actinomycetes, and fungi. However, the extent of the growth response by these three groups differed, being greatest for the bacterial and least for the actinomycetal population. An upward shift in temperature after 14 days or maintenance of temperature at the same level for the full 28 days caused an increase in microbial numbers and increased the rate of N transformations. But when the incubation temperature was shifted downward, microbial population levels decreased markedly whereas ammonification and nitrification rates increased significantly, resulting in a temporary flush of mineral-N. This "kill" of microbial cells was much more pronounced with fluctuating than with constant temperature conditions. The validity of this phenomenon was supported by 4 yr field data that showed that the onset of the first cold spell each fall and late frosts in spring resulted in sudden flushes in NO3-N production. Microbial population levels and N transformation rates were generally higher in NH4-N-amended than in unamended soil.