Seasonal Fluctuation Patterns of Microbial Numbers in the Surface Soil of a Grassland

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Abstract
Numbers of bacteria and fungi as well as hyphal length were measured in the upper layers of a sown grassland soil in the northern district of Hokkaido during the period 1982–1983. The site, on a heavy clay soil, was a mowed orchard grass sward rejuvenated in 1976. The numbers of bacteria in the 0–2 cm soil layer fluctuated, with peaks occurring twice a year in May and August or September. The fungal spore counts performed by the dilution plate method indicated maximum numbers in August in the 0–2 cm soil layer with a concomitant decrease in hyphal length. These patterns were generally identical over the two years, although the climatic conditions of both years were quite different. And the microbial populations in the deeper layers were small and constant. The changes in the amount of labile organic substrate were similar to those of the bacterial numbers, and the extremely dry soil conditions which occurred in the summer of 1982 seemed to delay the appearance of the second peak. Based on these results, it is assumed that the factor responsible for the peaks of bacterial numbers was the supply of substrates from the vegetation and that soil water status also controlled the appearance of these peaks.