Abstract
This paper reviews our studies on the protozoan distribution and their behaviour within and among soil aggregates. Our studies were performed with the idea that the aggregate structure involving pores of various size is the key concept in the analysis of microenvironment of soil microorganisms. Aggregates (1-2mm in diameter) taken from sandy clay loam soil were used throughout these studies. The main results are as follows: (1) Detected protozoan groups differed noticeably from aggregate to aggregate. This suggests that protozoa cannot migrate easily from aggregate to aggregate under field conditions. (2) As most protozoan cells were easily washed out into the water, we consider that these organisms usually inhabit on the surface or in non-capillary pores opened to the surface. Some of amoebae, flagellates and small ciliates cells, however, persisted in being washed out. These cells are considered to have inhabited in closed non-capillary pores inside the aggregates. (3) When Colpoda sp. (protozoa) and Klebsiella pneumoniae (bacterium) were introduced to sterilized aggregates, Colpoda sp. ate the bacterial cells anchored at the surface and in the open pores. The protozoa were not able to consume the bacterial cells in non-capillary pores. (4) In sterilized aggregates, the percentage of eaten bacterial cells at the surface and in the open pores increased with the amount of added protozoan cells in a range from ca. 0.3 to ca. 50 cells per aggregate at 60% W. H. C. Protozoan movement between and within soil aggregates is limited under this moisture condition. (5) Under higher conditions of moisture, however, the protozoa ate a much higher percentage of bacterial cells than at 60% W. H. C. The increase in the moisture content must facilitate the movement of the protozoa within and among aggregates.
