Notes on protozoa in agricultural soil with emphasis on heterotrophic flagellates and naked amoebae and their ecology.

Heterotrophic flagellates and naked amoebae are usually very numerous in agricultural soils; with numbers in the magnitude of 10,000 to 100,000 (active+encysted) cells per gram of soil. In 'hotspots' influenced by living roots or by dead organic material, the number may occasionally be as high as several millions per gram of soil. An exact enumeration of these organisms is virtually impossible. As they most often adhere closely to the soil particles, direct counting will underestimate numbers since the organisms will be masked. The method usually applied for enumeration of these organisms, the 'most probable number (MPN) method', is based on the ability of the organisms to grow on particular culture media. This method will in many cases underestimate the total protozoan number (active+encysted). It is uncertain how many of the heterotrophic flagellates and naked amoebae are actively moving and how many are encysted at a particular time; the 'HCl-method' which has usually been used to discriminate between active and encysted has proven to be highly unreliable. Despite the methodological difficulties many investigations of these organisms indicate that they play an important role in agricultural soils as bacterial consumers, and to a minor extent as consumers of fungi. Because of their small size and their flexible body they are able to graze bacteria in small pores in the soil in which larger organisms are precluded from coming. Key factors restricting the number and activity of heterotrophic flagellates and naked amoebae in soils seem to be water potential and soil structure and texture. In micro-cosm experiments, small heterotrophic flagellates and naked amoebae regulate the size and composition of the bacterial community. Bacterial activity seems to be stimulated by these organisms in most cases as well as the mineralization of carbon and nitrogen and possibly other mineral nutrients. In the rhizosphere of living plants the activity of protozoa has proven to stimulate uptake of nitrogen in pot experiments, and it has been hypothesized that organic matter liberated by plants in the root zone will stimulate bacterial and protozoan activity, leading to mineralization of organic soil nitrogen which is subsequently taken up by the plants.