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Abstract:  Protozoa are important predators of bacteria in soil and protozoan predation is one of the main factors responsible for the decline of bacterial populations introduced into soil. Bacteria, however, are not equally susceptible to protozoan predation. We have studied the response of indigenous protozoan populations to the introduction of the polychlorophenol-degrading bacterium Mycobacterium chlorophenolicum PCP-1 (DSM 43826), and Pseudomonas chlororaphis (ATCC 43928), into soil microcosms. Introduction of P. chlororaphis to the soil resulted in a huge increase in the numbers of heterotrophic flagellates and naked amoebae during the first 8 days of the experiment. Addition of M. chlorophenolicum to soil caused only a slight increase in protozoan numbers, which was similar to the increase caused by addition of water. There was no indication that addition of M. chlorophenolicum to soil resulted in any increase in the number of protozoa able to feed on this bacterium. The number of colony forming units (CFU) decreased rapidly in the treatment amended with P. chlororaphis cells, whereas there was no decrease in CFUs in the M. chlorophenolicum treatment. The only slight increase in protozoan numbers in the M. chlorophenolicum treatment, as well as the apparently low mortality rate of M. chlorophenolicum in the soil microcosms, coincided with significantly lower soil respiration in the soil microcosms amended with M. chlorophenolicum compared to those amended with P. chlororaphis. The results suggest that the indigenous soil protozoa did not graze on M. chlorophenolicum at all, presumably because it is not a suitable food source.

Abstract: The effects of incorporation of elm leaves (Ulmus glabra) into an agricultural sandy loam soil by earthworms (Lumbricus festivus) on the bacterial and protozoan populations were investigated. Three model systems consisting of soil, soil with leaves, and soil with leaves and earthworms, respectively, were compared. The total, viable, and culturable number of bacteria, the metabolic potentials of bacterial populations, and the number of protozoa and nematodes were determined in soil size fractions. Significant differences between soil fractions were shown by all assays. The highest number of microorganisms was found in microaggregates of 2–53 μm and the lowest in the <0.2μm fraction. A major part of the bacteria in the latter fraction was viable, but non-culturable, while a relatively higher number of culturable bacteria was found in the macroaggregates. The number of colony-forming units and 5-cyano-2,3-ditolyl tetrazolim chloride (CTC)-reducing bacteria explained a major part of the variation in the number of protozoa. High protozoan activity and predation thus coincided with high bacterial activity. In soil with elm leaves, fungal growth is assumed to inhibit bacterial and protozoan activity. In soil with elm leaves and earthworms, earthworm activity led to increased culturability of bacteria, activity of protozoa, number of nematodes, changed metabolic potentials of the bacteria, and decreased differences in metabolic potentials between bacterial populations in the soil fractions. The effects of earthworms can be mediated by mechanical mixing of the soil constituents and incorporation of organic matter into the soil, but as the earthworms have only consumed a minor part of the soil, priming effects are believed partly to explain the increased microbial activity.

Adding fresh roots to intact soil cores resulted in marked increases in microbial and microfaunal activity at the resource islands. Microbial activity increased in two phases following root addition. Respiratory activity and concentration of respiratory enzyme (dehydrogenase) in soil adhering to the roots was very high during the first three weeks resulting in anaerobic conditions in the soil. After a period of low respiratory activity and enzyme content, these quantities increased from 6 to 20 weeks, but not enough to maintain anaerobic conditions. Numbers of protozoa peaked earlier than the nematodes. Based on yield coefficients of microbes and bacterivores, the increase in bacterivores was in accordance with root-induced respiration activity. In soil adhering to roots, numbers of bacterial grazers (protozoa and nematodes) were up to 80 and 30 times higher, respectively, than in the surrounding soil. This effect is up to 20 times higher than observed around live root systems, which may suggest that the rhizosphere effect on microbivores could for the major part result from the decomposition of dead segments of the root system.