Daily changes in bacterial-feeding nematode populations oscillate with similar periods as bacterial populations after a nutrient impulse in soil

Abstract

Previously, we showed that bacterial populations oscillated in a regular manner in response to a nutrient impulse in soil. For this paper we investigated if the wave-like fluctuations in bacterial populations could be explained by their interactions with populations of bacterial-feeding nematodes (BFN). In two microcosm experiments, soil bacterial populations—colony forming units (CFUs) and microscopic counts of stained bacteria and nematode populations in 22 families were monitored daily for 25 or 30 days after incorporation of clover+grass (CG) plant material into soil. In another microcosm experiment, dynamics of bacteria and nematode populations were monitored in response to gamma-irradiated plant material added to gamma-irradiated soil mixed with filtered bacterial suspensions and in non-irradiated soil. In the first experiment, soil bacterial populations fluctuated significantly after incorporation of the plant material with two peaks within the first week and three or four smaller peaks thereafter. Populations of total nematodes and BFN started to increase in the second week after CG incorporation, but the proportion of BFN increased within 1 week. Inactive juvenile BFN (dauerlarvae) seemed to be activated after 2 days (as the percentage of Rhabditidae increased and dauerlarvae decreased), followed by stepwise increases in dauerlarvae every 4 days, indicating that there was a new generation every 4 days. There were significant wave-like fluctuations in daily population changes of BFN, but not for those of total nematode communities, over the duration of these experiments. These fluctuations had similar periods (5 days) as those of bacterial populations, but were shifted about 3 days relative to the bacterial fluctuations. Gamma-irradiation of soil significantly increased the periods and amplitudes of bacterial oscillations. Nematode populations were eliminated in gamma-irradiated soils, but small numbers of protozoa were accidentally introduced in the irradiated soil, and may have been partially responsible for the delayed regulation of bacterial growth. We conclude that fluctuations in bacterial populations were not directly related to similar fluctuations in populations of BFN, as expected from classical Lotka–Volterra equations for predator–prey relationships, but were related to changes in growth rates of BFN. An alternation in active and inactive stages in a synchronized predator community after a disturbance could allow periods of bacterial growth alternated with periods of death. Fluctuations in bacterial populations were dampened after a much longer period when the soil fauna was largely eliminated.