Top-down control of microbial activity and biomass in an Arctic soil ecosystem

Globally, soil microbes preside over vast carbon stores, and both microbial biomass and activity are known to be regulated by bottom-up controls, that is, limitation by nutrients and energy. However, there is evidence that grazing by protozoans exerts top-down controls on biomass. Here, we investigate top-down control by phage on soil microbes using an experimental site near Barrow, Alaska (71 degrees N, 157 degrees W) during the 2007 growing season. Soil measurements were taken from sites that covered a range of microtopographical features within a drained and thawed lake basin including high- and low-centred ice-wedge polygons to estimate the availability of carbon and nitrogen for microbes. Using both field and laboratory experiments, we successfully increased both microbial biomass and respiration by decreasing phage populations. The addition of carbon and nutrients to soils had no significant effects on biomass or respiration, indicating a lack of bottom-up controls. Additionally, we present the first use of tea extracts as a potent anti-phage agent in soils. Our results suggest that top-down controls, such as phage predation, are critical to regulation of microbial activities in Arctic soils.