Dynamics of autochthonous soil viral communities parallels dynamics of host communities under nutrient stimulation

Viruses are highly abundant in soils with their numbers exceeding those of cooccurring bacterial cells by 10- to over 1000-fold. Water and organic matter content influence the magnitude of the viral-to-bacterial ratio in soils; thus, ecosystem type and land use shape interactions between viral and host microbial communities in soils. Less understood are the shorter term interactions between viral and host communities that ultimately maintain the large viral standing stock within soils. This study examined short-term dynamics of viral and bacterial communities in soils to determine whether the growth of soil bacterial communities results in the production of soil viruses, and if viral community responses occur within specific populations. In microcosms amended with different carbon sources, increases in viral abundance (VA) accompanied increases in bacterial abundance (BA) and bacterial respiration rate (BRR). The timing and intensity of increases in BA, VA and BRR were different across C sources suggesting differences in the predominant mode of viral replication within growth-stimulated bacterial populations. Moreover, compositional changes occurred in soil bacterial and viral communities indicating that new viral production arose from a subset of host populations. To our knowledge, these are the first observations of soil viral populations responding to short-term changes in soil bacterial communities.