Effects of experimentally elevated virus abundance on soil carbon cycling across varying ecosystem types

Viruses are abundant and diverse members of soil communities, but their influences on soil biogeochemical cycling are poorly understood. To assess the potential for viruses to influence soil carbon (C) cycling in varying environmental contexts, we sampled soils from four contrasting ecosystem types across the continental United States: conifer forest, broadleaf deciduous forest, tallgrass prairie, and agricultural cropland. We then experimentally increased virus abundance in the soils by inoculating microcosms with virus concentrates isolated from the same original soils and incubated the soils for 14 days. The virus-treated conifer forest and prairie soils respired significantly less C (14 μg and 10 μg less C per gram of soil, respectively) over the course of the 14-day incubation compared with control soils, though the effects were proportionally small in magnitude (3% and 6% reductions in cumulative respiration, respectively). Following the initial 14-day incubation, we conducted a¹³C-glucose tracer incubation. In contrast to the initial incubation, after glucose addition we observed effects on respiration only in the agricultural soil, where respiration of soil organic matter-derived C nearly doubled in the virus-treated soils compared with control soils. We also observed overall reduced incorporation of ¹³C into microbial biomass (i.e., lower growth yield) and lower carbon use efficiency on average in all virus-treated soils. These results demonstrate that viruses can influence overall microbial metabolism but with different aggregate effects on soil C balance across soil types depending on soil physicochemical properties. Overall, our study demonstrates that viral influences on soil microorganisms can manifest in altered fates of soil C, with either increased or decreased respiratory C loss depending on ecosystem type.