High frequency of virus-infected bacterial cells in a sheep grazed pasture soil in New Zealand

Viruses cause significant mortality of microbes in aquatic environments and thus play a major role in biogeochemical cycling. While viruses are known to be present in soil, the rate of infection of bacteria, and therefore the potential for mortality and lysis, are unknown. In this study we measured the frequency of viral infected of bacterial cells in soils from a New Zealand sheep grazed pasture under ambient or elevated atmospheric carbon dioxide levels at the New Zealand Free Air Carbon Dioxide Enrichment facility (NZ-FACE). Averaged across the CO2 treatments, the frequency of visibly infected cells (FVIC) was 23%. This is far greater than the reported FVIC values for marine or fresh water environments. The average FVIC of soils under ambient and elevated atmospheric CO2 conditions was 28% and 18% respectively. These results are discussed in relation to nutrient cycling in grazed pastures.