Phosphate amendment drives bloom of RNA viruses after soil wet-up

Soil rewetting after a long dry season results in a burst of microbial activity accompanied by succession of both microbial and DNA viral communities. We hypothesized that RNA viruses, like DNA viruses, would exhibit temporal succession following rewetting. Moreover, we expected their response would change with the addition of phosphate, since viral proliferation may lead to phosphorus (P) limitation due to their low C:N:P ratio. We used a replicated time-series of soil metatranscriptomes collected after rewetting to identify parameters affecting RNA viral community composition over three weeks. P amendment led to a decrease in RNA viral community diversity and evenness, significantly impacting beta diversity over time. As has been observed for DNA viruses, the spatial distribution of RNA viruses in dry soil was highly heterogeneous. Most viruses were predicted to infect bacteria or fungi, and a small fraction was predicted to infect protists, plants, and animals. The amount of RNA extracted from phages of the class Leviviricetes increased significantly after one week in P-amended soil, contrasting with unamended soil. This suggests that P availability plays an important role in RNA phage proliferation. We estimate that the number of bacteria infected by RNA phages is on the order of 10⁷–10⁹ per gram soil, comparable to the range of total cells in soil. This implies that RNA phages likely have a profound effect on the bacterial community following soil wet-up when P is not a limiting factor.