Viruses in Soil Ecosystems: An Unknown Quantity Within an Unexplored Territory

Viral abundance in soils can range from below detection limits in hot deserts to over 1 billion per gram in wetlands. Abundance appears to be strongly influenced by water availability and temperature, but a lack of informational standards creates difficulties for cross-study analysis. Soil viral diversity is severely underestimated and undersampled, although current measures of viral richness are higher for soils than for aquatic ecosystems. Both morphometric and metagenomic analyses have raised questions about the prevalence of nontailed, ssDNA viruses in soils. Soil is complex and critically important to terrestrial biodiversity and human civilization, but impacts of viral activities on soil ecosystem services are poorly understood. While information from aquatic systems and medical microbiology suggests the potential for viral influences on nutrient cycles, food web interactions, gene transfer, and other key processes in soils, very few empirical data are available. To understand the soil virome, much work remains.