Reducing atmospheric CO2 is a global priority to which soil organic matter (SOM) could contribute greatly. Microorganisms have emerged as key drivers of stable SOM formation, through the stabilization of microbial remains (necromass). Understanding microbial death is therefore essential for effective management of soil carbon stocks. Viruses (phages) inflict heavy mortality on aquatic bacteria, yet, despite their abundance, they have scarcely been studied in soil. I will uncover the functional significance of soil phages for bacterial death and carbon stabilization by (i) determining bacterial mortality due to phages; (ii) assessing the effect of phages on necromass accumulation; and (iii) testing how phages affect incorporation of carbon into stable SOM fractions. To test the influence of phages, soil samples with natural phage abundance will be compared with two increments of increased phage by adding phage extract from a subsample of the same soil. I will estimate phage-induced death and SOM formation using a novel combination of molecular and chemical techniques. I will quantify bacterial mortality rates by 18O stable isotope probing of DNA. Necromass accumulation will be assessed by tracing 13C from synthetic root exudate into biomarker components of microbe-derived SOM. Finally, I will measure incorporation of 13C into occluded and mineral-associated SOM fractions, which represent the two main mechanisms of SOM stabilization. This project brings together my expertise in soil carbon cycling and compound-specific stable isotope probing, and the excellent molecular ecology capabilities of the host institute. This will provide the first quantification of phage-induced mortality in soil and determine its functional significance as a previously overlooked driver of SOM formation. Methodological advances in the quantification of growth and death will be of broad interest to microbial ecologists. Results will be disseminated through workshops, presentations and publications via both scientific and professional channels.