\section{Significance} Soil microorganisms drive C flux through the terrestrial
biosphere, and models that predict terrestrial C flux can benefit by accounting
for microbial ecophysiology in soils. However, characterizing the ecophysiology
of microbes that mediate C decomposition in soil has proven difficult due to
their overwhelming diversity. We characterized microbial C metabolism in soil
and show that different types of C have distinct decomposition dynamics
governed by different microbial lineages. For example, we found that
uncharacterized microbial taxa, which are cosmopolitan in soils, assimilated
cellulose-C into DNA. These microbes may drive cellulose decomposition on
a global scale. We identify microbial lineages engaging in labile and
structural C decomposition and explore their ecological properties.
