Frey 2013 Nature Climate Change, The temperature response of soil microbial efficiency and its feedback to climate

http://www.nature.com/nclimate/journal/vaop/ncurrent/pdf/nclimate1796.pdf

Soils are the largest repository of organic carbon (C) in the
terrestrial biosphere and represent an important source of carbon
dioxide (CO2) to the atmosphere, releasing 60–75 Pg C annually
through microbial decomposition of organic materials1,2.
A primary control on soil CO2 flux is the efficiency with which
the microbial community uses C. Despite its critical importance
to soil–atmosphere CO2 exchange, relatively few studies have
examined the factors controlling soil microbial efficiency. Here,
we measured the temperature response of microbial efficiency
in soils amended with substrates varying in lability. We also
examined the temperature sensitivity of microbial efficiency
in response to chronic soil warming in situ. We find that the
efficiency with which soil microorganisms use organic matter
is dependent on both temperature and substrate quality, with
efficiency declining with increasing temperatures for more
recalcitrant substrates. However, the utilization efficiency of a
more recalcitrant substrate increased at higher temperatures
in soils exposed to almost two decades of warming 5 C above
ambient. Our work suggests that climate warming could alter
the decay dynamics of more stable organic matter compounds,
thereby having a positive feedback to climate that is attenuated
by a shift towards a more efficient microbial community
in the longer term.