Although the primary effect of these Ni-Bi grain
boundary superstructures is the embrittlement of
the metal, grain boundary superstructures could
potentially play a role in electronic, magnetic, and
diffusion-related properties as well. It is already
known that dopant-based grain boundary complexions can change the electrical resistivity of
thick-film resistors ( 45) and the coercivity of Nd-Fe-B magnets ( 46) and could affect the giant spin
Hall effect in the Cu-Bi system ( 47). If the complexions in these systems form ordered superstructures, the change of two-dimensional translational
symmetry at the grain boundaries would have an
impact on related physical properties. Moreover,
drawing an analogy to the well-known diffusion
anisotropy that occurs on reconstructed metal
surfaces ( 48), diffusion through superstructures
at grain boundaries will likely be anisotropic,
and this behavior could potentially be exploited
to engineer anisotropic microstructures with
The discovery of Bi segregation–induced superstructures at general grain boundaries greatly
enriches our limited knowledge of the atomic structure of complexions and may offer new insights
into a spectrum of structure-related grain boundary properties such as plasticity, diffusivity, and
conductivity. We suggest that ordered grain boundary superstructures may indeed be a general,
although not necessarily universal, feature of
polycrystalline materials. This suggestion is based
on an analogy to the reconstruction behavior of
free surfaces, in which adsorbates often form periodic structures [e.g., Bi on Cu ( 49)] but can also
form disordered overlayers [e.g., S on Cu ( 50)].
Additional studies using TEM and complementary
techniques such as APT are needed to determine
whether segregation-induced grain boundary
superstructures exist in other polycrystalline
metals, especially in systems with strong attractive
adsorbate-metal pair interactions.
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Additional data and images are available in the supplementary
materials. We thank W. Zhang (School of Materials Science and
Engineering, Tsinghua University) and X. Gu (School of Materials
Science and Engineering, University of Science and Technology
Beijing) for valuable discussions. Supported by Office of Naval
Research Multidisciplinary University Research Initiatives program
grant N00014-11-0678 and by U.S. Department of Energy grant
Materials and Methods
Figs. S1 to S30
Tables S1 and S2
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22 January 2017; accepted 29 August 2017
GLOBAL CARBON CYCLE
Long-term pattern and magnitude of
soil carbon feedback to the climate
system in a warming world
J. M. Melillo,1 S.D. Frey,2 K. M. DeAngelis, 3 W. J. Werner,1 M. J. Bernard,1
F. P. Bowles, 4 G. Pold, 5 M. A. Knorr,2 A. S. Grandy2
In a 26-year soil warming experiment in a mid-latitude hardwood forest, we documented
changes in soil carbon cycling to investigate the potential consequences for the climate
system. We found that soil warming results in a four-phase pattern of soil organic matter
decay and carbon dioxide fluxes to the atmosphere, with phases of substantial soil carbon
loss alternating with phases of no detectable loss. Several factors combine to affect the
timing, magnitude, and thermal acclimation of soil carbon loss. These include depletion of
microbially accessible carbon pools, reductions in microbial biomass, a shift in microbial
carbon use efficiency, and changes in microbial community composition. Our results support
projections of a long-term, self-reinforcing carbon feedback from mid-latitude forests to the
climate system as the world warms.
Alarge and poorly understood component of global warming is the terrestrial carbon cycle feedback to the climate system (1). Simulation experiments with fully coupled, three-dimensional carbon-climate models
suggest that carbon cycle feedbacks could substantially accelerate or slow climate change over
the 21st century (2– 4). Both the sign and magnitude of these feedbacks in the real Earth system
are still highly uncertain because of gaps in basic
understanding of terrestrial ecosystem processes.
For example, the potential switch of the terres-
trial biosphere from its current role as a carbon
sink to a carbon source is critically dependent on
the long-term temperature sensitivity of soil or-
ganic matter (SOM) decay ( 5–7) and complex
carbon-nitrogen interactions that will likely occur
in a warmer world ( 8–12). However, without long-
term field-based experiments, the sign of the
feedback cannot be determined, the complex
mechanisms regulating that feedback cannot be
quantified, and models that incorporate the soil’s
role in carbon feedbacks to the climate system
cannot be tested. Here, we present results from