Persistence of soil organic matter as an ecosystem property Page: 1 of 10

                                
Persistence of soil organic matter as an
ecosystem property
Michael W. I. Schmidt'*, Margaret S. Torn23*, Samuel Abiven', Thorsten Dittmar4'5, Georg Guggenberger, Ivan A. Janssens7,
Markus Kleber8, Ingrid Kdgel-Knabner9, Johannes Lehmann'0, David A. C. Manning", Paolo Nannipieri2, Daniel P. Rasse,
Steve Weiner14 & Susan E. Trumbore5
Globally, soil organic matter (SOM) contains more than three times as much carbon as either the atmosphere or
terrestrial vegetation. Yet it remains largely unknown why some SOM persists for millennia whereas other SOM
decomposes readily-and this limits our ability to predict how soils will respond to climate change. Recent analytical
and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact,
environmental and biological controls predominate. Here we propose ways to include this understanding in a new
generation of experiments and soil carbon models, thereby improving predictions of the SOM response to global
warming.

Understanding soil biogeochemistry is essential to the stewardship of
ecosystem services provided by soils, such as soil fertility (for food,
fibre and fuel production), water quality, resistance to erosion and
climate mitigation through reduced feedbacks to climate change. Soils
store at least three times as much carbon (in SOM) as is found in either
the atmosphere or in living plants1. This major pool of organic carbon
is sensitive to changes in climate or local environment, but how and
on what timescale will it respond to such changes? The feedbacks
between soil organic carbon and climate are not fully understood,
so we are not fully able to answer these questions"-, but we can
explore them using numerical models of soil-organic-carbon cycling.
We can not only simulate feedbacks between climate change and
ecosystems, but also evaluate management options and analyse carbon
sequestration and biofuel strategies. These models, however, rest on
some assumptions that have been challenged and even disproved by
recent research arising from new isotopic, spectroscopic and molecular-
marker techniques and long-term field experiments.
Here we describe how recent evidence has led to a framework for
understanding SOM cycling, and we highlight new approaches that
could lead us to a new generation of soil carbon models, which could
better reflect observations and inform predictions and policies.
The conundrum of SOM
About a decade ago, a fundamental conundrum was articulated': why,
when organic matter is thermodynamically unstable, does it persist in
soils, sometimes for thousands of years? Recent advances in physics,
material sciences, genomics and computation have enabled a new
generation of research on this topic. This in turn has led to a new
view of soil-organic-carbon dynamics-that organic matter persists
not because of the intrinsic properties of the organic matter itself, but
because of physicochemical and biological influences from the sur-
rounding environment that reduce the probability (and therefore

rate) of decomposition, thereby allowing the organic matter to persist.
In other words, the persistence of soil organic carbon is primarily not
a molecular property, but an ecosystem property.
This emerging view has not been fully implemented in global models
or research design, for a variety ofreasons. First, the knowledge gathered
in the past decade has often been published in outlets of traditionally
separated disciplines. As a result, confusion has arisen because these
different disciplines can use the same vocabulary to mean different
things, or vice versa. For example, 'decomposition rates' may mean
the rate of mass loss of fresh litter, the production rate of CO2 in a
laboratory incubation, or the rate inferred from input and loss of an
isotopic tracer present in plant inputs to soil10. Second, the complexity
of the soil system is difficult to incorporate into one conceptual model or
to translate into a tractable yet accurate numerical model. Soil is a realm
in which solid, liquid, gas and biology all interact, and the scale of spatial
structures spans many orders of magnitude (from nanometre minerals
to football-sized soil clods). Indeed, the spatial heterogeneity of biota,
environmental conditions and organic matter may have a dominant
influence on carbon turnover and trace gas production in soils. Last,
the new knowledge remains more qualitative than quantitative. In many
cases, it tells us what is important and suggests new model structures,
but not how to parameterize them.
Recent insights into carbon cycling
Since pioneering work in the 1980s", new insights gathered across
disciplines (ranging from soil science to marine science, micro-
biology, material science and archaeology) have challenged several
foundational principles of soil biogeochemistry and ecosystem models;
in particular, the perceived importance of the 'recalcitrance' of the
input biomass (the idea that molecular structure alone can create stable
organic matter) and of humic substances (biotic or abiotic condensa-
tion products). New observations show these to be only marginally

'Department of Geography, University of Zurich, 8050 Zurich, Switzerland. 'Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. 'Energy and Resources
Group, University of California, Berkeley, California 94720, USA. 4Max Planck Research Group for Marine Geochemistry, University of Oldenburg, Institute for Chemistry and Biology of the Marine
Environment, 26129 Oldenburg, Germany. 5Max Planck Research Group for Marine Geochemistry, Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany. 'Institute of Soil Science, Leibniz
Universitat Hannover, 30419 Hannover, Germany. Department of Biology, University of Antwerp, 2610 Wilrijk, Belgium. 'Department of Crop and Soil Science, Oregon State University, Corvallis, Oregon
97331, USA. Lehrstuhl fur Bodenkunde, Technische Universitat Munchen, 85354 Freising, Germany. '0Department of Crop and Soil Sciences, Cornell Center for a Sustainable Future, Cornell University,
Ithaca, New York 14853, USA. "School of Civil Engineering and Geosciences, Institute for Research on Environment and Sustainability, Newcastle University, Newcastle NEl 7RU, UK. '2Department of
Plant, Soil and Environmental Sciences, University of Firenze, 50144 Firenze, Italy. '3Norwegian Institute for Agricultural and Environmental Research, 1432 As, Norway. 4Structural Biology, Weizmann
Institute, 76100 Rehovot, Israel. 5Max Planck Institute for Biogeochemistry, 07745 Jena, Germany.
*These authors contributed equally to this work.

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