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Are Interactions between Organic Compounds and Nanoscale Weathering Minerals the Key Drivers of Carbon Storage in Soils? Isabelle Basile-Doelsch,*,† Jérôme Balesdent,‡ and Jérôme Rose† †
Aix-Marseille Université, CNRS, IRD, CEREGE UM34, 13545 Aix en Provence, France INRA, UR1119 GSE, F-13100 Aix-en-Provence, France intensive agricultural practices is not solely caused by a decrease in the input flux, but rather by a decrease in the carbon residence time. These two levers can be used to increase OM stabilization and thus enhance the C-stock. Agro-ecological techniques developed so far have mainly been focused on the first lever. However, greater insight into the mechanisms of soil OM stabilization and destabilization is required to act on the Cresidence time lever. To understand stabilization mechanisms, it should be kept in mind that the average mass proportion of minerals and OM in soil is roughly 95% versus 5%. Mineral phases include both primary minerals from parent rocks and secondary minerals formed via primary mineral weathering. Soil OM includes about 15% living biomass and 85% dead biomass. Dead OM is formed by an assembly of low molecular weight compounds. These organic compounds have their own dynamics but it remains unknown why some OM persists for millennia whereas other OM decomposes readily.2 Among OM stabilization mechanisms, mineral surfaces are suspected to govern organic compound stabilization through organo-mineral interactions at the molecular scale. Physical-chemical protection by mineral phases involves and use disrupts the natural functioning of soils, leading to intermolecular interactions between organic compounds and soil resource degradation. Forecasts also estimate that inorganic phases (mineral surfaces, secondary nanosized phases agricultural production has to be increased by 1.85-fold to meet or metal ions). A generally recognized conceptual model of the the food demand of 9 billion people by 2050.1 Agro-ecological organo-mineral interface in soil proposes that organic practices thus have to fulfill two main objectivesminimize soil compounds form a ∼4 nm thick discrete zonal sequence degradation while improving ecosystem services. Agrodependent on the hydrophobicity of the molecules3 (Figure 1). ecological strategies for restoring soil functioning mainly strive In this model, mineral phases are considered to be (i) stable, (ii) of much larger size than that of the organic compounds, to enhance the soil organic matter pool. In this Viewpoint, we and (iii) formed prior to the occurrence of organic compounds argue that future agro-ecological techniques should also be that form complexes on their surface. However, this pattern is geared toward increasing the residence time of organic matter probably too simplistic because the fate of the mineral phases in soil. A better understanding of organic matter stabilization closely depends on the soil biophysical−chemical fluctuations. mechanisms is therefore needed.2 We propose an alternative Indeed, primary rock minerals are subjected to weathering model of organo-mineral interactions that no longer considers conditions that upset their stable thermodynamic conditions. soil minerals as stable surfaces but instead take nanoscale Weathering is especially enhanced at points of contact with weathering mineral dynamics into account. bioactive zones, such as plant roots and in the vicinity of microThe soil organic matter (OM) pool is one of the main targets organisms. of agro-ecological strategies for three reasons: (i) soil OM plays The formation of secondary weathered phases in soils has key roles in soil system functioning (plant nutrient, soil been challenged over the last 10−15 years, particularly in soils structure, energy supply for microbes and pollutant adsorpdeveloped on volcanic rocks. Weathering of the latter rocks tion), (ii) soil OM represents the largest continental carbon rapidly produces poorly crystallized nanometric-sized secon(C) stock, and (iii) intensive use of arable land has led to the dary phases (allophane type). These nanoscale phases are destabilization of significant amounts of soil OM. Restoring the characterized by high surface reactivity and have a very high OM pool in cultivated soils could thus address two major affinity to organic compounds.4 Andosolsknown for issues: food security and climate change mitigation. The C-stock in a soil (t.ha−1) results from a combination of flux (t·ha−1·yr−1) and residence time (yr). It has been Received: February 5, 2015 Published: March 11, 2015 demonstrated that a severe decrease in OM content following ‡
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© 2015 American Chemical Society
3997
DOI: 10.1021/acs.est.5b00650 Environ. Sci. Technol. 2015, 49, 3997−3998
Viewpoint
Environmental Science & Technology
Figure 1. Conceptual organo-mineral interaction models contrasting in their view of the state of the mineral surface. In classical models (A), organic compounds form a zonal sequence. The proposed model (B) considers that mineral weathering generates nanosized amorphous minerals. Their high reactivity and specific surface area5 enhance interactions with organic compounds.
ecological practices based on the residence time levers, with the aim of addressing both food security and climate change issues.
stabilizing high amounts of OMare formed in these geological contexts. Nanoscale mechanisms observed in Andosols represent a typical case, highlighting the role of silicate phases weathering in stabilizing soil OM, but such processes are not unique as they also occur in other soil types. The formation of a ∼100 nm thick amorphous layer at the surface of weathering minerals has been described in batch and mesocosm experiments, as well as at the surface of minerals altered in natural environments. In any soil, secondary nanosized phases newly formed by the weathering of surface minerals may give rise to nanosized organo-mineral complexes and stabilize organic compounds. The conceptual breakthrough that we propose is thus to no longer regard soil minerals as stable surfaces on a pedogenesis time scale. Mineral surfaces should instead be considered as unstable, subject to weathering, and generating nanometric amorphous (or short-range ordered) polymers consisting mainly of hydrolyzed Al, Si, and Fe (Figure 1). Although the composition, degree of crystallinity and rate of formation of the inorganic polymers depend on the nature of the weathering mineral, these nanosized particles should have the specific properties of finely divided materials, that is, large specific surface area and high surface reactivity.5 In natural weathering systems without organic compounds (or low amounts), nanopolymers gradually become organized into minerals through crystalline growth and form phyllosilicates and/or oxy-hydroxides. However, in soils, nanosized particles may interact with organic compounds. These interactions would both hamper the crystalline growth of secondary minerals and enhance organic compound stabilization. Upcoming novel research on soil OM stabilization should thus lead to the development of a new paradigm. On one hand OM research papers have given rise to interesting new concepts on OM structure and reactivity while considering mineral mainly as stable surfaces, and on the other hand mineral weathering research has revealed the formation of nanosized surface secondary phases while considering OM oversimplistically. Our aim is to reconcile both concepts in a unified model of OM−mineral interactions. Enhancing knowledge on soil OM stabilization-destabilization processes could open new fields of research for engineering C stocks in soils through agro-
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Funding
I.B.D. is supported by Institut Universitaire de France funds. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank OSU-Institut Pythéas. REFERENCES
(1) Paillard, S.; Treyer, S.; Dorin, B. Agrimonde. Scenarios and Challenges for Feeding the World in 2050; Quae: Versailles, 2010; p 295. (2) Schmidt, M. W. I.; Torn, M. S.; Abiven, S.; Dittmar, T.; Guggenberger, G.; Janssens, I. A.; Kleber, M.; Kogel-Knabner, I.; Lehmann, J.; Manning, D. A. C.; Nannipieri, P.; Rasse, D. P.; Weiner, S.; Trumbore, S. E. Persistence of soil organic matter as an ecosystem property. Nature 2011, 478 (7367), 49−56. (3) Kleber, M.; Sollins, P.; Sutton, R. A conceptual model of organomineral interactions in soils: self assembly of organic molecular fragments into zonal structures on mineral surfaces. Biogeochemistry 2007, 85, 9−24. (4) Levard, C.; Doelsch, E.; Basile-Doelsch, I.; Abidin, Z.; Miche, H.; Masion, A.; Rose, J.; Borschneck, D.; Bottero, J. Y. Structure and distribution of allophanes, imogolite and proto-imogolite in volcanic soils. Geoderma 2012, 183−184, 100−108. (5) Auffan, M.; Rose, J.; Bottero, J.-Y.; Lowry, G. V.; Jolivet, J.-P.; Wiesner, M. R. Towards a definition of inorganic nanoparticles from an environmental, health and safety perspective. Nat. Nano 2009, 4, 1−8.
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DOI: 10.1021/acs.est.5b00650 Environ. Sci. Technol. 2015, 49, 3997−3998
