Micro- and nano-environments of C sequestration: Multi-element spectromicroscopy assessment of soil organomineral assemblages

Soil, the complex biomaterial that promotes the growth of terrestrial organisms, is an integral component of the Earth's environment, and the central organizer of terrestrial ecosystem processes; interacting constantly with the atmosphere, biosphere, and the hydrosphere. It represents the largest reservoir of terrestrial organic C, and plays a critical role in global C cycling, storing six times more C than is contained in all living plants and four times more than the atmosphere. Hence, comparatively small increases in soil C would make a large difference in atmospheric CO2 contents and for the Earth's climate. In light of predicted climate changes and a more unified approach to greenhouse gas emissions at the global scale, the soil's ability to accumulate and sequester C, and thus to act as a sink for increasing CO2 concentrations in the atmosphere, has received growing interest in the last decades. The first investigations of soil organic C were documented in the 18th century, and the importance of the interaction between C and minerals in soil have been recognized more than one hundred years ago. We know that most organic matter enters the soil as readily recognizable plant and animal detritus, and is mineralized within short timescales of one or two years. The remaining organic C, however, is stabilized for longer timescales of up to thousands of years in the presence of Fe, Al, Mn oxide and hydroxide species, phyllosilicates and other soil minerals. Changes in mineralogy can enhance the soil C storage potential several fold. It is, therefore, all the more astonishing that the precise processes for the interaction between minerals and organic C in organomineral assemblages are still not yet well understood; and the underlying biogeochemical mechanisms for long-term stabilization of this vast amount of C in soils and the full potential for C sequestration in the Earth's surface remains unknown. Possible mechanisms for soil C sequestration appear to be the results of a combination of basic processes occurring at a micro-, nano- and atomistic level involving. One of the major limiting factors to past investigations of soil C sequestration is, therefore, the fact that these processes operate well below the scale that most researchers have been able to observe. Although our investigation is at its early stage, we have made progress over the past years to examine the interactive properties of organic C and mineral particles present in organomineral assemblages at sub-micrometer level using FTIR, multi-element STXM-NEXAFS and STEM-EELS spectromicroscopy assessment of soil organomineral assemblages and soil nanoparticles. The unprecedented ability rendered by these advanced techniques to investigate the different phases present in complex matrices such as soils, separated probably by only an atomically-thin wall, enables the generation of a wealth of new atomic- and nanoscale-level information of associated organic and oxide communities. Here we will use the first results of our investigation as a proof of concept and attempt to identify the mechanisms for C sequestration in soil relevant for agricultural sustainability and climate change mitigation strategies.