Importance of inner-sphere P-O-Fe bonds in natural and synthetic mineral-organic associations.

Sorption of organic molecules on mineral surfaces can occur through several binding mechanisms of varying strength. Here, we investigated the importance of inner-sphere P-O-Fe bonds in synthetic and natural mineral-organic associations. Natural organic matter such as water extracted soil organic matter (WESOM) and extracellular polymeric substances (EPS) from liquid bacterial cultures were adsorbed to goethite and examined by FTIR spectroscopy and P K-edge NEXAFS spectroscopy. Natural particles from a Bg soil horizon (Gleysol) were subjected to X-ray fluorescence (XRF) mapping, NanoSIMS imaging, and NEXAFS spectro-microscopy at the P K-edge. Inner-sphere P-O-Fe bonds were identified for both, adsorbed EPS extracts and adsorbed WESOMs. Characteristic infrared peaks for P-O-Fe stretching vibrations are present but cannot unambiguously be interpreted due to possible interferences with mono- and polysaccharides. For the Bg horizon, P was only found on Fe oxides, covering the entire surface at different concentrations, but not on clay minerals. Linear combination fitting of NEXAFS spectra indicates that this adsorbed P is mainly a mixture of orthophosphate and organic P compounds. By combining atomic force microscopy (AFM) images with STXM-generated C and Fe distribution maps, we show that the Fe oxide surfaces were fully coated with organic matter. In contrast, clay minerals revealed a much lower C signal. The C NEXAFS spectra taken on the Fe oxides had a substantial contribution of carboxylic C, aliphatic C, and O-alkyl C, which is a composition clearly different from pure adsorbed EPS or aromatic-rich lignin-derived compounds. Our data show that inner-sphere P-O-Fe bonds are important for the association of Fe oxides with soil organic matter. In the Bg horizon, carboxyl groups and orthophosphate compete with the organic P compounds for adsorption sites.

Identification of Distinct Functional Microstructural Domains Controlling C Storage in Soil.

The physical, chemical, and biological processes forming the backbone of important soil functions (e.g., carbon sequestration, nutrient and contaminant storage, and water transport) take place at reactive interfaces of soil particles and pores. The accessibility of these interfaces is determined by the spatial arrangement of the solid mineral and organic soil components, and the resulting pore system. Despite the development and application of novel imaging techniques operating at the micrometer and even nanometer scale, the microstructure of soils is still considered as a random arrangement of mineral and organic components. Using nanoscale secondary ion mass spectroscopy (NanoSIMS) and a novel digital image processing routine adapted from remote sensing (consisting of image preprocessing, endmember extraction, and a supervised classification), we extensively analyzed the spatial distribution of secondary ions that are characteristic of mineral and organic soil components on the submicrometer scale in an intact soil aggregate (40 measurements, each covering an area of 30 µm × 30 µm with a lateral resolution of 100 nm × 100 nm). We were surprised that the 40 spatially independent measurements clustered in just two complementary types of micrometer-sized domains. Each domain is characterized by a microarchitecture built of a definite mineral assemblage with various organic matter forms and a specific pore system, each fulfilling different functions in soil. Our results demonstrate that these microarchitectures form due to self-organization of the manifold mineral and organic soil components to distinct mineral assemblages, which are in turn stabilized by biophysical feedback mechanisms acting through pore characteristics and microbial accessibility. These microdomains are the smallest units in soil that fulfill specific functionalities.

Novel Sample Preparation Technique To Improve Spectromicroscopic Analyses of Micrometer-Sized Particles.

Microscale processes occurring at biogeochemical interfaces in soils and sediments have fundamental impacts on phenomena at larger scales. To obtain the organo-mineral associations necessary for the study of biogeochemical interfaces, bulk samples are usually fractionated into microaggregates or micrometer-sized single particles. Such fine-grained mineral particles are often prepared for nanoscale secondary ion mass spectroscopy (NanoSIMS) investigations by depositing them on a carrier. This introduces topographic differences, which can strongly affect local sputtering efficiencies. Embedding in resin causes undesired C impurities. We present a novel method for preparing polished cross-sections of micrometer-sized primary soil particles that overcomes the problems of topography and C contamination. The particles are coated with a marker layer, embedded, and well-polished. The interpretation of NanoSIMS data is assisted by energy-dispersive X-ray spectroscopy on cross-sections prepared by a focused ion beam. In the cross-sections, organic assemblages on the primary soil particles become visible. This novel method significantly improves the quality of NanoSIMS measurements on grainy mineral samples, enabling better characterization of soil biogeochemical interfaces. In addition, this sample preparation technique may also improve results from other (spectro-) microscopic techniques.