4:40 PM - 4:55 PM
[MZZ42-10] Promoting Soil Aggregate Formation Toward Carbon Neutrality: Spatial Distribution of Chemical Species of Iron and Soil Organic Matter in Forest Soil Microaggregate
Keywords:Carbon neutral, Soil aggregate, Soil organic matter, STXM, XAFS, Carbon stabilization
Efforts to increase the CO2 sink capacity of terrestrial ecosystems are attracting attention as a part of efforts to achieve carbon neutrality by 2050. Soil aggregate formation could contribute to increase the sink capacity because (i) organic matter (OM) stored in soil aggregates is relatively stable against microbial degradation, and (ii) aggregation can lead to better nutrient retention, thereby promoting primary productivity. While soil aggregates are known to have hierarchical structures, the interaction of OM with clay minerals and short-range-order metal oxides that have high specific surface areas (e.g., ferrihydrite and amorphous gibbsite) is considered critical in the formation of micro-scale structural units at the lower hierarchy level. However, the direct observation of these soil components at the micro scale is still limited in the literature.
In this study, we aimed to gain insights on the association between iron (Fe) phases and OM at micro scale by analyzing the chemical species and local distribution of Fe and OM in sub-micron sized aggregates in a forest soil. We sampled a surface soil (A-horizon) from the forest at Toyota Technical Center Shimoyama in Toyota City, Aichi Prefecture and isolated micron- to submicron-sized aggregates by mechanical shaking. To identify Fe chemical species, we obtained X-ray Absorption Fine Structure (XAFS) data and compared it with XAFS spectra of standard samples to identify and quantify Fe chemical species in the samples. Furthermore, by combining transmission and electron yield methods, we compared the Fe chemical species between bulk and the aggregate surface. Additionally, we prepared resin thin sections embedding the soil samples and it was used by Scanning Transmission X-ray Microscopy (STXM) to obtain Near Edge X-ray Absorption Fine Structure (NEXAFS) of Carbon and Iron. By utilizing the spectra in micro scale, we analyzed the spatial distributions of OM and Iron in the sub-micron aggregates.
We found that the dominant Fe chemical species in the aggregates were ferrihydrite and clay minerals such as illite and biotite from the transmission XAFS, suggesting the involvement of ferrihydrite with in the microaggregate formation. The analysis combining transmission and electron yield methods further revealed that the proportion of ferrihydrite to total Fe was higher on the aggregate surface than in the bulk aggregate. These results implied that ferrihydrite may be localized on the surface of the aggregates, possibly acting as binding agents to allow physical association with less reactive minerals such as biotite. Mapping of C and Fe by STXM showed the colocalization of soil OM and ferrihydrite within the selected submicron aggregates, with ferrihydrite appeared to surround biotite particle. These STXM observations are consistent with the findings from Fe-XAFS, implying that ferrihydrite can bind to OM as well as clay minerals thereby potentially acting as a nucleus in the aggregate formation at the micro scale. The mechanistic insights inferred from these findings may be applicable to the development of technology aimed at promoting soil aggregate formation as part of broader efforts to achieve carbon neutrality.
In this study, we aimed to gain insights on the association between iron (Fe) phases and OM at micro scale by analyzing the chemical species and local distribution of Fe and OM in sub-micron sized aggregates in a forest soil. We sampled a surface soil (A-horizon) from the forest at Toyota Technical Center Shimoyama in Toyota City, Aichi Prefecture and isolated micron- to submicron-sized aggregates by mechanical shaking. To identify Fe chemical species, we obtained X-ray Absorption Fine Structure (XAFS) data and compared it with XAFS spectra of standard samples to identify and quantify Fe chemical species in the samples. Furthermore, by combining transmission and electron yield methods, we compared the Fe chemical species between bulk and the aggregate surface. Additionally, we prepared resin thin sections embedding the soil samples and it was used by Scanning Transmission X-ray Microscopy (STXM) to obtain Near Edge X-ray Absorption Fine Structure (NEXAFS) of Carbon and Iron. By utilizing the spectra in micro scale, we analyzed the spatial distributions of OM and Iron in the sub-micron aggregates.
We found that the dominant Fe chemical species in the aggregates were ferrihydrite and clay minerals such as illite and biotite from the transmission XAFS, suggesting the involvement of ferrihydrite with in the microaggregate formation. The analysis combining transmission and electron yield methods further revealed that the proportion of ferrihydrite to total Fe was higher on the aggregate surface than in the bulk aggregate. These results implied that ferrihydrite may be localized on the surface of the aggregates, possibly acting as binding agents to allow physical association with less reactive minerals such as biotite. Mapping of C and Fe by STXM showed the colocalization of soil OM and ferrihydrite within the selected submicron aggregates, with ferrihydrite appeared to surround biotite particle. These STXM observations are consistent with the findings from Fe-XAFS, implying that ferrihydrite can bind to OM as well as clay minerals thereby potentially acting as a nucleus in the aggregate formation at the micro scale. The mechanistic insights inferred from these findings may be applicable to the development of technology aimed at promoting soil aggregate formation as part of broader efforts to achieve carbon neutrality.