Resolving the spatial arrangement governing soil organic matter dynamics at the microscale based on NanoSIMS analysis

Abstract:

Recent studies have shed light on the functioning of organic matter (OM) dynamics at the microscale, thanks to advanced spectromicroscopic analyses like NanoSIMS. By resolving scales that capture individual organic and mineral components, it was possible to locate OM turnover and storage processes in the soil and investigate how these are related with mineral components. In this contribution, the novel understanding gained through individual studies is compared and put into a larger perspective where the spatial arrangement is a major factor driving OM dynamics at the microscale. Stable isotope labelling enabled tracing enriched OM and quantify how far it is incorporated into the mineral matrix and stable soil aggregates. The architecture of mineral and organic soil components influences the arrangement of newly associated OM which are mostly located in pores with a diameter <1-2 µm. While OM at the edge of the mineral matrix exhibits a highly variable C and N composition, increasing distance leads to a more N-rich composition. The comparison of soils with different composition demonstrated that Fe and Al oxides can be co-localized with increased associations of OM. However, the arrangement of mineral and organic soil components dictates which surfaces are exposed and can potentially associate with OM while other reactive surfaces might not be accessible. By reviewing various study results, we hypothesize that the OM dynamics in soils do not equal the sum of individual mineral or OM properties, but are governed by their spatial arrangement at the microscale. These findings frame a new perspective of spatially resolved interactions that determine the fate of OM locally at the soil microscale.