Linking atomic force microscopy with nanothermal analysis to assess microspatial distribution of material characteristics in young soils

Coupling of atomic force microscopy (AFM) with nanothermal analysis (nTA) has the potential to assess material characteristics in soils on the lower mm-scale, but has been shown to require additional characteristics for clear distinction of materials. The objective of this study was to evaluate to which extent the combination of AFM-nTA with AFM adhesion force analysis and structural features allows distinction of organic materials in soils. Using soil samples from a chronosequence from the Damma Glacier forefield, Switzerland, as example, we tested a grid analysis approach for assessing distribution of adhesion forces and nanothermal characteristics. This approach was compared with an approach involving pre-selection of structural features of interest via morphological criteria. Only three types of nanothermograms were distinguished in the soil samples based on different thermal expansion-compression characteristics and phase transition temperatures. Combined evaluation of nanothermal characteristics, adhesion forces and morphological characteristics allowed distinction of a larger set of materials than using nanothermal analysis, adhesion force distribution or morphological characteristics separately. Part of the analyzed features showed a combination of characteristics similar to that of fresh bacterial cells which we analyzed as a potential reference material. Their stronger occurrence in the regions of interest of older samples than in those of younger samples may underline their relevance in soil development. Achieving the long-term objective of identification of materials still requires more information on reference materials, understanding the impact of mixed layering of materials on thermal profiles and the assessment of variability of the characteristics within and between different material groups.