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The biogeochemical functioning of soils (e.g., soil carbon stabilization and nutrient cycling) is determined at the interfaces of specific soil structures (e.g., aggregates, particulate organic matter (POM) and organo‐mineral associations). With the growing accessibility of spectromicroscopic techniques, there is an increase in nano‐ to microscale analyses of biogeochemical interfaces at the process scale, reaching from the distribution of elements and isotopes to the localization of microorganisms. A widely used approach to study intact soil structures is the fixation and embedding of intact soil samples in resin and the subsequent analyses of soil cross‐sections using spectromicroscopic techniques. However, it is still challenging to link such microscale approaches to larger scales at which normally bulk soil analyses are conducted. Here we report on the use of laboratory imaging Vis–NIR spectroscopy on resin embedded soil sections and a procedure for supervised image classification to determine the microscale soil structure arrangement, including the quantification of soil organic matter fractions. This approach will help to upscale from microscale spectromicroscopic techniques to the centimetre and possibly pedon scale. Thus, we demonstrate a new approach to integrate microscale soil analyses into pedon‐scale conceptual and experimental approaches. Highlights: Quantification of soil constituents using Vis‐NIR spectroscopy.New approach to use resin embedded soil core sections with intact structure.Reproducible quantification of soil constituents important for soil carbon storage.Vis‐NIR as promising tool for upscaling from microscale to pdeon scale. [ABSTRACT FROM AUTHOR]

Organic carbon (OC) stored in permafrost affected soils of the higher northern latitudes is known to be highly vulnerable to ongoing climatic change. Although the ways to quantify soil OC and to study connected C dynamics from ecosystem to global scale in the Arctic has improved substantially over the last years, the basic mechanisms of OC sequestration are still not well understood. Here we demonstrate a first approach to directly study micro scale soil structures mainly responsible for soil OC (SOC) stabilization using nano scale secondary ion mass spectrometry (NanoSIMS). A cross section from a permafrost layer of a Cryosol from Northern Alaska was analysed using a cascade of imaging techniques from reflectance light microscopy (RLM) to scanning electron microscopy (SEM) to NanoSIMS. This allowed for the direct evaluation of micro scale soil structures known to be hot spots for microbial activity and SOC stabilization in temperate soils. The imaging techniques were supported by classical soil analyses. Using this unique set of techniques we are able to evidence the formation of micro-aggregate structures in the vicinity of plant residues in permafrost soils. This clearly indicates biogeochemical interfaces at plant surfaces as important spheres for the formation of more complex soil structures in permafrost soils. Organo-mineral associations from these hot spots of microbial activity were recovered from plant residues (free particulate organic matter, fPOM) as fine grained mineral fraction with a typically low C/N ratio. This nicely illustrates the link between classical bulk analysis and state of the art spectromicroscopic techniques. [ABSTRACT FROM AUTHOR]

We still lack crucial knowledge about the contribution of plant vs. microbial residues to specific SOM pools, particularly the relative contribution of arbuscular (AM), ectomycorrhizal (ECM), and saprotrophic (SAP) fungi. We investigated sources of particulate and mineral-associated organic matter (POM and MAOM) around trees with distinct mycorrhizal types, Liriodendron tulipifera (AM-association) and Quercus alba (ECM-association), in a temperate deciduous forest in Indiana, USA. Combining 13C and 15N natural abundance analyses with measurements of microbial residues using amino sugars, the isotope signatures of large, medium and small-sized POM and MAOM fractions were compared with those of leaves, roots and biomass of mycorrhizal and saprotrophic fungi. A Bayesian inference isotope mixing model calculated sources of C and N to SOM fractions. While the isotope composition of POM resembled that of plants, MAOM was close to fungal values. This was confirmed by mixing model calculations and microbial residue analysis, which additionally and independent from tree partner suggested saprobic fungi contributing with 4–53% to POM and 23–42% to MAOM, as opposed to ECM contributions. Our results suggest fungal, not plant residues, as the source of the most putatively stable OM pool; thus, altering fungal communities may enhance efforts to increase long-term soil C storage. • Plant vs. microbial input to SOM via Bayesian Inference Isotopic Mixing Model. • Ectomycorrhizal and saprotrophic fungi are main contributors to MAOM. • Amino sugar data confirmed a high contribution of fungal residues to MAOM. • Consideration of the role of fungal functional groups in soil carbon storage. [ABSTRACT FROM AUTHOR]

The connection between litter chemistry and the pathways controlling soil organic matter (SOM) formation and decay in forest ecosystems remains poorly understood, particularly in tropical soils. We addressed this question by incubating samples of a Ferralsol for 200 days with typical forest litter (leaves, twigs, bark, and roots) obtained from 13C-enriched Eucalyptus seedlings. Throughout the incubation, we monitored 13C/12C–CO 2 evolved from the soil to quantify the microbial respiration of the 13C-labeled fresh plant litter and of the native SOM. Afterwards, we used density fractionation to obtain particulate organic matter (POM) with density <1.8 g cm−3, and the soil material remaining was wet-sieved to obtain SOM with particle-size >53 μm and mineral-associated SOM (MAOM, with particle-size <53 μm). We used solid-state 13C-CPMAS-NMR spectroscopy to assess the molecular composition of plant material and SOM fractions and quantified microbial amino sugars in bulk soil using gas chromatography. Our 13C/12C–CO 2 results indicate that leaves, twigs, and bark (aboveground litter) were respired at higher rates but led to lower degradation of native SOM as compared to root tissues. On average, aboveground litter promoted net C gains in both POM and MAOM, whereas root litter only led to net C gains in POM. Overall, SOM formation via microbial incorporation of aboveground litter through in vivo pathways appears to be more efficient and causes less degradation of native MAOM than roots. Moreover, a reduction in microbial amino sugars in bulk soils suggests that in vivo pathways also favored the formation of POM, which had more microbial-derived protein than forest litter. Therefore, the connection between litter chemistry and the pathways controlling SOM formation in tropical forest ecosystems must be included in a framework that also considers the mineralization of native SOM and the vertical separation of aboveground and belowground C inputs to soils. • Pathways through which 13C-labeled forest litter enters soil C pools were assessed. • Litter chemistry impacts both biotic (in vivo) and selective preservation (ex vivo). • Aboveground litter enters into soil C pools predominantly via in vivo pathways. • Ex vivo pathways are much more relevant for roots. • Priming effect is larger in the ex vivo than in the in vivo pathway. [ABSTRACT FROM AUTHOR]