Quantitative mapping and spectroscopic characterization of particulate organic matter fractions in soil profiles with imaging VisNIR spectroscopy

Abstract

Organic matter is an important constituent of soils that controls many soil functions and is of vital importance for ecosystem services like climate regulation and food security. Soil organic matter (SOM consists of a wide spectrum of different organic substances that are highly heterogeneous in terms of chemical composition, stability against microbial decomposition and turnover time. SOM is heterogeneously distributed in the soil profile impeding its fast assessment. A technique to accurately measure SOM quality and quantity with a high spatial resolution in the soil profile is presently lacking. Imaging visible light and near infrared spectroscopy (imVisIR) is a promising technique for the fast and spatially resolved assessment of SOM quality and quantity. In this study, we evaluate the potential of imVisIR to quantitatively map the labile particulate organic matter fraction in undisturbed cores from mineral soils.

Mercury Accumulation and Sequestration in a Deglaciated Forest Chronosequence: Insights from Particulate and Mineral-Associated Forms of Organic Matter

“…One possible reason for the enhanced Hg accumulation in MAOM is the longer exposure to Hg deposition considering its decades to centuries residence time, as compared to the <10-year residence time of POM. − Another possible reason is the selective Hg sorption to unique organic matter speciation of MAOM that are more conducive for Hg absorption and protective from litter decomposition via association with mineral surfaces. , Furthermore, the mineralogical and chemical compositions (Figures and S2–S10) of POM and MAOM show clear differences in their physiochemical characteristics and persistence. Specifically, small size clay minerals and/or iron oxides which with large specific surface areas are enriched in MAOM, thus absorbing organic compounds complexed with Hg onto the surfaces. ,, Minerals can serve as catalysts for Hg 0 oxidation through direct electron transfer or by generating reactive oxygen species, which oxidize Hg 0 and stored the produced Hg 2+ . , …”

“…Soil organic matter components are operationally defined by size and density. − POM is defined as particulate organic matter with a size ranging from 53 to 2000 μm and a density less than 1.8 g cm –3 . Dissolved organic matter (DOM) is defined as water-soluble fraction that can pass through 0.45 μm filters.…”

“…Soil organic matter is a complex mixture that can be separated into multiple components of diverse properties. , Analyzing the difference of soil organic matter in particulate (POM) and mineral-associated (MAOM) has demonstrated potential in process understanding since the two organic components form, behave, and function distinctly. − POM is largely made up of lightweight fragments decomposed from litters and has a residence time of <10 years in soils. − MAOM consists of single molecules and microscopic fragments of organic material formed from leached fraction of decomposing litters, including compounds transformed by soil biota with decades to centuries residence time. , The differentiation of POM from MAOM enables a more accurate prediction of persistence, dynamics, and cycling in organic soil − and therefore provides further understanding of Hg complexed with soil organic matter. Compared to POM, MAOM is nutrient-rich, has a lower C/N ratio, and contains fewer plant-derived compounds and more microbial products. − Previous studies suggested enhanced Hg accumulation in “old soil organic matter” and speculated that Hg is preferentially sorbed to high-density metal-binding functional groups. ,,, This indicates that MAOM has a higher adsorption ability of Hg compared to POM.…”

Spatial molecular heterogeneity of POM during decomposition at different soil depths resolved by VNIR hyperspectral imaging

“…The successful modelling results for both the alkyl ratio and C/N of POM we achieved by linking VNIR with C/N measurements and 13 C-CPMAS-NMR spectroscopic data in machine learning ensemble models validate this method as appropriate for studying the decomposition of organic amendments and plant residues in soils at a sub-millimetre scale. Steffens et al (2014) were already able to distinguish three different classes of OM at different stage of decomposition based on the composition determined by 13 C-CPMAS-NMR spectroscopy and on their VNIR reflectance spectra, and Steffens et al (2021) recently the feasibility of particulate organic matter fractions mapping in soil profiles, but we went further by predicting molecular indicators, that is, alkyl ratio and C/N, of POM at a very fine scale (Figures 3 and 4). Xu et al (2020) achieved the mapping of so-called different soil OM fractions, namely soil organic carbon, salt-extractable organic carbon and readily oxidisable organic C. These variables are strongly correlated with total organic carbon contents in the soil and do not reveal much about the composition of soil OM.…”

Topographic correction of visible near‐infrared reflectance spectra for horizon‐scale soil organic carbon mapping

“…Driven by the need for methods capable of nondestructively studying intact soil samples, proximal sensing approaches have been developed that open the door to high resolution, spatially explicit characterization of soil constituents and soil structure at scales relevant to soil hydrologic processes like preferential flow (e.g., horizon to pedon scales) (Hirmas et al., 2016a). One of these methods, visible near‐infrared (VNIR) hyperspectral imaging (HSI) spectroscopy, offers a promising way forward in quantifying the horizon‐scale spatial distribution of soil chemical properties (Buddenbaum & Steffens, 2012a, 2012b; Hobley et al., 2018, 2021; Steffens & Buddenbaum, 2013; Steffens et al., 2014). HSI collects reflectance spectra from a soil surface at high spatial resolution (e.g., sub‐millimeter) without disrupting the arrangement of constituents.…”