Potential origin and formation for molecular components of humic acids in soils

Abstract

Soil humic acids are the base soluble/acid insoluble organic components of soil organic matter. Most of what we know about humic acids comes from studies of their bulk molecular properties or analysis of individual fractions after extraction from soils. This work attempts to better define humic acids and explain similarities and differences for several soils varying in degrees of humification using advanced molecular level techniques. Our investigation using electrospray ionization coupled to Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) and nuclear magnetic resonance spectroscopy (NMR) has given new insight into the distinctive molecular characteristics of humic acids which suggest a possible pathway for their formation. Humic acids from various ecosystems, climate regions and soil textural classes are distinguished by the presence of three predominant molecular components: lignin-like molecules, carboxyl-containing aliphatic molecules and condensed aromatic molecules that bear similarity to black carbon. Results show that humification may be linked to the relative abundance of these three types of molecules as well as the relative abundance of carboxyl groups in each molecular type. This work also demonstrates evidence for lignin as the primary source of soil organic matter, particularly condensed aromatic organic molecules often categorized as black carbon and is the first report of the non-pyrogenic source for these compounds in soils. We also suggest that much of the carboxyl-containing aliphatic molecules are sourced from lignin. Abbreviations ESI-FTICR-MS, electrospray ionization coupled to Fourier transform ion cyclotron resonance mass spectrometry; NMR, Nuclear Magnetic Resonance Spectroscopy; DBE, double bond equivalents; CRAM, carboxyl-rich alicyclic molecules; CCAM: carboxyl containing aliphatic molecules; BC, Black carbon; CPMAS, cross-polarization magic angle spinning; DPMAS, direct polarization magic angle spinning; KMD, Kendrick mass defect; Ai mod , modified aromaticity index

Trial by Fire: On the Terminology and Methods Used in Pyrogenic Organic Carbon Research

“…Other non-pyrogenic OM sources of condensed aromatic OM are abundant in the geosphere, including woody peat, coal, kerogen, and oil (Yoshioka and Ishiwatari, 2005;Hammes et al, 2007;Wang et al, 2012;Hartman et al, 2015;Li et al, 2017). Still other studies have found that nonpyrogenic OM can be readily transformed to condensed aromatic OM, which would appear to be pyrogenic, through photolytic, microbial, or chemical degradative processes (Glaser and Knorr, 2008;Chen et al, 2014;Waggoner et al, 2015;DiDonato et al, 2016).…”

Qualitative and quantitative soil organic matter estimation for sustainable soil management

“…Although based on electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS) data, it was shown that carboxylic acids bound to both aliphatic and condensed aromatic molecules are common components of HAs (DiDonato and Hatcher 2016;DiDonato et al 2016). In their opinion, the vicinities of a variety of carboxylic groups to an assortment of structural components would indicate that Blignin-like^compounds resent the third major molecular group common to HAs (DiDonato et al 2016).…”

Riparian reforestation with a single exotic species restores soil aggregation and porosity but not humic substances

“…Humic substances (HSs) composed of humic and fulvic acids are produced via the decay of plants rich in lignin (Di Donato et al ., ), forming supramolecular associations of heterogeneous molecules held together by hydrophobic interactions (Nebbioso & Piccolo, ). According to Di Donato et al .…”

“…According to Di Donato et al . (), HSs can be classified into the following molecular groups based on the Van Krevelen () diagram: lignin‐like molecules, carboxyl‐containing aliphatic molecules and condensed aromatic molecules related to black carbon.…”