Evidence for the formation of fused aromatic ring structures in an organic soil profile in the early diagenesis

Abstract

The presence of fused aromatic ring (FAR) structures in soil define the stability of the recalcitrant soil organic matter (RSOM). FAR are important skeletal features in RSOM that contribute to its extended residence time. During the early diagenesis, FAR are formed through condensation and polymerization of biomolecules produced during plant residue and microbial product decay. Molecular level characterization of the RSOM extracted from an organic soil profile gives important insights into the formation of FAR. Advanced solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, including recoupled long-range C−H dipolar dephasing experiments on extracted humic acids (HA) showed that they contain diagenetically formed FAR different from charcoal and lignin. Peaks characteristic of FAR are observed at all depths in the soil profile, with a greater prevalence observed in the HA extracts from the clay soil layer at the bottom. In the clay soil layer, 78% of the aromatic carbon was non-protonated, and this was 2.2-fold higher than the topsoil. These data further strengthen our understanding of the humification process that could occur in early diagenesis and help explain the importance of incorporating diagenesis as an important phenomenon for long-term carbon sequestration in soil.

Potentially Massive and Global Non-Pyrogenic Production of Condensed “Black” Carbon through Biomass Oxidation

“…28,29 Another recent study observed non-pyrogenic formation of ConAC in the aerobic microbial incubation of wheat straw, 30 an example of a microbiological system where microbes exude extracellular enzymes, which release ROS. These mechanistic studies explain the observation of nonpyrogenic ConAC formation during soil humification 31 and photochemical DOC irradiation. 26 However, due to the use of qualitative (electrospray ionization−mass spectrometry) or less conventional methods (spectral editing nuclear magnetic resonance (NMR) spectroscopy) in these studies, their results "warrant further investigation using quantitative methods such as the benzenepolycarboxylic acids (BPCA) method [sic]".…”

“…The lack of correlation is consistent with our proposition that ConAC in soil is primarily derived from a non-pyrogenic process, which has been also shown empirically recently. 31 Furthermore, ConAC is strongly correlated with SOC regionally 18 −21 and globally 8,22 (Figure 5B). The current interpretation for this ConAC-SOC correlation is that charcoal from ancient fires (referred to as legacy BC) is equally distributed globally in SOC pools.…”

“…The quantitative results in our study show that ConAC may be largely of non-pyrogenic origin as previously suggested by qualitative studies. 30,31,56 The BC or pyC terminologies would have been inappropriate for our study as they would have imposed a pyrogenic source onto non-pyrogenically produced ConAC. It has been also shown that ConAC is present in petroleum, 80 asphalt, 81 and hydrothermal vent exhaust, 81 which exemplifies how, if the BC or pyC terminologies were used for ConAC in such samples, they would imposed a pyrogenic source to petrogenic molecules.…”

“…22,82,83 In light of our findings, assuming that BPCA (and other) methods quantify BC would lead to major overestimations of environmental reservoirs and fluxes of "true" fire-derived carbon. 56 This is especially notable for soils, where the non-pyrogenic process of soil formation (soil humification) produces ConAC, 31 with biomass oxidation being a key process. Thus, it is very likely that the majority of reported BC quantities in soil systems (Figure 5A,B), in fact, correspond to refractory humic ConAC and not BC.…”