Increased Electron-Accepting and Decreased Electron-Donating Capacities of Soil Humic Substances in Response to Increasing Temperature

Abstract

The electron transfer capacities (ETCs) of soil humic substances (HSs) are linked to the type and abundance of redox-active functional moieties in their structure. Natural temperature can affect the chemical structure of natural organic matter by regulating their oxidative transformation and degradation in soil. However, it is unclear if there is a direct correlation between ETC of soil HS and mean annual temperature. In this study, we assess the response of the electron-accepting and -donating capacities (EAC and EDC) of soil HSs to temperature by analyzing HSs extracted from soil set along glacial-interglacial cycles through loess-palaeosol sequences and along natural temperature gradients through latitude and altitude transects. We show that the EAC and EDC of soil HSs increase and decrease, respectively, with increasing temperature. Increased temperature facilitates the prevalence of oxidative degradation and transformation of HS in soils, thus potentially promoting the preferentially oxidative degradation of phenol moieties of HS or the oxidative transformation of electron-donating phenol moieties to electron-accepting quinone moieties in the HS structure. Consequently, the EAC and EDC of HSs in soil increase and decrease, respectively. The results of this study could help to understand biogeochemical processes, wherein the redox functionality of soil organic matter is involved in the context of increasing temperature.

Molecular‐weight‐dependent redox cycling of humic substances of paddy soils over successive anoxic and oxic alternations

“…The above information indicates that the C:H, HIX, humic‐like component, and E 4 :E 6 of HS can be utilised as indicators for quinonoid structures. Therefore, our results on the significant correlations between these chemical indicators and RCs of HSs confirm the previous viewpoint that the aromatic structures, such as quinones, can function as redox‐active groups in HSs (Aeschbacher et al, ; Lovley et al, ; Scott et al, ; Tan et al, ). Although HAs and FAs show remarkable differences in degree polymerisation, MW, and solubility at a strong acidic pH (Stevenson, ), our results showed that the significant relationships between microbial RCs and chemical properties were present both in individual data sets of HAs and FAs and in the integrated data set including HAs and FAs (Figure S5), thus indicating that the responsibility of aromatic systems for the microbial RCs of HSs was independent of HS fraction.…”

“…Our recent works demonstrated the significant relationships between the redox properties of HSs and their chemical structures using a variety of soils along latitude and altitude transects and from agriculture lands (Tan et al, ; Tan et al, ); the results of these studies indicated that the causation of chemical structures with redox properties of soil HSs is independent of the soil source. Thus, although we demonstrated the significantly negative association of the reversible extent of HSs after microbial reduction and then O 2 reoxidation with their AMWs merely using paddy soils, our result is also applicable for HSs originating from other types of soils and matrices.…”

“…Reducing capacities (RCs) of native HSs, of HSs after microbial reduction but before reoxidation, and of HSs after microbial reduction and reoxidation were determined by mediated electrochemical oxidation (MEO) according to the method proposed in previous studies (Aeschbacher, Sander, & Schwarzenbach, ; Tan et al, ). After sterile filtration, the MEO of HS samples in different redox states was performed in a glassy carbon container, which served as a working electrode (WE), under anoxic conditions (N 2 , 25 ± 1°C).…”

“…Reducing capacities (RCs) of native HSs, of HSs after microbial reduction but before reoxidation, and of HSs after microbial reduction and reoxidation were determined by mediated electrochemical oxidation (MEO) according to the method proposed in previous studies (Aeschbacher, Sander, & Schwarzenbach, 2010;Tan et al, 2017).…”

“…Direct comparisons of FT ion cyclotron resonance mass spectrometry and fluorescence analyses have indicated that humic-like fluorescence component generally covaried with aromatic moieties such as quinones and hydroquinones (Boyle, Guerriero, Thiallet, Del Vecchio, & Blough, 2009;Del Vecchio & Blough, 2004;Herzsprung et al, 2012). A low value of E 4 :E 6 is mainly due to absorption by the moieties with carbon double bond of benzene ring (Ikeya, Sleighter, Hatcher, & Watanabe, 2015;Kleber & Johnson, 2010;Li & Hur, 2017 (Aeschbacher et al, 2010;Lovley et al, 1996;Scott et al, 1998;Tan et al, 2017). Although HAs and FAs show remarkable differences in degree polymerisation, MW, and solubility at a strong acidic pH (Stevenson, 1994), our results showed that the significant relationships between microbial RCs and chemical properties were present both in individual data sets of HAs and FAs and in the integrated data set including HAs and FAs ( Figure S5), thus indicating that the responsibility of aromatic systems for the microbial RCs of HSs was independent of HS fraction.…”