Preprints
https://doi.org/10.5194/egusphere-2023-2994
https://doi.org/10.5194/egusphere-2023-2994
06 Jun 2024
 | 06 Jun 2024
Status: this preprint is open for discussion.

Solubility characteristics of soil humic substances as a function of pH

Xuemei Yang, Jie Zhang, Khan M. G. Mostofa, Mohammad Mohinuzzaman, H. Henry Teng, Nicola Senesi, Giorgio S. Senesi, Jie Yuan, Yu Liu, Si-Liang Li, Xiaodong Li, Baoli Wang, and Cong-Qiang Liu

Abstract. This study investigated the solubility features, environmental consequences, and mechanisms of humic substances (HS), including humic acids (HA), fulvic acids (FA), and protein-like substances (PLS), in two soils in the pH range of 1–12. The pH-dependent presence or absence of fluorescence peaks in the individual HS components reflected their functional group proton/electron exchange features at both low and high pH values, which were related to their solubility or insolubility. In particular, alkaline pH (≥ pH 9) yielded the anionic forms (‒O‒ and ‒COO‒) of phenolic OH and carboxyl groups of HACS resulted in decreased electron/proton transfer from HS functionalities, as indicated by the decline of fluorescence peak maxima, whereas the protonic functionalities (e.g., −COOH, −OH) of HS at lower pH resulted in the formation of highly available and remains uncomplexed HS forms. The solubility of HA fractions increases with increasing pH, whereas their insolubility increases with decreasing pH, which determines their initial precipitation at pH 6 and final precipitation at pH 1, amounting approximately to 39.1–49.2 % and 3.1–24.1 % of the total DOM, respectively, in the two soils. HS insolubility arises via organo-metal and organo-mineral interactions at alkaline pH, along with HApH6 insolubility via rainwater/water discharge, whereas HApH2+FA+PLS appears to be soluble at acidic pH, thereby being transported in ambient waters via rainwater/water discharge and groundwater infiltration. These results were supported by the corresponding elemental compositions and FTIR data. Therefore, the pH-dependent behaviour of soil HS greatly contributes to a better understanding of the progressive transformation, mobility/transportation, and immobility/accumulation of HS components under various environmental conditions, with relevant implications for sustainable soil management practices and soil DOM dynamics.

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Xuemei Yang, Jie Zhang, Khan M. G. Mostofa, Mohammad Mohinuzzaman, H. Henry Teng, Nicola Senesi, Giorgio S. Senesi, Jie Yuan, Yu Liu, Si-Liang Li, Xiaodong Li, Baoli Wang, and Cong-Qiang Liu

Status: open (until 18 Jul 2024)

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Xuemei Yang, Jie Zhang, Khan M. G. Mostofa, Mohammad Mohinuzzaman, H. Henry Teng, Nicola Senesi, Giorgio S. Senesi, Jie Yuan, Yu Liu, Si-Liang Li, Xiaodong Li, Baoli Wang, and Cong-Qiang Liu
Xuemei Yang, Jie Zhang, Khan M. G. Mostofa, Mohammad Mohinuzzaman, H. Henry Teng, Nicola Senesi, Giorgio S. Senesi, Jie Yuan, Yu Liu, Si-Liang Li, Xiaodong Li, Baoli Wang, and Cong-Qiang Liu

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Short summary
The solubility characteristics of soil humic acids (HA), fulvic acids (FA), and protein-like substances (PLS) at different pH values remain uncertain. The key findings includes: HA solubility increases with increasing pH and decreases with decreasing pH; HApH6 and HApH1 contribute to 39.1–49.2 % and 3.1–24.1 % of total DOM, respectively; and HApH2, FA, and PLS are highly soluble at acidic pH values and are transported by ambient water. These issues are vital for sustainable soil management.