the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Solubility characteristics of soil humic substances as a function of pH
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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CC1: 'Comment on egusphere-2023-2994', Mohammad Mahbub Kabir, 26 Jul 2024
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-This paper presents some excellent views on the solubility of soil humic substances depending on pH
variations and their solubility mechanisms, together with important biogeochemical perspectives.
Considering this issue, I suggest adding “: mechanisms and biogeochemical perspectives” to the
end of the current title.-Another important issue is that the author should shortly describe the important information
regarding the results of elemental compositions and FTIR data” by replacing “These results were
supported by the corresponding elemental compositions and FTIR data” from the abstract.-In the manuscript, type settings need to be corrected by adding a gap between the ending sentence and
the reference in many cases.Citation: https://doi.org/10.5194/egusphere-2023-2994-CC1 -
AC1: 'Reply on CC1', Khan M. G. Mostofa, 29 Jul 2024
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Interactive review comments
Based on the interactive review comments, we are submitting the manuscript revised duly using the blue color for all revisions made.
-This paper presents some excellent views on the solubility of soil humic substances depending on pH variations and their solubility mechanisms, together with important biogeochemical perspectives. Considering this issue, I suggest adding “: mechanisms and biogeochemical perspectives” to the end of the current title.
Response: Thank you for reviewer thoughtful suggestion. We agreed with reviewer suggestion. As suggested new title “Solubility characteristics of soil humic substances as a function of pH: mechanisms and biogeochemical perspectives” are added in the revised manuscript.
-Another important issue is that the author should shortly describe the important information regarding the results of elemental compositions and FTIR data” by replacing “These results were supported by the corresponding elemental compositions and FTIR data” from the abstract.
Response: Thank you for reviewer thoughtful comment. Based on this comment, we have replaced “These results were supported by the corresponding elemental compositions and FTIR data” by adding ‘Elemental anlysis results demonstrated that the C and N contents of HALS-pH6 were lower and those of O, S and H higher than those of HACS-pH6, suggesting the preservation of C and N without S acquisition in HACS-pH6 possibly because of their complexed with minerals, which, in turn, would determine the insolubility of the HACS-pH6 fraction. FACS+PLSCS showed relatively higher C and S contents, and lower O% with respect to FALS+PLSLS, impling that FACS+PLSCS would remain under mineral protection. Fourier Transform Infrared (FTIR) results show significantly reduced infrared absorptions (e.g. 3300‒3600 and 800-1200 cm−1) of HACS-pH6 with respect to HALS-pH6, suggesting the existence of strong intermolecular interactions among HA functional groups, possibly due to insoluble forms originally complexed with minerals. But FALS+PLSLS exhibited stronger bands at 3414-3429 cm−1 and 1008-1018 cm−1 than FACS+PLSCS, implying a strong interaction among functional groups possibly derived from various organo-mineral complexes in FACS+PLSCS. These results would indicate that’
-In the manuscript, type settings need to be corrected by adding a gap between the ending sentence and the reference in many cases.
Response: Revised accordingly in the manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-2994-AC1 -
CC2: 'Reply on AC1', Mohammad Mahbub Kabir, 29 Jul 2024
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The authors addressed all my recommendations very effectively. I recommend to accept this paper in its current form.
Citation: https://doi.org/10.5194/egusphere-2023-2994-CC2
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CC2: 'Reply on AC1', Mohammad Mahbub Kabir, 29 Jul 2024
reply
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AC1: 'Reply on CC1', Khan M. G. Mostofa, 29 Jul 2024
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