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 HA_CS 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 HA_pH2+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.