Solubility characteristics of soil humic substances as a function of pH: mechanisms and biogeochemical perspectives

Abstract. Soil humic substances (HSs) typically alter their electrochemical behaviours in the pH range of 1–12, which simultaneously regulates the stability of organo-minerals by modifying the HS functionalities. This process facilitates both biotic and abiotic transformations, which consequently leads to the export of degradative byproducts (e.g. HS components, nutrients) from soils into surrounding aquatic environments through water and/or rainwater discharges. However, the solubility features, environmental consequences, and mechanisms of HSs, including humic acids (HAs), fulvic acids (FAs), and protein-like substances (PLSs), under different pHs remain unclear. To respond to these issues, we used two soil extracts which were fractionated in the pH range from 12–1. 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, resulting 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 HSs at lower pH resulted in the formation of highly available and remaining 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 dissolved organic matter (DOM), respectively, in the two soils. Elemental analysis results demonstrated that the C and N contents of HALS-pH 6 were lower and that those of O, S, and H were higher than those of HACS-pH 6, suggesting the preservation of C and N without S acquisition in HACS-pH 6, possibly because of their being complexed with minerals, which, in turn, would determine the insolubility of the HACS-pH 6 fraction. FACS + PLSCS showed relatively higher C and S contents and lower O% with respect to FALS + PLSLS, implying 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-pH 6 with respect to HALS-pH 6, suggesting the existence of strong intermolecular interactions among HA functional groups, possibly due to insoluble forms originally complexed with minerals. However, FALS + PLSLS exhibited stronger bands at 3414–3429 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 HS insolubility arises via organo-metal and organo-mineral interactions at alkaline pH, along with HApH 6 insolubility via rainwater/water discharge, whereas HApH 2 + FA + PLS appears to be soluble at acidic pH, thereby being transported in ambient waters via rainwater/water discharge and groundwater infiltration. Therefore, the pH-dependent behaviour of soil HSs 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.