Quantity and quality variations of dissolved organic matter (DOM) in column leaching process from agricultural soil: Hydrochemical effects and DOM fractionation

Leaching dynamics of dissolved organic matter (DOM) from agricultural soil are influenced by the chemistry of irrigation water. This study used multiple spectroscopy methods to investigate the dynamics of DOM in column leaching process by utilizing selected solutions. DOCwas leached out by Na solutions more effectively than by Ca, despite the additional contributions of the ligand exchange of SO42- and the hydrolysis of HCO3-. The concentration ranking corresponding to solutions is Na2SO4 > NaHCO3 > NaCl > CaCl2. Furthermore, the aromaticity and molecular weights of DOM leached by Na solutions increased with the leaching, whereas contrary trends were reared by Ca. Thus, specific cation substantially affects DOM concentration and composition. Parallel factor analysis of the three-dimensional fluorescence spectra (EEM-PARAFAC) determined the characteristics of DOM components. (1) The long-wavelength, humic-like components (HLC, i.e., C1, C3, and C5) were largely accumulated at the early water saturating stage but attenuated toward dynamic stability in the salt-solution leaching (SSL) stage. (2) Reactive functional group-related HLC (i.e., C2, and C4) remarkably increased in response to the solutions chemistry. (3) The protein-like component (PLC, i.e., C6) was released with insignificant dynamic differences between the solutions. Despite approximately stable DOC concentration at the late stage of each SSL, HLC exhibited accelerating release in sodium salts but declining trend in calcium salt whereas PLC showed a roughly constant dynamic in the both. Ca-bridging reactions with the detachment of OM-Ca-clay complex prompting HLC release and its formation to stabilize the HLC well explain the contrast dynamics of HLC with Na or Ca leaching. The diffusion process that was defined for the slow, lasting and small concentration of leaching of inactive components could contribute to the steady dynamic of PLC, given its insensitivity to either the salt cation. This study identified distinct release kinetics of DOM compositions from those of DOC by EEM-PARAFAC and correspondingly the potential mechanisms, which offers promising advantages in monitoring DOM transport and fractionation in dynamic leaching process. (c) 2019 Elsevier B.V. All rights reserved.