Ca/Na concentration-constrained variations of dissolved organic matter leaching from groundwater-irrigation area soil in North China Plain

Abstract

This study investigates the quantity and quality variations of dissolved organic matter (DOM) leaching from the soil in groundwater irrigation area of the North China Plain, constrained by the concentration of Ca/Na. Soil samples with dominant humic-like (HLC) and protein-like (PLC) components were paired with parallel concentration gradients of Ca/Na extractants for equilibrium experiments. Fluorescence-PARAFAC, UV-visible spectroscopy, and multiple statistical analyses were combined for data analysis and interpretation. The results reveal that the primary DOM components remained dominant for specific soil sample, with a higher relative abundance of PLC (HLC) in Ca (Na) extract. HLC preferentially binds to soil phase in all extractions, while PLC is readily released into the solution. However, Ca inhibits HLC desorption and promotes PLC release more significantly than Na, as indicated by stronger ion/proton reaction (IPR) and electrostatic effect (ESE). The strongest IPR and ESE are seen in the HLC-dominated DOM extracted with Ca, suggesting a condition where Ca bridges to HLC and forms total dissolved organic carbon (DOC) that decreases. In contrast, Na extraction exhibits only a weaker ESE that is offset by soil-contained HLC and exchangeable Ca, resulting in subtle DOC decrease. The trends in leaching of HLC and PLC are self-dependent, and the level of variation in either component correlates with the increasing concentration of specific cations present. These findings underscore the crucial role of soil organic matter (SOM) composition and its interaction with leaching cations in soil management in large-scale groundwater irrigation areas, where SOM quality and groundwater chemistry vary.

Keywords: Ca/Na concentration; Fluorescence-PARAFAC; Groundwater; Ion/proton reaction; Soil DOM.