Dual-Domain Regulation of Dissolved Organic Matter Release in Soil: The Role of pH and Calcium

Dissolved organic matter (DOM), being the most reactive soil organic matter (SOM) fraction, affects key biogeochemical processes in soil like nutrient cycling, pollutant transport, and organic carbon sequestration. Quantitative understanding of physical-chemical processes regulating the release of DOM in response to variation in factors such as pH and Ca concentration is still lacking. Here, we conducted batch experiments and employed the Natural Organic Matter-Charge Distribution (NOM-CD) model and the Non-Ideal Consistent Competitive Adsorption-Donnan (NICA-Donnan) model to investigate the physical-chemical processes controlling DOM release in seven agricultural topsoils under varying pH (3–9) and Ca concentration (0–10 mM). The DOM fractionation results showed that while hydrophilic acid (Hy), fulvic acid (FA), and humic acid (HA) concentrations increased with pH, their contribution to total DOM differed: Hy and HA dominated at respectively low pH (∼4–6) and high pH (∼8–9), whereas FA peaked at near-neutral pH (∼6.5–7). Our NOM-CD model calculations revealed that changes in the DOM concentration at low pH (pH < ∼5–6.5) were mainly due to OM desorption from soil minerals. Changes in the DOM concentration at high pH (pH > ∼5–6.5) were predominantly controlled by OM dissolution, as demonstrated by the relation between the DOM concentration and Donnan potential (φ_D) of DOM calculated with the NICA-Donnan model. Based on these findings, we propose a conceptual Dual-Domain Desorption Dissolution model in which the relative importance of these two controlling mechanisms is quantitatively assessed for the first time. These insights will be helpful to better quantify soil management effects on the stability and functioning of SOM.