Soil Salts Affect MAOC Content and the Adsorption of Organic Matter on Clay Minerals in Coastal Saline Soils

ABSTRACT The composition of dissolved salts is a key factor controlling the formation of mineral‐associated organic carbon (MAOC) in coastal saline soils, but how salt type and concentration quantitatively affect the adsorption of organic matter to clay minerals remains unclear. In this study, a field experiment was conducted to investigate the effect of long‐term organic amendment on the MAOC of coastal saline soil and the correlation between MAOC and clay minerals. In addition, laboratory experiments were conducted to examine the adsorption–desorption behaviors of potassium humate (KH), glycine (G), and chitosan (CTS) on Na‐ and Ca‐saturated montmorillonite (Na‐M and Ca‐M, respectively), illite, and kaolinite in electrolytes with varying cationic and anionic compositions. The results of the field experiment showed a significant positive correlation between MAOC and clay mineral content. The adsorption data of KH and G by clay minerals were well fitted by the Langmuir model, suggesting a monolayer chemical adsorption mechanism. Among the tested clay minerals, Ca‐M exhibited the highest Langmuir‐derived maximum adsorption capacities (Q m ) for KH and G, with values of 571 and 631 mg g −1 , respectively, and the measured desorption rates ranged from 8.6% to 20%. In contrast, Na‐M showed the highest Q m for CTS (432 mg g −1 ) among all clay minerals. The adsorptions of KH and G were 53–318% higher in Ca 2+ and Al 3+ electrolytes than in Na + systems. Increasing Na + concentration gradually from 0.01 to 0.05 M in electrolyte enhanced KH adsorption by 29%–72% but reduced CTS adsorption by 6.8%–15% on the Na‐saturated clays, while it decreased KH adsorption by 1.3%–11% yet increased G adsorption by 8.3–20% on the Ca‐saturated clays. G adsorption was significantly correlated with the cation exchange capacity (CEC) ( r = 0.81) and specific surface area (SSA) ( r = 0.87) of the clay minerals. With the adsorption of KH, the SSAs and CECs of clay minerals increased by 5.1%–42% and 4.5%–96%, respectively. CTS adsorption increased the interlayer spacings of Na‐M and Ca‐M. Saturation by Ca 2+ and Na + changed the adsorption capacities of the clay minerals for organic matter in different electrolytes. This study demonstrated that compared to monovalent cations, the presence of bivalent or multivalent cations is more favorable for the formation of organo‐mineral complex in coastal saline soils. Therefore, it is suggested that Ca‐based mineral conditioner be applied together with organic material to better improve SOC in coastal saline soils.