Soil particle aggregation and aggregate stability associated with ion specificity and organic matter content

Soil aggregate formation and stability are profoundly influenced by soil organic matter (SOM) content and electrolyte conditions by altering particle interactions during soil particle aggregation and aggregate breakdown. However, the effect of the ion-interface reaction, especially the newly discovered asymmetric hybridization of counterions, and its coupling with SOM on particle interactions and accordingly the particle aggregation and aggregate breakdown remain unclear. In this study, four black soils with different SOM contents were adopted to examine the particle aggregation kinetics using dynamic laser scattering method and aggregate stability using aggregate breakdown intensity in Li+, Na+, and K+ solutions with different concentrations. Combining experimental results with theoretical calculation of particle interactions, it found that different patterns of asymmetric hybridization between the three counterions produce ion specificity and lead to an ion sequence of Li+ < Na+ < K+ in promoting soil particle aggregation and reducing soil aggregate breakdown by decreasing the electrostatic repulsive force between soil particles. An increase in SOM, particularly its dissolvable fraction, increases the electrostatic repulsive force and steric repulsion and then inhibits soil particle aggregation, which is contrary to the effect of the ion sequence. The effects of ion specificity and SOM on particle aggregation could cancel each other in some specific cases of ion species and SOM contents. Moreover, similar to the above ion sequence, the increase in SOM also reduces soil aggregate breakdown and improves soil aggregate stability, because SOM greatly increases the van der Waals attractive force and short-range bonding interactions but rarely changes the electrostatic repulsive force between particles in soil aggregates.