Effect of CaCl2 on the kinetics of dissolved organic matter release from a sandy soil�

The dissolution of organic matter in soil is of fundamental relevance for the fate of organic contaminants associated with organic matter and for the microbial availability of organic matter. In this study, the kinetics of soil organic matter (SOM) dissolution from a sandy forest soil was investigated under different electrolyte conditions, using a continuous extraction method. The mathematical analysis of the concentration signal obtained from extractions with constant flow rates and after sudden flow rate changes showed that the dissolution of SOM is diffusion limited. The dissolution rate was lower during extraction with 0.01 M CaCl₂. The reaction on sudden flow rate changes was slower when extracting with 0.01 M CaCl₂ as compared to water, and the mechanism was different. These observations were explained by a gel phase developing in the swelling SOM. The lower dissolution rates found for extractions with 0.01 M CaCl₂ could indicate a more stable gel structure in the presence Ca²⁺. The development of the gel phase may be influenced by mechanical strain due to increased flow rates.