Effect of Polyelectrolytes on Soil Organic Matter in Model Experiments

The influence of polyelectrolyte-based soil conditioners on the qualitative and quantitative composition of soil organic matter (SOM) was studied for two soils: sandy Dystric Arenosol (Aric) with 1% of C and silt loamy Abruptic Luvisol (Siltic, Cutanic) with 2% of C. The investigated polymer formulations included a synthetic polycation poly(diallyldimethylammonium chloride) (PDADMAC), a humic-based polyanion (commercial humic product Lignohumate, LH), and an interpolyelectrolyte complex (IPEC) prepared from PDADMAC and LH. In laboratory experiments, soils were treated with polymers and the composition of SOM was analyzed and compared with the control. The effect of polyelectrolytes on the molecular weight distribution and amphiphilic properties of SOM was evaluated using low-pressure size-exclusion chromatography and reversed-phase hydrophobic interaction chromatography. It was shown that the effect of polyelectrolytes differs depending both on the chemical nature and composition of polymers’ functional groups and on the soil properties. Polyanionic LH promoted mobilization of labile humic fractions, whereas polycationic PDADMAC was adsorbed in non-labile fractions. Positively charged IPEC occupied an intermediate position increasing the yield of mobile fractions only in sandy soil. The weighted average molecular weight of SOM slightly decreased with the application of all the polymers. The impact of IPEC on the distribution of hydrophobic and hydrophilic fractions depended on the native humus properties. In clay loam, it increased the proportion of the hydrophilic fraction, whereas in sandy soil the proportion of hydrophobic fractions was higher. Soil organic matter in the sandy low-humus soil provided more distinct effects of polymer application. In contrast, in a more humified loamy soil, polymers were immobilized by binding with the soil organomineral matrix and their effect on the SOM was weak.