Effects of different soil organic amendments (OAs) on extracellular polymeric substances (EPS)

Extracellular polymeric substances (EPS) synthesized by soil microorganisms play a crucial role in maintaining soil structure by acting as binding agents of soil aggregates. Microbial EPS production is governed by C sources, soil nutrient availability, pH, and other local environmental factors. Another important factor is soil management, and particularly, the addition of organic amendments (OAs), has the potential to influence soil EPS as it can change the biotic and abiotic properties of the soil. Yet the response of soil EPS to the addition of OAs, especially in field trials, and its subsequent impact on soil aggregation remains unclear. This study aimed to elucidate the influence of OAs (including compost from organic residues, mown grass from roadsides and parks, and cattle manure) on soil EPS content and aggregate stability in a three-year field experiment with annual OA application. We further investigated factors that govern EPS production in the soil by exploring the relationship between soil EPS (i.e., polysaccharide and protein content), soil physicochemical properties (i.e., pH, dissolved organic carbon, available and total amount of nutrients), and the soil microbial community (i.e., microbial abundance and taxonomic structure). We found that the addition of grass, manure, and the combination of grass and manure led to an increase in soil EPS content compared to unamended and compost-amended soils. EPS content was correlated with soil variables; in particular, a significant positive correlation was observed between EPS concentration and available N in the soil. Furthermore, bacterial and fungal biomass contributed to soil EPS. Specific bacteria (e.g., members of Proteobacteria, Bacteroidetes, and Chloroflexi) and fungi (e.g., members of Ascomycota and Basidiomycota) demonstrated strong and significant correlations with EPS in the soil. The direction of correlation, whether positive or negative, varied at the order level. In addition, our study revealed significant positive correlations between EPS concentration and soil aggregate stability. These findings offer insights into designing sustainable agricultural management practices, and whether the application of appropriate OAs can enhance soil EPS content and, consequently, soil aggregate stability.