Dissolved organic matter may induce water-stable aggregates in various soil textures

The interactions between mineral particles and soil organic matter (SOM) are an important factor for soil structure formation. Percolating dissolved organic matter (DOM) from upper soil horizons is considered an important input pathway of organic carbon (OC) into subsoils. While DOM sorption processes have been extensively studied, the effect of DOM input on soil structure formation has rarely been looked at systematically. We conducted a 30-day laboratory incubation experiment to investigate the process of DOM-induced structure formation in artificial model soils with three contrasting textures (clay loam, loam, sandy loam).

The soil texture defined the pore system and the flow characteristics of the soil solution, leading to a lower liquid retention and faster soil solution turnover in the sand-rich soils. In contrast, the OC retention was unaffected by the soil texture, indicating that only the clay minerals and iron oxides, but not the texture-defining quartz grains, contributed to the OC sorption.

The total microbial biomass, as well as the CO₂-release were unaffected by the texture. In contrast, the microbial community composition showed a texture-dependent development with a higher proportion of fungi and gram-positive bacteria in the sand-rich mixtures. This suggests that texture-related architectural features of the pore space shape the microbial community composition.

It could be shown that the biochemical processing of the percolating DOM solution was sufficient to induce the formation of large macroaggregates in all textures without requiring mechanical stress or the presence of physical OM nuclei. Very low OC concentrations (< 0.8 mg g^-1) could support the water-stability of the formed aggregates, although they were not sufficient to provide any meaningful stability against mechanical loads.