Complementary effects of sorption and biochemical processing of dissolved organic matter for emerging structure formation controlled by soil texture

Background: Percolating dissolved organic matter (DOM) from the topsoil is considered the main source of subsoil organic carbon (OC) in temperate soils, but knowledge about its influence on OC storage and structure-forming processes is limited. Aims: We conducted a 30-day incubation experiment with artificial soils to study the effects of percolating DOM and soil texture on OC turnover and initial structure formation. Methods: Artificial soils with contrasting texture, but identical mineral composition, were used to mimic subsoil conditions, where mineral surfaces free of OM come into contact with percolating DOM. After the incubation, we assessed the solution exchange, OM covers on minerals, microbial community and OC turnover, and aggregate formation and stability. Results: A higher sand content caused a lower porosity, accompanied by a lower moisture content. In contrast, the OC retention (21% of the OC input), microbial activity, and community size were unaffected by soil texture. The OM covered 10% of the mineral surfaces within an otherwise OC-free mineral matrix. The formation of large, water-stable aggregates occurred in all soils, but was pronounced in the clay-rich soils (58% mass contribution), which also supported a higher mechanical stability of the aggregates. Conclusions: The initial retention and microbial mineralization of DOM are decoupled from pore sizes and soil solution exchange but are driven by the mineral composition and OC input. The biochemical processing of the percolating DOM can induce large aggregates. Here, the presence of fine mineral particles enhances the formation and mechanical stability of the aggregates, irrespective of their surface charge or sorptive properties.