Microbial carbon use efficiency of mineral-associated organic matter is related to its desorbability

Abstract

Interactions between organic substances, minerals, and microorganisms are crucial for organic carbon (OC) stabilization in soil. We hypothesized that thresholds of sorption strength (described by the sorption coefficient of the Freundlich isotherms) and desorbability (i.e., the ratio of the amount desorbed to the amount sorbed) of organic monomers control the extent of their microbial processing.Freundlich sorption isotherms and desorbability of uniformly ¹⁴C-labeled glucose, acetylglucosamine, phenylalanine, salicylic acid, and citric acid onto goethite, kaolinite, and illite were studied in batch experiments. Monomers adsorbed to minerals were mixed with loamy and sandy arable topsoil and incubated at 25°C. Mineralization of mineral-adsorbed monomers was observed over three weeks, after which the assimilation into microbial biomass, and the ¹⁴C remaining in soil were quantified. Subsequently, the mineralization of incubated soils was observed for additional three weeks after glucose priming.The adsorption of carboxylic acids onto minerals exceeded that of (amino) sugars and phenylalanine, with the overall highest amounts both adsorbed and retained after desorption with water for goethite. Assimilation of monomer ¹⁴C into microbial biomass and the microbial carbon use efficiency (CUE) of mineral-adsorbed monomers in both soils increased linearly with the monomer desorbability from mineral phases. Furthermore, the CUEs of monomers adsorbed to goethite were lower than those of the same monomers adsorbed to clay minerals. In terms of total amount of carbon retained in the soil, carboxylic acids adsorbed on goethite showed highest values, emphasizing the significance of oxides for the stabilization of OC within soils. Priming of incubated soil with non-labeled glucose caused an additional mineralization of monomer-C, with the priming effect decreasing from goethite to clay minerals.We conclude that sorption strength and desorbability shape microbial utilization of mineral-bound organic compounds, but no universal thresholds determine bio-accessibility of sorbed organic compounds.