Sorption thermodymamics and aggregation as controls of substrate use and mineralization

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Sorption strength of organic molecules to minerals and the stability of aggregates protecting organic substrates are among the boundary conditions that “shape the channel for energy and matter use” by the microbial community of soil systems (overall Hypothesis C of SPP 2223). Although the stabilization of organic matter against microbial use and mineralization in soil has been linked to sorption, its relationship to the thermodynamics of sorption processes has not been quantified so far. Furthermore, the impact of soil aggregation and soil microbial diversity on this relationship is unknown. To analyze the boundary conditions shaping the channel for energy and matter use, we will test the following hypotheses: (H I) sorption and desorption energies increase with increasing polarity and degree of oxidation of the sorbate and increasing number of surface hydroxyl groups of the sorbent, (H II) microbial substrate use decreases with decreasing deltaG, increasing thermodynamic sorption hysteresis and activation energy of desorption with thermodynamic thresholds for the use of sorbed substrates, (H III) thermodynamic thresholds for microbial use of sorbed substrates increases with increasing functional diversity of the soil microbial community and (H IV) high physical protection of substrates within aggregates undermines the importance of sorption thermodynamics for microbial substrate use. We will test these hypotheses in four corresponding workpackages. In WP1 we will be determine Gibbs free energies deltaG of sorption reactions, sorption enthalpies (deltaH) and entropies (deltaS), activation energies Ea of desorption reactions and losses of molar free energy due to sorption hysteresis (Tii) for 6 model substrates and three model minerals via sorption-desorption experiments at different temperatures, Arrhenius plots and microcalorimety. WP2 will use miniaturized incubation experiments with SPP soil samples to determine use and mineralization of 14C labelled model substrates adsorbed to model mineral mixed into 4 SPP soils. The role of functional diversity in mineralization will be studied in WP3 by a second incubation experiment with SPP soils spiked with extracted microbes to manipulate microbial diversity or nutrients based on enzymatic analyses using ecoplates. Finally, in WP 4, the mineralization of selected 13C-labelled model substances adsorbed to dispersed and aggregated soil systems that were characterized morphologically and physically will be measured. Combining the expected results, our project will deliver essential insights for the integration of sorption processes and aggregation into thermodynamic concepts of soil ecosystem functioning and development.