Galactosamine can be used as an indicator for microbial extracellular polymeric substances (EPS), an important source of stable organic matter in agricultural soils

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Four experiments form a consecutive programme for elucidating the role of EPS (extracellular polymeric substances) in agricultural soil environments, based on the following two central hypotheses: (1) EPS are an important component of microbial necromass, which can be extracted from soil with cation exchange resins (CER). (2) Galactosamine (GalN) indicates microbial EPS in soil. In the first experiment, microbial EPS formation will be investigated by cultivating soil bacteria and soil fungi, using glycerol and starch as a C source in the absence or presence of SOM-free sterile sand. In the second experiment, simple (glycerol or maize-derived starch) and complex (maize-derived cellulose or maize straw) 13C labelled substrates will be tested for their ability to form bacterial and fungal EPS in four soils, differing in their fungi to bacteria ratio due to differences in soil pH and fertilization history. In the third experiment, glucose and cellulose decomposition will be monitored to investigate carbon use efficiency (CUE) and thermodynamic efficiency in two soils, differing in their fungi to bacteria ratio due to differences in fertilization history and due to the absence or presence of EPS. In the fourth experiment, microbial EPS formation in soil will be induced by applying to one soil a simple and easily available substrate (maize-derived starch) that promotes bacterial EPS formation and a more recalcitrant substrate (maize-derived cellulose) that promotes fungal EPS formation. The persistence and aggregate stabilizing effect of the freshly formed EPS will be monitored for 70 days. The C/N ratio of the substrates will be generally adjusted to 40, using (NH4)2SO4 in experiment 1 and (15NH4)2SO4 in experiments 2, 3, and 4. In all experiments, CER extracted EPS will be analysed for their concentration of amino sugars, combined with their compound-specific δ13C and δ13N analysis. EPS extracts will be additionally checked for total organic 13C, total δ15N, total carbohydrates, total protein, and total deoxy-ribonucleic acid (DNA). In the soil, soil organic 13C, microbial biomass 13C, total 15N, microbial biomass 15N, enzyme activity (N-acetyl--D-glucosaminidase and tyrosine-peptidase) and amino sugars will be measured, combined with their compound-specific δ13C and δ13N analysis. In the soil atmosphere, the evolution of total CO2 and 13CO2 will be measured. In experiment 3, heat productions will be additionally monitored using microcalorimetry.