Chemical and microbial mass balances in microbial turnover of two easily degradable carbon substrates

Recent research indicates that soil microbes play a significant role in the formation and turnover of soil organic matter (SOM). Thus, OM is metabolized by microorganisms through intracellular and extracellular enzymatic activity, with one portion of it being converted into biomass and another being respired for energy. This causes an energy and matter flux that is adjusted and slowed down by ongoing recycling of the matter and residual energy. Matter and energy are conserved as much as possible throughout repeating microbial growth cycles, resulting in an "energy use channel," and/or storage as necromass. Soil fertility and several other soil functions depend on the activity of diverse soil microbial populations and, consequently, on continual energy and carbon flows within the soil system. Fluxes and stoichiometry concerns must be considered for the maintenance of microbial diversity and ecosystem activities in soil, including C storage. To comprehend C turnover and sequestration in terrestrial ecosystems, further knowledge of the relationship between element cycling and energy fluxes is required. In this project, we present a conceptual overview of microorganisms as mediators of SOM production, we do that by investigating seven carbon substrates with varying complexity with the same model soil (fertilized Dikopshof) in five different incubation experiments.

In the first experiment, we study the effect of substrate size (Glucose — 180 Da, α — 1,4-maltotetraose — 666,6 Da). We hypothesize that exoenzymes would be required to degrade any substrate greater in size than 600 Da, meaning different CUE/EUE due to a change in the process type from growth-oriented processes — high energy flux for glucose degradation to the adaption-oriented processes for the larger substrate, i.e., maltotetraose in this case. The substrates were labelled with ¹³C to determine various carbon pools in the samples. Destructive sampling was used to obtain subsamples from 6 different time points. Aminosugars and acids were used as markers of microbial biomass/necromass. Chloroform fumigation extraction was performed to determine microbial biomass of carbon and nitrogen. In combination with further data to calculate the microbial quotient (C_mic/OC), the respiratory quotient (qCO2= resp./C_mic), and CUE. Gas flux sampling and isotope selective CO₂ analysis to determine the differences in the turnover of the substrates (Energy consumption respiration) The energy accumulation includes the formation of additional biomass, necromass, and metabolites. Analysis of C, H, N, S, O, and P to calculate the stoichiometry of OM.