Experimental access to cellulose oxidation and the dynamics of microbial carbon and energy use in artificial soil under varying temperature, water content, and C/N ratio

Soil is one of the most important natural carbon sinks, alongside its many other critical functions. The ratio of carbon incorporated into the soil system for instance as biomass as a proportion of carbon consumption from a given substrate is defined as carbon use efficiency (CUE). CUE is assumed to depend on environmental conditions. However, the conservation of energy within the soil system, quantified as energy use efficiency (EUE), has not yet been thoroughly studied related to CUE. To study the effects of environmental conditions on cellulose degradation in the absence of soil organic matter background, we utilized artificial soil to analyse their impact on CUE and EUE. We quantified CO2 evolution rate, heat production rate, and cellulose degradation, at three water contents (10%, 14.4% and 19%), two C/N ratios (9 and 18), and two temperatures (7 degrees C and 20 degrees C). Environmental conditions significantly influenced the primary parameters such as CO2 evolution, heat production, and cellulose degradation rate while the derived parameters CUE and in particular EUE were less sensitive. Among the studied factors, water content had the least influence on ultimate CUE and EUE. Conversely, decreases in nitrogen supply and temperature tended to increase CUE without significantly affecting EUE, which is generally higher than CUE. Over the degradation process, CUE often started from high values > 90% and then decreased to lower ones around 50%, whereas the EUE remained more constant on a higher level. The variations of CUE and EUE suggest alterations in the stoichiometry of the microbial growth reaction and thus add additional factors (e. g. time lapse of the degradation process) to be considered for their assessment.