Soil organic matter (SOM) represents a continuum of progressively decomposing organic compounds mainly provided by primary producers and predominantly metabolized by adapted dynamic microbial communities. The carbon (C) in SOM flows through the microbial biomass, which needs – beside C and nutrients – Gibbs energy for growth and maintenance. The microbial metabolism and thus the degradation and stabilization of SOM follow thermodynamic laws. The thermodynamic perspective on soil systems is increasingly becoming the focus of research and has the potential to take us a substantial step towards a mechanistic understanding of SOM turnover and stabilization. An integral part of new bioenergetic concepts and models is the energy content of SOM, but the number of empirical studies dealing with soil C cycling or storage in relation to energy contents and flux is small.

In this study, topsoil profiles (comprising organic forest floor horizons OL, OF, OH and the mineral soil layer 0-5 cm) at an afforested post-mining site were investigated to evaluate the influence of (i) soil depth – representing different stages of organic matter (OM) turnover – and (ii) litter quantity and quality (litterfall and fine root tissues) provided by different tree species (Douglas fir – Pseudotsuga menziesii, black pine – Pinus nigra, European beech – Fagus sylvatica, red oak – Quercus rubra) on the energy contents of SOM. The total energy content stored in soils and plant litter was determined using two calorimetric approaches: bomb calorimetry and differential scanning calorimetry combined with thermogravimetry (DSC-TG).

The results of the litter inputs obtained with both methods showed the same trends: the C cycle in the soil was fueled by aboveground and belowground litter inputs, with energy-richer litterfall tissues (needles > leaves) compared to fine root tissues. However, with bomb calorimetry higher energy contents were generally observed in plant litter but also in the upper two forest floor horizons (OL, OF) of the soil profiles. The energy content per unit C (calorific value) changed with increasing depth due to the progressive turnover and stabilization of organic compounds but surprisingly, we identified opposite depth trends with both methods: bomb calorimetry revealed decreasing calorific values, while with DSC-TG increasing calorific values were determined. The few existing studies reported either the one trend or the other with ongoing decomposition, leading to different interpretations of the energetic driven microbial modulated formation and turnover of SOM.

It is mandatory to overcome this fundamental challenge to achieve a reliable integration of the promising bioenergetic approaches into conceptual and modelling frameworks to assess SOC turnover and persistence based on robust empirical data.