Marcel Lorenz, Thomas Maskow, Sören Thiele-Bruhn

• The turnover and stabilization of organic matter (OM) in soils depend on mass and energy fluxes. Understanding the energy content of soil organic matter (SOM) is therefore of crucial importance, but this has hardly been studied so far, especially in mineral soils. In this study, combustion calorimetry (bomb calorimetry) was applied to determine the energy content (combustion enthalpy, ΔCH) of various materials: litter inputs, forest floor layers (OL, OF, OH), and bulk soil and particulate organic matter (POM) from topsoils (0–5 cm). Samples were taken from 35-year-old monocultural stands of Douglas fir (Pseudotsuga menziesii), black pine (Pinus nigra), European beech (Fagus sylvatica), and red oak (Quercus rubra) grown under highly similar soil, landscape and boundary conditions. This allowed to investigate the influence of the degree of transformation and litter quality on the ΔCH of SOM. Tree species fuel the soil C cycle with high-energy litter (38.9 ± 1.1 kJ g−1C) and fine root biomass (35.9 ± 1.1 kJ g−1C). As plant material is transformed to SOM, ΔCH decreases in the order: OL (36.8 ± 1.6 kJ g−1C) ≥ OF (35.9 ± 3.7 kJ g−1C) > OH (30.6 ± 7.0 kJ g−1C) > 0–5 cm bulk soil (22.9 ± 8.2 kJ g−1C). It indicates that the energy content of OM decreases with transformation and stabilization, as microorganisms extract energy from organic compounds for growth and maintenance, resulting in lower-energy bulk SOM. The POM fraction has 1.6-fold higher ΔCH compared to the bulk SOM. Tree species significantly affect ΔCH of SOM in the mineral soil with the lowest values under beech (12.7 ± 3.4 kJ g−1C). The energy contents corresponded to stoichiometric and isotopic parameters as proxies for the degree of transformation. In conclusion, litter quality, in terms of elemental composition and energy content, defines the pathway and degree of the energy-driven microbially mediated transformation and stabilization of SOM.