Bioenergetic stability of organic matter in forest and grassland soil (Haplic luvisol)

The stability of soil organic matter (SOM) is the trade-off between its bioenergetic profits for microorganisms and the energy barrier for its decomposition. The energy density and activation energy, which determine the SOM stability in topsoil (0–30 cm), belowground biomass and litter in forest and grassland ecosystems, were determined by thermogravimetryand differential scanning calorimetry. Organic matter was fractionated into pools based on their thermal stability — labile, stable, and persistent. The activation energy of thermal decomposition of persistent SOM was 2.8 times higher than for labile SOM. There was more labile SOM per unit of thermally stable SOM in forest soil than in grassland. Therefore, the higher bioenergetic availability of SOM in forest ecosystems suggested a faster carbon turnover compared to grassland soils. Forest soil accumulated 1.5 times more organic carbon than grassland soil. The ratio of energy density to activation energy characterized the potential bioenergetic advantage of organic substrate decomposition by microorganisms, could be used to characterize the SOM quality as an addition to the widely used C:N ratio. The transformation of forest and grassland litter and belowground biomass into SOM was accompanied by a decrease in activation energy and an increase in energy density. These identified processes underline the leading role of progressive decomposition and selective stabilization of microbial metabolites for the SOM formation