Microbial and thermal indices of organic matter stability in lowland soils with variable texture

Soil organic carbon (SOC) stabilization and turnover are important processes of global carbon cycle. This study evaluates microbial activity and thermal stability of soil organic matter (SOM) in lowland soils of central Europe during relatively dry summer period. We observed that the intensity of soil respiration and substrate availability are positively affected by soil moisture. Soil water availability was correlated with clay content that supported water retention and capillary elevation from the subsurface. Besides moisture, soil respiration was partially controlled also by nutrient availability (Ca, Mg, K, P). Microbial C content was not correlated with any of the properties that were measured. To assess SOM stability, we used thermogravimetry combined with differential scanning calorimetry (TG-DSC). Correlations between TG-DSC data and soil respiration indicate that heterotrophic microorganisms degrade SOM without significant preference for either labile, intermediate, or stable organic fraction. The heat released per mass unit of labile SOM correlated positively with the ratio of microbial vs. total organic C in the sample. This suggests that the energy density of labile OM affects positively the C amount that is converted to microbial biomass. More heat was released per mass of labile OM from the samples that showed higher SOC/SOM ratio and organic C/O value. This indicates that higher portion of C bound in SOM affects positively the energy density of labile fraction. The samples with higher clay content released relatively more heat at higher temperatures in comparison to more coarse-textured soils. Considering the effect of clay on SOC stabilization, the findings of this study pointed on the two contradicting aspects. Although the results suggest that interaction of OM with inorganic clay components results in formation of stable C fraction, fine-textured soils showed higher respiration rate, during the drier period, due to the higher water availability.