Conversion from Conventional to No Tillage Alters Thermal Stability of Organic Matter in Soil Aggregates

The quantity and stability of soil organic matter (SOM) associated with soil aggregates are affected by tillage management, which can be characterized potentially using the technique of thermal analysis. In this study, we evaluated the concentration and thermal stability of SOM occulted with various aggregate classes under no-tillage (NT) and moldboard plow (MP) treatments using thermogravimetry (TG) and differential scanning calorimetry (DSC). Soil samples were collected 10 yr after tillage experiment was started. The results showed that conversion from MP to NT significantly increased SOM concentration and the proportion of large macro-aggregates (>2 mm) in the 0- to 5-cm soil layer. For all aggregate classes the derivatives of thermogravimetry curves (DTG) had three weight loss peaks near 100, 350, and 500 degrees C, and one endothermic peak and three exothermic peaks in the DSC curves. No differences in ignition temperature, peak position, and ending temperature of SOM combustion were observed between tillage treatments and among the soil layers. For both tillage systems, the proportion of thermal labile SOM (weight loss in 200-400 degrees C accounting for that in 200-550 degrees C, Exo(1)/Exo(tot)) and energy densities (ED) of SOM (energy release per unit SOM) declined with decreasing aggregate size in the 0- to 20-cm soil layer. Moreover, TG-T-50 (the temperature resulting in 50% of SOM loss) correlated negatively to aggregate size, but DSC-T-50 (the temperature at which 50% of energy resulting from organic matter combustion release) correlated positively to aggregate size. Compared with MP management, NT management improved quantity but decreased thermal stability of SOM in aggregates in the 0-to 5-cm layer, which was indicated by the greater weight loss at combustion, higher Exo(1)/Exo(tot) ratio, greater energy densities and lower TG-T-50. No tillage also led to increased stratification ratios of thermal labile and thermal recalcitrant SOM.