Abstract

HELFRICH, M., FLESSA, H., DREVES, A. and LUDWIG, B.

J. Plant Nutr. Soil Sci. 173, 61-66 (2010)

Findings of previous studies suggest that there are relations between thermal stability of soil organic matter (SOM), organo-mineral associations, and stability of SOM against microbial decay. We aimed to test whether thermal oxidation at various temperatures (200°C, 225°C, 275°C, 300°C, 400°C, or 500°C) is capable of isolating SOM fractions with increasing stability against microbial degradation. The investigation was carried out on soils (Phaeozem and Luvisol) under different land-use regimes (field, grassland, forest). The stability of the obtained soil organic carbon (SOC) fractions was determined using the natural-¹³C approach for continuously maize-cropped soils and radiocarbon dating. In the Luvisol, thermal oxidation with increasing temperatures did not yield residual SOC fractions of increasing microbial stability. Even the SOC fraction resistant to thermal oxidation at 300°C contained considerable amounts of young, maize-derived C. In the Phaeozem, the mean ¹⁴C age increased considerably (from 3473 y BP in the mineral-associated SOC fraction to 9116 y BP in the residual SOC fraction after thermal oxidation at 300°C). An increasing proportion of fossil C (calculated based on ¹⁴C data) in residual SOC fractions after thermal oxidation with increasing temperatures indicated that this was mainly due to the relative accumulation of thermally stable fossil C. We conclude that thermal oxidation with increasing temperature was not generally suitable to isolate mineral-associated SOC fractions of increasing microbial stability.