Hyphenated techniques of thermal analysis for characterisation of soil humic substances

Our aim was to investigate the thermal behaviour of humic substances extracted from temperate and tropical soils by means of hyphenated techniques of thermal analysis (e.g. simultaneous thermal analysis DSC/TG coupled with mass spectrometry, MS, for the analysis of evolved gas, EGA) in order (i) to verify whether the chemical composition of isolated humic substances also reflected the differences in microbial parameters previously measured in related soil samples and (ii) to identify suitable. indices of thermal stability. Our results show that the investigation of humic substances by thermal methods can provide information on soil organic matter dynamics. Differences in thermal behaviour between the two groups of soils were found. The indices of thermal stability here proposed, IR (index of thermal recalcitrance), and ID (index of thermal decomposability) clearly showed that in humic substances from tropical soils the thermally recalcitrant organic fraction dominated, whilst in temperate humic substances the opposite held. This agrees with previous results on the microbial dynamics and organic matter turnover of the respective soils and indicates that these indices of thermal stability could represent a useful tool in soil environmental quality investigations.