PYROLYSIS-FIELD IONIZATION MASS-SPECTROMETRY OF AGRICULTURAL SOILS AND HUMIC SUBSTANCES - EFFECT OF CROPPING SYSTEMS AND INFLUENCE OF THE MINERAL MATRIX

Abstract

Whole soil samples, extracted humic substances, the corresponding fulvic (FA) and humic acids (HA) and the extraction residues (humins) from long-term, agricultural test plots were investigated by in-source pyrolysis-field ionization mass spectrometry (Py-FIMS). For the soils distinct differences in the chemical composition of the organic matter in differently managed fields were observed. The FI mass spectra of the extracted humic substances gave complementary chemical information, as they cover a larger mass range compared to the whole soil spectra. The chemical, structural information of the conventional alkaline extraction residues was demonstrated by Py-FIMS spectra to be similar to that of the related soil samples. Influences of mineral matrix to organic matter ratios were studied on mixtures of extracted humic substances with defined mineral components such as quartz, basalt, iron oxide (Fe2O3), Ca-montmorillonite, kaolinite and illite. It was shown that in these mixtures the number of mass signals detected and the covered mass range decreased, when organic carbon concentrations (C(org)) in this synthetic mineral matrix dropped below 2% (w/w). Limitations in the direct application of Py-FIMS might arise in the case of natural soil samples with C(org) concentrations below 0.5% (w/w), high contents of swelling clay minerals and iron oxides.