Age, turnover and molecular diversity of soil organic matter in aggregates of a Gleysol

We used an integrated approach to describe soil organic matter (SOM) dynamics through known inorganic and organic components in aggregates of adjacent forested and cultivated Gleysolic soil. Mineral and SOM components were examined in water stable macroaggregates (>250 mu m), microaggregates 1 (50-250 mu m) and microaggregates 2 (<50 mu m) fractions. SOM was characterized by pyrolysis-field ionization mass spectrometry (Py-FIMS), and soil minerals by X-ray diffraction analysis. The mean residence time of organic-C (OC) was determined using radiocarbon dating. OC turnover was determined using the natural abundance of native C-13 and that derived from corn residue. We found that OC in macroaggregates was young (<100 yr), turned over in 14 yr, and consisted of OM typical of that found in tissues of plants and soil organisms. Chemical classes of compounds in macroaggregates consisted mainly of carbohydrates, lignin monomers and phenols, lignin dimers, lipids (alkanes, alkenes, n-alkyl esters), fatty acids, sterols, suberin and aliphatic and aromatic N compounds. The fast turnover time of OC in larger size aggregates supports the hypothesis that the initial decline in SOM after breaking native land is associated with losses of SOM stored in macroaggregates. OC in microaggregates 1 was young (<100 yr) and turned over in 61 yr. OC in microaggregates 2 was old, turned over in 275 yr, and consisted of highly humified macromolecules. Pyrolyzable SOM products representing plant and microbial components like lignin dimers, sterols, suberin and fatty acids were absent from microaggregates 2 containing old OC. The turnover time of OC correlated directly with the amount of smectite and Al extracted with ammonium oxalate, inversely with non-expandable phyllosilicates, and weakly with the total clay content of aggregates. Thermolabile and thermostable molecular components in aggregates indicated degree of association between SOM and clay minerals. Carbohydrates, peptides and alkylaromatics appeared to be less affected by abiotic stabilization reactions.