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) d ynamics through known inorganic and organic components in aggregates o f adjacent forested and cultivated Gleysolic soil. Mineral and SOM com ponents were examined in water stable macroaggregates (>250 mu m), mic roaggregates 1 (50-250 mu m) and microaggregates 2 (<50 mu m) fraction s. SOM was characterized by pyrolysis-field ionization mass spectromet ry (Py-FIMS), and soil minerals by X-ray diffraction analysis. The mea n residence time of organic-C (OC) was determined using radiocarbon da ting. OC turnover was determined using the natural abundance of native C-13 and that derived from corn residue. We found that OC in macroagg regates was young (<100 yr), turned over in 14 yr, and consisted of OM typical of that found in tissues of plants and soil organisms. Chemic al classes of compounds in macroaggregates consisted mainly of carbohy drates, lignin monomers and phenols, lignin dimers, lipids (alkanes, a lkenes, n-alkyl esters), fatty acids, sterols, suberin and aliphatic a nd aromatic N compounds. The fast turnover time of OC in larger size a ggregates supports the hypothesis that the initial decline in SOM afte r breaking native land is associated with losses of SOM stored in macr oaggregates. OC in microaggregates 1 was young (<100 yr) and turned ov er in 61 yr. OC in microaggregates 2 was old, turned over in 275 yr, a nd consisted of highly humified macromolecules. Pyrolyzable SOM produc ts representing plant and microbial components like lignin dimers, ste rols, suberin and fatty acids were absent from microaggregates 2 conta ining old OC. The turnover time of OC correlated directly with the amo unt of smectite and Al extracted with ammonium oxalate, inversely with non-expandable phyllosilicates, and weakly with the total clay conten t of aggregates. Thermolabile and thermostable molecular components in aggregates indicated degree of association between SOM and clay miner als. Carbohydrates, peptides and alkylaromatics appeared to be less af fected by abiotic stabilization reactions.