EFFECTS OF ADDITION OF CLAY-MINERALS ON THE FATTY-ACID FRACTION OF A PODZOL SOIL

The A1 horizon of a sandy podzol soil, with a very small proportion of clay-sized material, was sampled and homogenized. Samples, both with and without supplementation with 150 g clay minerals kg(-1), natural heteroionic bentonite (saturated with Na, Ca, Mg, K), Mg-bentonite or Fe-kaolinite, were moistened to two-thirds of water-holding capacity and incubated at 28 degrees C for 16 weeks. After incubation, lipids were extracted with 3:1 light petroleum: ethylacetate mixture in a Soxhlet apparatus as unbound components (directly extractable by organic solvents) and bound components (only extractable after treatment with acids). Monoacids (free and esterified), diacids, ketoacids and hydroxyacids were isolated from the two lipidic fractions (unbound and bound) by column chromatography. The different components of each family were analyzed by gas chromatography (GC) or gas chromatography-mass spectrometry (GC-MS). Incubation without supplementation generated a selective decrease in abundance of free monoacids in unbound and bound fractions. The decrease was in inverse proportion to the chain-length of components. Soil supplementation with bentonite (homoionic or heteroionic) did not induce an overall quantitative change, but promoted a transfer of fatty acids from the unbound to the bound fraction, increased the exchange between free and esterified monoacids (transesterification) and stimulated hydrolysis of glycerides. In contrast, soil supplementation with Fe-kaolinite caused much decomposition or biotransformation of monoacids but it did not induce change either in the intensity of transesterification in the wax ester fraction or in the intensity of hydrolysis in the glyceride fraction.