SOIL ORGANIC-MATTER MOBILIZATION BY ROOT EXUDATES OF 3 MAIZE HYBRIDS

In order to study the macromolecular behaviour of humic substances, we treated humic material (from the Ah horizon of an Eutric Cambisol) ei ther with exudates from 10 day-old seedlings obtained in axenic condit ions from three maize cultivars (Zea mays L.; cv. Paolo, cv. Polaris a nd cv. Sandek) or with the organic acids (succinic and fumaric) presen t in the exudates and followed the change in molecular size distributi on. Succinic acid, Polaris and Sandek exudates shifted the humic absor bance from high to low molecular weight in size exclusion chromatogram s, while Paolo exudates were not able to produce the same shift and ga ve an absorbance profile like that of humic substance alone. Our resul ts are evidence of the micellar behaviour of humic substances in solut ion and of the importance of hydrophobic bonds in holding humic molecu lar together. Exudates were also used to separate soil organic fractio ns from the same soil. Soil organic extracts were characterized by low pressure gel permeation chromatography, hormone-like activities, and H-1-NMR. The data, compared with water extract and KOH extract, indica te that maize exudates, fumaric and succinic acids mobilize soil organ ic fractions in small quantities, but these extracts are endowed with auxin- and/or gibberellin-like activity. Water extract and maize exuda tes exhibit no hormone-like activity, while KOH extract shows a neglig ible one. Moreover, only Polaris and Sandek exudates and succinic acid are able to disaggregate an organic fraction with low apparent molecu lar size (<13KDs) from the bulk of the soil. The H-1-NMR data indicate that water and succinic extracts are largely composed of carbohydrate and aliphatic constituents, Sandek exudate and fumaric acid can liber ate aromatic components, Polaris exudate releases carbohydrates from s oil, while Paolo slightly influences the aliphatic constituents. The r ole of organic acids released by maize seedlings in mobilising soil or ganic fractions is presented. (C) 1997 Elsevier Science Ltd.