Soil organic matter mobilization by root exudates of three maize hybrids

In order to study the macromolecular behaviour of humic substances, we treated humic material (from the Ah horizon of an Eutric Cambisol) either with exudates From 10 day-old seedlings obtained in axenic conditions from three maize cultivars (Zea mays L. ; cv. Paolo, cv. Polaris and cv. Sandek) or with the organic acids (succinic and fumaric) present in the exudates and followed the change in molecular size distribution. Succinic acid , Polaris and Sandek exudates shifted the humic absorbance from high to low molecular weight in size exclusion chromatogram,s while Paolo exudates were not able to produce the same shift and gave an absorbance profile like that of humic substance alone. Our results are evidence of the micellar behaviour of humic substances in solution and of the importance of hydrophobic bonds in holding humic molecular together. Exudates were also used to separate soil organic fractions from the same soil. Soil organic extracts were characterized by low pressure gel permeation chromatography, hormone-like activities, and ‘H-NMR. The data, compared with water extract and KOH extract, indicate that maize exudates, fumaric and succinic acids mobilize soil organic fractions in small quantities, but these extracts are endowed with auxin- and/or gibberellin-like activity. Water extract and maize exudates exhibit no hormone-like activity, while KOH extract shows a negligible one. Moreover, only Polaris and Sandek exudates and succinic acid are able to disaggregate an organic fraction with low apparent molecular size (<13KDs) from the bulk of the soil. The ‘H-NMR data indicate that water and succinic extracts are largely composed of carbohydrate and aliphatic constituents, Sandek exudate and fitmaric acid can liberate aromatic components, Polaris exudate releases carbohydrates from soil, while Paolo slightly influences the aliphatic constituents. The role of organic acids released by maize seedlings in mobilising soil organic fractions is presented.