Dynamique de la matière organique en relation avec les propriétés chimiques et structurales des sols dans une chronoséquence forêt-prairies de graminées en Amazonie orientale (état de Para)
Abstract
Studies of SOM changes along this forest-tc-pasture sequence were based on total
carbon and 13C measurements, which allowed to calculate the distribution of carbon derived from forest and pasture throughout the profiles down to 1 rn depth. Carbon contents reached 116 Tha-l under forest and 12 year old pasture, 123 T ha-1 under 7 year pasture and 126 T ha-1 under 17 year old pasture. In the topsoil (0-0.10 rn) carbon derived from pasture increased from 12% in the 7 year old pasture to 38% in the 17 year old pasture. Moreover a degradation of physical-chernical properties was noted after pasture installation : the total porosity and rnacroporosity decreased in the topsoil, the storage porosity generally decreased, whereas the surface negative charges and clay dispersability increased in the 0-0,40m layers. Pasture establishment has induced an improvernent of aggregate stability in the topsoil, in relation with the change of SOM contents and nature and the increase of faunal activity such as earthworrns. In the topsoil, the changes of SOM quality was evaluated by a decrease of the proportion of light fraction (raw organic matter) and an increase of heavy fraction (organe-mineral macroaggregates) with increasing age of pasture. In addition, temporary increase of SOM biological and chernical s.tability was observed in the 7 and 12
year old pastures.
Après déforestation et installation des prairies dans des oxisols d'une chronosequence forêt-prairie de 7, 12 et 17 ans dans l'état de Para (Amazonie orientale), on a pu observer une légère augmentation de la quantité de carbone de 116 T ha-1(forêt) a 126 T ha-1(prairie de 17 ans). En revanche, les proportions du carbone d'origine graminéenne augmentent notamment en surface de 12 % sous prairie de 7 ans a 38 % sous prairie de 17 ans. Une dégradation des propriétés physico-chimiques est observée, se traduisant par une baisse de la porosité totale, de la porosité de stockage, tandis qu'une augmentation des charges de surface et de la dispersabilité des argiles est notée dans les couches organiques (0-0,40n) un accroissement temporairie de la stabilité chimique et surtout biologique de la matière organique de surface est observée sous prairies de 7 et 12 ans.
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