6533b836fe1ef96bd12a06f7

RESEARCH PRODUCT

Effets à long terme de l'apport répété de déchets organiques sur l'évolution de la matière organique et des éléments trace métalliques dans un sol sableux acide (Couhins, 33)

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The distribution of trace metal elements (ETM) in soils depends on physico-chemical characteristics of the soil (pH, redox potential …) and does not remain fixed during time. The objective of this work was to study the long-term effects of the organic waste applications (sewage sludge and manure) on the evolution of the ETM and the organic matter in a cultivated soil. This study was made on the acid sandy soil of the experimental site of Ambarès (Gironde) set up in 1974 by the INRA of Bordeaux. During 20 years, this soil, cultivated in maize, received various organic applications (manure 10 t ha-1 year-1, sludges 10 t ha-1 year-1 and sludges 100 t ha-1 year-1), which stopped in 1993. This study showed that it existed two critical periods as for the fate of the ETM brought by the organic wastes: the period of applications, because of the new physico-chemical conditions to which these contributions are exposed in the soil, and the stop of the applications which caused again a destabilization of the soil. However, the observed stocks seem very lower than the expected theoretical stocks. The application of manure shows losses mainly in Cu and in Fe, of the order of 50 %. The Sludge 10 plot records moderate losses (< 25 %) contrary to the Sludge 100 plot which shows losses of 23 % for Cu and 63 % for Cd and Zn. This period is also marked by the forming of macro-aggregates in the sludge-amended plots. It seemed that these macro-aggregates had not only a protector role of the organic matter of origin maïsicole against the biodegradation, but was also responsible for the accumulation of the ETM in the grain-size fraction SG (200-2000 µm). The stop of the applications in 1993 is marked by a fast decline of the stocks of organic carbon as well as a degradation of the macro-aggregates in micro-aggregates, coming along with a redistribution of most of the ETM of the fraction SG towards the fraction LA (0-50 µm). The stocks of ETM remain constant on the manure plot after the stop of the applications, contrary to the sludge-amended plots which show a strong decrease (50 %) of stocks of Cu and Pb, suggesting their export in the form of organo-metal complexes. Losses of the order of 15 % are observed in the case of Cd and Zn.