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Valorisation d'un compost de boues urbaines en garrigue pour le reboisement : comportement des jeunes arbres d'une plantation et modifications de la dynamique de la vegetation naturelle apres amendement

Abstract : Since sewage sludge dumping has been banned in the EEC, there is renewed interest in studying new ways of recycling it. Mediterranean areas are prone to recurrent fires and violent precipitation events that favour erosion and soil impoverishment. Calcareous degraded soils are often colonized by stable Quercus coccifera garrigues that are adapted to low resource availability, and consequently tree installation is delayed. There, organic amendment could increase soil fertility level, modify the vegetation dynamics and the dominance relationships, and could improve succession and resilience processes, as well as reforestation. Two experiments were settled in Provence (South-eastern France) to study the effect of co-composted sewage sludge and green wastes on the dynamics of the garrigue ecosystem. First, we superficially spread 0, 50 and 100 Mg.ha-1of fresh compost in the natural garrigue biotope, and observed the consequences on ecosystem functioning. Soil, vegetation, and litter microorganisms compartments were monitored during the two years following amendment (2002-2004). Secondly, three rates of fresh compost (0, 20, and 40 kg.m-2) were incorporated into the soil at the stem of planted seedlings. The tree density was 400 seedlings.ha-1, and so 8 and 16 Mg.ha-1 of fresh compost were globally spread. The evolution of soil properties and the development of trees were followed respectively for three years (2001-2003) and two years (2002-2003). Amendment durably (2 years) and efficiently improved soil fertility. However, the soil was greatly enriched in P and Zn, which could lead to serious water pollution problems and to toxic effects on microorganisms. These two elements limit sewage-sludge compost use in natural areas, and repeated amendment at similar rates are not considerable. Compost amendment in the plantation globally improved reforestation by enhancing seedling growth and nutrition, and particularly by increasing their survival under drought. In addition, compost spreading generated fertile patches in garrigue biotope, in which succession processes could locally be enhanced and tree installation later favoured, especially Pinus halepensis'. Moreover, functional diversity is increased by compost spreading, as seeders are improved, which could increase the global resilience of ecosystem after fire. These beneficial effects of compost rather occurred for the intermediate rates. Moreover, the maximal rates should not be used because the fungal colonization of Quercus coccifera litter was depleted on D100, and because P and Zn pollution risks increased with amendment rate. However, the calcareous nature of the experimental soil and the high compost maturity limited water exportations and the plant contamination by potentially toxic elements. Compost superficial spreading rather increased plant sensitivity to drought, while it decreased when compost and soil were mixed. Finally, the compost spreading in garrigue had multiple effect (fertilization, crushing and mulching) and highlighted plant specific strategies in nutrient management and productivity. This work showed the relative importance of species on ecosystem functioning, identified key species for element cycling, and pointed out the complementarity resource use that exist in such natural ecosystem.
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Submitted on : Thursday, March 17, 2005 - 8:02:12 PM
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Marie Guittonny - Larcheveque. Valorisation d'un compost de boues urbaines en garrigue pour le reboisement : comportement des jeunes arbres d'une plantation et modifications de la dynamique de la vegetation naturelle apres amendement. Ecologie, Environnement. Université de Provence - Aix-Marseille I, 2004. Français. ⟨tel-00008812⟩

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