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Impact of short rotation forestry on soil fertility assessed by ecosystem input-output nutrient budgets

Laclau J.P., Ranger J., Nzila J.D.D., Bouillet J.P., Deleporte P.. 2002. In : World congress of soil science. Bangkok : WCSS, 1 Cd-Rom. World Congress of Soil Science. 17, 2002-08-14/2002-08-20, Bangkok (Thaïlande).

Clonal plantations of Eucalyptus have been introduced since 1978 on savanna soils of the coastal plain of Congo. These high productive stands grow on Ferralic Arenosols of very low chemical fertility. Sustainable management of such short rotation forests depends both on enhancement of stand productivity (constant genetic improvement) and maintenance of the capacity of soil for long term production (sustainability). Nutrient storage in soils and trees and main nutrient fluxes were estimated from the following measurements: i) nutrient content throughout stand development, uptake, internal translocation, litterfall, and forest floor decomposition, using a chronosequence of 5 stands and, ii) atmospheric deposition, canopy exchange and transfer through the soil on the whole rooting depth (6 m) over three years in the native savanna and in the plantation. Nutrient input-output budgets were calculated for the whole rotation of Eucalyptus and compared with the native ecosystem of savanna. Results showed that: - Strong changes in the ecosystem mineral functioning occurred since afforestation. In particular dry depositions of Na+, Ca2+ and Cl- as well as water and nutrient uptake were largely higher in the Eucalyptus stand than in savanna. High amounts of litter fall led to the accumulation of a forest floor in the plantation whereas dead material was burnt every year during the dry season in savanna. - High production was achieved in this alien plantation due to an effective nutrient cycling both in the plant (internal translocations), large restitutions by litterfall, and weak losses by deep drainage. External inputs (atmospheric inputs and fertilization) and mineralization of organic matter played a crucial role for the supply of nutrients to trees in this highly weathered soil. However weeding in the plantations led to the destruction of a leguminous species which was responsible for a N input of about 20 kg ha-1 y-1 by symbiotic fixation in savanna. - Fertilization practices must be reconsidered from these results for the present time and for the future. A N budget clearly unbalanced in the Eucalyptus ecosystems points out that inputs by fertilization will have to increase over successive rotations. By contrast the budgets are rather well balanced for the other nutrients when compared to the amounts available in the soil. These results are consistent with field trials of fertilization and show that introduction of a leguminous understory in these plantations might allow to reduce the cost of fertilizations.

Mots-clés : eucalyptus; arénosol; savane; litière forestière; cycle de l'azote; minéralisation; fixation de l'azote; matière organique du sol; fertilisation; engrais azoté; dose d'application; légumineuse; congo

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