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Wageningen University (2002)

Tree effects on crop growth on a phosphorus-fixing Ferralsol

Radersma, S.

Titre : Tree effects on crop growth on a phosphorus-fixing Ferralsol

Auteur : Radersma, S.

Université de soutenance : Wageningen University

Grade : PhD thesis 2002

Résumé
In agroforestry systems trees affect crops. This study aimed at understanding how trees grown in lines for pole production affect maize on an unfertilized, P-fixing Ferralsol, in the agricultural area of Western Kenya. I hypothesized that, because maize production was primarily limited by P-deficiency, trees would affect maize growth by effects on P-uptake of maize, as determined by P-availability of the soil, soil water contents and root-length densities.

Field-based experiments were used to investigate 1) rhizosphere modifications by trees and maize roots and their effects on P-availability, 2) tree water-use and extraction from the soil profile, and 3) root distributions of trees and maize. The tree-crop interaction model WaNuLCAS was adjusted and used to compute the effects on maize production of soil drying and rhizosphere modifications as caused by the trees.

Soil drying occurs if water-use of trees is larger than water-inputs by rainfall. High water-use of trees in tree lines was mainly related to high leaf-area and low leaf-efficiency. Meteorological variables influenced water-use of tree lines less. The importance of leaf-area as determinant of water-use of tree lines was due to the large range over which water-use of tree-lines responds linearly to increases in leaf-area.

Water extraction from the soil with distance to the tree line (horizontal dimension) was highest in the zones near the tree line, and was not clearly related to relative root length density. This was due to the decreasing water-potential gradient, between root and soil, with increasing distance to the stem-base. Vertically, root length densities decreased more strongly than water-potential gradient, and determined the water-extraction profile.

Intermediate to high organic anion contents and pH increases in rhizosphere soil of Cassia spectabilis (syn. Senna spectabilis ) and Grevillea robusta mobilized soil P, which was taken up by the trees themselves. Residual P-availability was not increased. The relatively long duration of pH-increases and organic anion exudation in the rhizosphere of Cassia , combined with the relatively high synlocation of Cassia and maize roots, suggests that maize can profit from P mobilized by Cassia roots.

Mots clés : zea mays / senna spectabilis / grevillea robusta / agroforestry / phosphorus / rhizosphere / ferralsols / crop yield / plant competition / plant interaction / soil water content / kenya / east africa

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