Effects of young agroforestry trees on soils in on-farm situations in western Kenya |
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| Authors: | G Nyberg P Högberg |
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| Institution: | (1) Section of Forest Soils, Department of Forest Ecology, Swedish University of Agricultural Sciences, S-901 83 Umeå, Sweden |
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| Abstract: | Soil samples were taken at different distances from the trunks of four- to five-year-old trees of six species planted on-farm in western Kenya. The tree species wereCordia africana, Croton megalocarpoides, Grevillea robusta, Acacia tortilis, Prosopis juliflora andSesbania sesban. Samples were also taken inside and outside a fenced plantation ofProsopis juliflora. Soil samples were analysed for total C%, total N%,13C abundance,15N abundance, extractable P and pH.The most sensitive indicator of effects of trees on soils was13C abundance, which reflected a shift in inputs of C from C4 grasses to C from C3 trees. All species except Prosopis lowered the13C abundance by 0.5–1  -units. This was equivalent to an increase of 3–5% of the percentage of C contributed by C3 species. Prosopis trees did not decrease the13C abundance because of the abundant grass-growth around them. Cordia, which had the most pronounced effects, raised the C%, N% and extractable P by 27%, 26% and 55%, respectively. Nitrogen and C were well correlated (r
2=0.97) in the whole material. Effects on soil pH were in both directions, i.e. it was raised under the Prosopis plantation by 0.33 units, while it was lowered under Acacia by 0.21 units.Effects of agroforestry trees were thus seen in as short time as five years in practical onfarm situations.13C abundance is recommended as a particularly sensitive indicator of the influence of trees on sites previously dominated by C4 crops and grasses. |
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| Keywords: | Africa soil properties 13C trees |
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