Effects of young agroforestry trees on soils in on-farm situations in western Kenya

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 were Cordia africana, Croton megalocarpoides, Grevillea robusta, Acacia tortilis, Prosopis juliflora and Sesbania sesban. Samples were also taken inside and outside a fenced plantation of Prosopis juliflora. Soil samples were analysed for total C%, total N%, (^13)C abundance, (^15)N abundance, extractable P and pH.
 

The most sensitive indicator of effects of trees on soils was (^13)C abundance, which reflected a shift in inputs of C from C4 grasses to C from C3 trees. All species except Prosopis lowered the (^13)C abundance by 0.5-1 δ-units. This was equivalent to an increase of 3-5% of the percentage of C contributed by C3 species. Prosopis trees did not decrease the (^13)C 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 (r2= 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 on- farm situations, (^13)C abundance is recommended as a particularly sensitive indicator of the influence of trees on sites previously dominated by C4 crops and grasses.
 

Nyberg, G. & Högberg, P.  Effects of young agroforestry trees on soils in on-farm situations in western Kenya. Agroforestry Systems 1995, 32:45-52.
 
Link to publication