Tuesday, 11 July 2006

N and P Cycling in Crop-Native Shrub Agroecoystems of the African Sahel.

Ekwe Dossa1, Richard Dick2, Mamadou Khouma3, Modou Sene3, Aminata Badiane4, and Ibrahima Diedhiou3. (1) Oregon State Univ, 3017 ALS, Soil Science, Corvallis OR, OR 97331, (2) Ohio State Univ, Sch. of Nat. Res., 2021 Coffey Road, Columbus, OH 43210-1085, (3) ISRA, BP Route de Hydrocarbons, Dakar, Senegal, (4) USAID, B.P. 49, Dakar, Senegal

The widely known Parkland agroforestry system, where farmers allow trees in their fields, can benefit soils and crops in Africa. However, there is another prominent Sahelian ecosystem component - native shrubs - that have largely been ignored in terms of research and extension relative to row crop production. In the Sahel, these shrubs coexist in cropped fields and have the curious characteristic of regrowing (~0.75 m height by 1-3 m canopy dia.) over the dry winter period after the cropping season. This is followed by coppicing of shrubs by farmers and undesirable burning of shrub residue in the spring to prepare fields for the rainy summer cropping season. Our recent work has shown that two dominant shrubs (Guiera senegalensis and Piliostigma reticulatum) have much greater biomass (peak-season of 2.1-3.2 and 3.0-4.6 Mg ha-1, respectively) and potential to provide organic inputs to soils than any other source (e.g. manure, composts or trees) if this material was not burned. G. senegalensis (200-600 mm) and P. reticulatum (700->1200 mm) are found throughout the Sahel and in farmers fields to varying degrees depending on population/agricultural intensity and landscape parameters. We hypothesized that the presence of shrubs in farmers' fields and an annual non-thermal return of shrub residues to soils would alter the N and P cycling (key nutrients in the Sahel) and nutrient efficiency of row crops systems. Therefore, the objective of this research was to determine the N and P budgets, and temporal dynamics of plant available NO3, NH4 and PO4 in shrub-crop (peanut and pearl millet) systems. Field plots were established in Senegal, West Africa, at two sites in 2003; one with P. reticulatum at Keur Ndary Ndiaye in the higher rainfall south (700-900 mm) and one with G. senegalensis at Keur Mata in the lower rainfall (200-400 mm) northern region. Both have a split-plot design with shrub (absence or presence) as a main plot and fertilizer (0, 0.5, 1 and 1.5 dose of recommended NPK) at subplot level. In 2004 it was shown that the presence of these 2 shrub species increased peanut yields by about 50% over non-shrub plots. The fertilizer efficiency was increased with shrubs and yields at all rates increased with presence of shrubs even when no fertilizer was applied. Preliminary analysis of millet data for 2005 shows we will have a similar crop yield response in 2004. At the ISRA Bambey station in central Senegal, the effect of shrub biomass is being studied in the absence of plants to isolate shrub residue effects on soil N and P cycling, C fractions and row crop yield. The treatments are shrub litter (at 1500 or 3000 kg ha-1) and fertilizer rate (0, 0.5 or 1.0 X the recommended dose) on soil chemical properties and crop yield. In 2004 there were no negative effects of shrub biomass on crop (peanut) establishment or yield. This is a positive outcome as this provides evidence that shrub residue does not need to be burned. Results are showing that addition of biomass for both shrub species, significantly increases particulate organic matter to soils which is an important fraction for improving soil quality in these sandy soils.

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