Long-Term Crop Productivity and Nutrient Dynamics in a Native Shrub (Guiera senegalensis) Cropping System of the Sahel.

Throughout the Sahel in regions of low natural soil fertility receiving 250-500 mm yearly precipitation, a native shrub, Guiera senegalensis, coexists at relatively low densities (0-300 shrubs ha^-1) with crops. A few studies have shown that this shrub positively impacts crop yields, but long-term experiments examining G. senegalensis grown together with millet and peanut as an intercropping system are lacking. Therefore, a 10-year study of an optimized (1500 shrubs ha^-1) shrub-millet intercropping system was initiated in 2004 in Senegal, West Africa with the objective to determine impacts of shrubs on soil quality, crop productivity, and drought stress resistance. The experiment was a split-plot factorial design with the main plot shrubs (with and without) and subplot of fertilizer rate (0, 0.5, 1, and 1.5 times the recommended N-P-K rate) at Keur Matar, Senegal that had a peanut-pearl millet crop rotation. Yield, biomass, total carbon, particulate organic matter (POM), soil and plant macro and micronutrient data from 2011-2015 were compared with data from 2004-2007.  Yields were consistently increased over the non-shrub plots at all fertilizer levels over the 10-year period.  Furthermore, millet stands become very difficult to achieve over 10 years without the presence of shrubs highlighting the importance of this intercropping system.  In addition, yields normalized for yearly rainfall (kg grain ha^-1 mm^-1 precipitation), showed that the presence of shrubs with or without fertilizer maintained yields even in very dry years compared to non-shrub plots where this yield ratio tracked rainfall.  There were also significantly higher levels of carbon and a majority of the macronutrients in shrub than no shrub plots.  Our data suggest that the advantage of shrub-intercropping is that it is local resource that subsistence farmers can adopt, and our data show that an optimized system regenerates degraded soils, increases crop productivity, and would be an ecological approach to buffer climate change.