Hydrologic Relations Between Native Woody Shrubs and Food Crops in the Peanut Basin, Senegal
The Sudano-Sahel is prone to frequent shortages of food supply. Agricultural productivity in this region is severely limited by soil degradation, desertification, and highly erratic rainfall patterns. The single most limiting factor for agricultural production in the Sudano-Sahel is water supply to the crop rooting zone. As the population increases, so too does pressure on the already marginal farmlands, and drought can have devastating effects. Agricultural practices that provide adequate nutrient and water supplies while maintaining crop yields and land conservation are urgently needed in the region. Local knowledge and prior research indicate that intercropping with the native woody shrubs Guiera senegalensis and Piliostigma reticulatum can increase crop yields by over 250% and avert drought in- duced crop failure. An important feature of these native shrubs is their ability to lift water from deep moist soil and redistribute it into shallow, dry soil. The goal of this study is to provide quantitative and mechanistic understanding of the effect of the native shrubs on water and nutrient availability to crops, and how the crops respond physiologically to being grown near shrubs. Prior work in the Peanut Basin has shown that planting food crops within the rooting zone of native woody shrubs has the potential to provide increased nutrients, water resources, and harvest yields compared to crops planted on bare soil. These shrubs are among the few plants in the region able to maintain photosynthesis even during periods of extreme drought. The specific research questions addressed here are how shrubs can affect the growth and vigor of crops during the growing season, how the presence of shrubs and their biomass have affected soil physical properties over ten years, and whether shrubs performing hydraulic redistribution provide water to nearby crops under drought stress conditions. The sites of Keur Matar Arame and Nioro Du Rip in the Central and Southern
Peanut Basin in Senegal have been under management since 2003 with a rotation of peanut (Arachis hypogea) and pearl millet (Penisetum glaucum) intercropped with native shrubs. In order to investigate the effect of ten years of biomass addition on soil properties, surface infiltration rate, hydraulic conductivity and water retention were measured on samples from the two fields. At a site with G. senegalensis the surface infiltration rate was 75% higher in the absence of shrubs than in plots with shrubs. At the Southern site crops planted alone had surface infiltration rates that were 28% higher than plots with intercropped P. reticulatum shrubs. Additionally soil moisture, transpiration rate, crop growth and physiological data were collected in the field during the growing seasons of 2012 and 2013 peanut and millet were grown, respectively. This dissertation shows that shrubs help protect shallow soil layers early in the season from potentially deadly drought and heat and despite lower soil moisture content at the end of the season, crops grown in association with shrubs have increased biomass production and a faster development cycle. At Keur Matar the presence of shrubs can increase plant available water in the shallow zones in the early season by up to 14%. Ad- ditionally, at Keur Matar the shrubs moderate the temperature in the early part of the season by keeping daily maximum temperature 5◦C cooler in the crop+shrub treatment. At Nioro the results showed decreased moisture content in the crop+shrub treatment compared to the crop only treatment at 10 cm depth, but an increase of moisture of 18-20% of plant available water at 20 cm depth in the crop+shrub plot compared to the crop only. The temperature differences were less pronounced at Nioro with the daily maximums were 0-5◦C cooler in the crop+shrub plot than in the crop only. Under irrigation in 2014 an isotopic tracer study investigating hydraulic redistribution was carried out by injecting deuterium-enriched water into the roots of three shrubs at one-meter depth and sampling shrubs and nearby crops for isotopic analysis of plant water. Deuterium enriched water was found in the crops of all three plots with δD in the crop tissue over 300‰. This provides strong support of the idea that water is lifted from >1 m by the shrubs and that the crops take advantage of some of this water. By keeping the rhizosphere hydrated hydraulic redistribution by the shrubs may be one way that shrubs can help crop roots avoid mortal desiccation under drought conditions. Using even the limited resources that farmers possess, this agroforestry technique can be expanded over wide swaths of the Sahel.