Arbuscular Mycorrhizal Colonization Rate Dynamics in a Guiera Senegalensis-Millet Agroforestry Intercropping System Across a Rainfall Gradient of Senegal.

The Sahel is a semi-arid region subject to frequent droughts and degraded soils where the woody shrub Guiera senegalensis coexists with crops and plays a positive role on millet growth throughout the region. The goal of this research was to establish part of the biological basis of this effect by studying arbuscular mycorrhizal (AM) fungal dynamics in the G. senegalensis rooting zone and their role in stimulating millet growth.  Accordingly, the first ever field scale AM community characterization study in the G. senegalensis-millet intercropping system was conducted in the three major precipitation regimes of Senegal during the 2013 growing season.  G. senegalensis and millet roots grown within and far from shrub influence were excavated from farmers’ fields and analyzed for extent of AM colonization.  We hypothesize that a common mycorrhizal network (CMN) forms between the roots of shrub and millet that is beneficial for millet growth. Results show very high congruent AM hyphal colonization rates (mean=81% shrub, 73% millet) in both G. senegalensis and millet within the shrub influence supporting the CMN hypothesis. In addition, a pot experiment was conducted to examine AM responses in millet grown in near shrub, far shrub and autoclaved soils uncoupled to the roots of the shrub. The rationale is to better understand whether AM responses in near shrub millet are a result of AM properties (e.g. propagule density) inherent in the soil itself or can be attributed to an already intact mycelial network emanating from the shrub that millet connect into when planted near the shrub. Results show significantly greater hyphal and arbuscular colonization rates (p<.05) in millet (of the same physiological age) grown in near shrub soils next to living shrub roots than in millet grown in near shrub soils without the living roots providing further evidence for the presence of a CMN.