Soil fertility and arbuscular mycorrhizal fungi related to trees growing on smallholder farms in Senegal

Tree cultivation in the dryland agroecosystems is increasingly advocated as a strategy to protect and reverse soil fertility decline, thus sustaining agricultural production. Woody legumes trees like the Ana tree Faidherbia albida (Del.) Chev. host N2-fixing bacteria as well as arbuscular mycorrhizal fungi (AMF), which may contribute to the soil organic carbon pool and soil fertility. The objective of this work was to determine how trees influenced AMF and soil fertility in the agroecosystems of two rural communities (Palmarin and Fimela) of the Saloum Agricultural Eco-Region of Senegal. Smallholder farmers typically cultivated 3-4 fields ranging in size from 0.5 to 2.0 ha with the major crop being millet (Pennisetum glaucum L.). Soil fertility was low to medium, with about 1% soil organic matter (SOM), 17±2.2 mg Bray-1 P kg−1 and 65±5.8 mg extractable K kg−1 in the fields studied. There were seven times more trees per hectare and greater tree diversity in fields around Palmarin, where the smallholder farmers resided, than in the fields they cultivated in Fimela. Social norms appeared to protect trees inside the residential village, while trees farther away are prone to being cut. The relationships between trees, AMF and soil fertility were examined using exploratory path analysis, a structural equation modeling technique. The path analysis model revealed a direct and significant (P<0.05) impact of trees on SOM and pH, which in turn affected the plant-available nitrogen and phosphorus concentrations. The hypothesized relationships between trees, AMF and soil fertility were not supported.