Cereal/legume rotations affect chemical properties and biological activities in two West African soils

A more widespread use of cereal/legume rotations has been suggested as a me ans to sustainably meet increasing food demands in sub-Saharan West Africa. Enhanced cereal yields following legumes have been attributed to chemical and biological factors such as higher levels of mineral nitrogen (N min) an d arbuscular mycorrhizae (AM) but also to lower amounts of plant parasitic nematodes. This study was conducted under controlled conditions to examine the relative contribution of AM, plant parasitic nematodes and increased ni trogen (N) and phosphorus (P) availability to cereal/legume rotation effect s on two West African soils. Sample soils were taken from field experiments at Gaya (Niger) and Fada (Burkina Faso) supporting continuous cereal and c ereal/legume rotation systems and analysed for chemical and biological para meters. Average increases in cereal shoot dry matter (DM) of rotation cerea ls compared with continuous cereals were 490% at Gaya and 550% at Fada. Sho ot P concentration of rotation millet was significantly higher than in cont inuous millet and P uptake in rotation cereals was on average 62.5-fold hig her than in continuous cereals. Rotation rhizosphere soils also had higher pH at both sites. For the Fada soil, large increases in Bray1-P and organic P were observed in bulk and rhizosphere soils. Plant parasitic nematodes i n roots of continuous cereals were 60-80-fold higher than in those of rotat ion cereals. In both cropping systems mycorrhizal infection rates were simi lar at 37 days after sowing (DAS) but at 57 DAS AM infection was 10-15% hig her in rotation sorghum than in continuous sorghum. This study provides str ong evidence that cereal/legume rotations can enhance P nutrition of cereal s through improved soil chemical P availability and microbiologically incre ased P uptake.