Arbuscular mycorrhizal fungi respond to rhizobial inoculation and croppingsystems in farmers' fields in the Guinea savanna

It has been difficult to explain the rotation effect based solely on N avai lability in maize-soybean cropping systems in the moist savanna zone of sub -Saharan Africa. Although arbuscular mycorrhizal fungi (AMF) can contribute to plant growth by reducing stresses resulting from other nutrient deficie ncies (mainly P) and drought, their role in the maize/soybean rotation crop ping systems in the Guinea savanna has not yet been determined. Pot and fie ld experiments were conducted for 2 years using 13 farmers' fields with dif ferent cropping histories in two agroecological zones (Zaria, northern Guin ea savanna and Zonkwa, southern Guinea savanna) in Nigeria. We quantified t he influence of cropping systems and rhizobial inoculation on plant growth, mycorrhizal colonization and diversity of promiscuous soybean and maize gr own in rotation. The relationships between these variables and selected soi l characteristics in farmers' fields were also examined. Percentage mycorrh izal colonization in promiscuous soybean roots ranged from 7% to 36%, while in maize it varied between 17% and 33%, depending on fields and the previo us cropping history. A large variation was also observed for mycorrhizal sp ores, but these were not correlated with mycorrhizal colonization and did n ot appear to be influenced by rotation systems. Soybean mycorrhizal coloniz ation was higher (13% increase) in Zonkwa, but not in Zaria, if the precedi ng crop was maize and not soybean. These differences were related to the so il P concentration, which was positively related to mycorrhizal colonizatio n in Zonkwa but negatively to this parameter in Zaria. The previous crop di d not affect mycorrhizal colonization of maize in both locations. Soybean c ultivars inoculated with rhizobia had a higher mycorrhizal colonization rat e (25%) and more AMF species than maize or uninoculated soybean (19%). Maiz e grown in plots previously under inoculated soybean also had higher percen tage mycorrhizal colonization than when grown after uninoculated soybean an d maize. Four AMF genera comprising 29 species were observed at Zaria and Z onkwa. Glomus was the dominant genus (56%) followed by Gigaspora (26%) and Acaulospora (14%). The genus Sclerocystis was the least represented (4%).