Determining vesicular–arbuscular mycorrhizal effectiveness by monitoring P status of leaf disks

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Abstract
A greenhouse experiment was conducted to monitor the development of symbiotic interaction between the vesicular–arbuscular mycorrhizal (VAM) fungus Glomus aggregatum and Vigna unguiculata grown in a typical Oxisol before and after the soil was subjected to simulated erosion and at various levels of phosphorus in the soil solution. VAM development monitored in terms of P status of cowpea leaf disks revealed that VAM activity was not detected in the eroded soil unless the soil was amended with P. When P was not limiting, VAM activity (effectiveness) was detected as early as 17 days from planting, the activity peaking 5–10 days thereafter. Peak VAM activity was observed at a soil solution P level of 0.026 μg/mL and the peak values were similar in the eroded and uneroded soil samples. Maximum mycorrhizal inoculation effect was also observed at this level of soil solution P. Our results illustrate the usefulness of the leaf-disk technique for monitoring the development of the VAM symbiosis and the significance of soil solution P in regulating host response to VAM inoculation.