Phosphorus uptake by cowpea plants from sparingly available or soluble sources as affected by nitrogen form and arbuscular-mycorrhiza-fungal inoculation

In most plant species, nutrient uptake is facilitated upon root association with symbiotic arbuscular mycorrhizal (AM) fungi. The aim of the present experiment was to test how the form in which nitrogen (N) is supplied to the growth medium affects substrate pH, AM development, and contribution of the symbiosis to phosphorus (P) uptake from sparingly available or soluble resources. Cowpea (Vigna unguiculata L. Walp) plants inoculated or noninoculated with AM fungi (Glomus sp.) were grown in pots with a sand substrate supplied with nutrient solution. The nutrient solution was prepared either with a high or a low concentration of soluble P, and NO3--N : NH4+-N ratios of 9:1 or 5:5. The substrate supplied with low-P nutrient solution was either or not additionally amended with ground rock phosphate. Despite a high level of root colonization, AM fungi used in the present study did not appear to increase plant availability of rock phosphate. It cannot be excluded that the ability of AM root systems to acquire P from sparingly available resources differs depending on the plant and fungal genotypes or environmental conditions. The absence from the growth substrate of P-solubilizing microorganisms able to associate with AM mycelia might also have been a reason for this observation in our study. Increased supply of NH4+ relative to NO3- improved plant P availability from rock phosphate, but also had a negative effect on the extent of AM-fungal root colonization, irrespective of the plant P-nutritional status. Whether increasing levels of NH4+ can also negatively affect the functioning of the AM symbiosis in terms of plant element uptake, pathogen protection or soil-structure stabilization deserves further investigation.