Rhizophagus manihotis promotes the growth of rhizobia-nodulated Vigna luteola L in phosphorus deficient acid montane soils devoid of ground cover vegetation

Abstract

The failure of Vigna luteola L. to colonize tropical montane regions of Venezuela with acid P-deficient soils that lack vegetation has been mainly attributed to the inability of indigenous arbuscular mycorrhizal fungi (AMFi) to be effective suppliers of P to this host plant. To test this hypothesis, Vigna luteola plants were grown in non sterile soil collected from this habitat. Plants became nodulated by indigenous rhizobia (Nod+) and the roots were colonized by AMFi (AMFi+). Some plants were inoculated with the arbuscular mycorrhizal fungus Rhizophagus manihotis (AMFg+). Other plants were fertilized with 6 mM nitrate and 2 mM P to inhibit nodulation (Nod-) and AMFi colonization (AMFi-), respectively and these served as controls. The Nod+AMFi+ plants displayed the smallest shoot and nodule dry weights upon harvest, the poorest AMF colonization, lowest foliar mineral content (N, P, Mg, Mn, Fe, Zn, and Cu), highest leaf ureide concentrations and lowest soil dehydrogenase, urease and acid phosphatase activities. Greater growth, nodulation, nutrient uptake, photosynthesis, catabolism of ureides in leaves, leaf superoxide dismutase and soil enzymatic activities were found in Nod+AMFg+ plants. The Nod-AMFg+ plants grew even better attributed to their higher P uptake that was allocated mainly to the photosynthetic apparatus rather than to N2-fixation. The results showed that V. luteola plants inoculated with R. manihotis and nodulated by indigenous rhizobia are capable successfully of colonizing open montane regions devoid of ground cover vegetation. The Nod+AMFg+ plants had greater growth, nodulation and root colonization by AMFg resulting in improved nutrient condition, enhanced uptake of nitrate and high catabolism of ureides in leaves than Nod+AMFi+ plants. However, more research is needed before the inoculation of open montane regions with AMFg can be recommended to land managers since a) the enhanced N2 fixation rate in Nod+AMFg+ plants have an extra cost of 1.2 mg P kg−1 leaf dry weight plant−1 which could places an extra burden on the plants grown in the P-deficient soils, and b) the possible impact of AMFg on the microbiology of these former forest soils must be assessed.