Mycorrhizas improve the absorption of non-available phosphorus by the green manure Tithonia diversifolia in poor soils

The use of Tithonia diversifolia (Tithonia) as green manure has spread over some parts of Africa, Latin America and Asia justified by its fast growth cycle, high nitrogen fixation capacity and high accumulation of phosphorus (P) in tissue, with improved capacity to explore nutrients like P in poor soils. We hypothesized here that P acquisition by Tithonia from poor soils is greatly facilitated by rapid root proliferation and by the formation of symbioses with native arbuscular mycorrhizal (AM) fungi; and that organic P sources are more suitable for Tithonia use than inorganic complexed ones. It was used a mesh exclusion technique in rhizotron to measure uptake of P by Tithonia plants from 33P-labelled sub-compartments into which either extra mycelial mycorrhizal hyphae and roots or only mycorrhizal hyphae could penetrate, in a poor P soil from Nepal. The sub-compartment labelled P was provided as either inorganic33P (33Pi), microbial33P or as 33Pi bound to either Ca or Fe. Root access to the sub-compartments containing 33Pi associated with added cations (Ca or Fe) did not increase 33Pi uptake relative to mycorrhizal hyphal access only. Uptake of microbial33P doubled when roots as well as mycorrhizal hyphae were present, indicating that extra mycelial mycorrhizal fungi associated with Tithonia contribute to uptake of mineral bound P, but do not participate in organic P cycles.