Occurrence and effect of arbuscular mycorrhizal propagules in wheat as affected by the source and amount of phosphorus fertilizer and fungal inoculation

Volcanic ash-derived soils in Chile show very unique behavior and properties as soil system due to their unusual composition characterized by high allophane and stabilized humus content. These soils constitute excellent models to study both natural and man-induced VA mycorrhizal effect over plant nutrition and soil ecology sustainability. This paper studies the effect of Glomus etunicatum inoculation of wheat in a natural volcanic soil fertilized with soluble P or with partially acidulated-rock phosphate (pa-RP) at two rates (17 and 86 kg P ha(-1)); yield, plant phosphorus acquisition, and mycorrhizal colonized root length, are measured. The influence of these treatments on mycorrhizal mycelium and spore production as well as on soil phosphatase (P-ase) activity was also determined. The inoculation of G. etunicatum, locally isolated, increased significantly the extent of P plant acquisition, spore number, length of extraradical mycelium, and P-ase activity when compared with indigenous arbuscular mycorrhizal (AM) fungi fertilized with pa-RP, at the level of 86 kg ha(-1). In concordance with these results, the remaining available P in the experimental soil without inoculation was depressed. A negative impact of soluble P application in G. etunicatum inoculated soil was noted in the P-ase activity, and also in the effectiveness of the applied inoculum in relation to P plant uptake. In soil AM inoculated and fertilized with pa-RP (86 kg ha(-1)), the enhancement of P-ase activity was related to high mycelium development and spore formation. P plant acquisition in G. etunicatunt inoculated plants ranged from 4.96 to 11.57 mg per pot when 86 kg ha(-1) of pa-RP is applied compared with the same amount of soluble P Surprisingly, adding pa-RP does not improve the amount of colonized root length. Fungal root AM colonization and AM propagules (mycelium and spores) were not depressed at higher soluble P supply, but the activity of G. etunicatum measured as P plant uptake was strongly affected. The inoculation with G. etunicatum enhances spore production, particularly at the lower soluble P and at the highest pa-RP levels. A close relationship (r = 0.938) between AM spores and P-ase activity was found. Only in pa-RP treatments the G. etunicatum inoculation reduced the external P wheat requirements. The improvement of P-ase activity by G. etunicatum inoculation, as a biological factor involved in P-cycling in soil, may be an important mechanism related to plant P acquisition. It is concluded that pa-RP is the best source of P, not only by increasing P uptake by interacting with G. etunicatum, but also by enhancing AM propagules (mycelium and spores) remaining in soil. (C) 2003 Elsevier Science B.V. All rights reserved.