Receptivity of an Argentinean Pampas Soil to Arbuscular Mycorrhizal Glomus and Acaulospora Strains

Soil receptivity to arbuscular mycorrhizal (AM) fungi tests the capacity of a soil to favour the mycorrhizal development after inoculation. Thus, receptivity is a key criterion to assess whether the introduction of non-indigenous AM fungi will successfully improve plant growth. Two experiments were set up to investigate the receptivity of a moderately acidic wheat-growing soil of the Argentinean Pampas (south America) to non indigenous Glomus and Acaulospora AM strains. Soil was collected from agricultural non fertilized wheat fields and native AM fungi were identified. At first, soil was tindalized and four AM strains were study in their capacity of colonizing and improving growth of a highly mycotrophic and mycorrhizal responsive test plant. Then, the most efficient AM strains were inoculated in the soil containing the native microflora and fertilized with phosphorus (P) and the mycorrhizal development and benefit were assessed for wheat plants. The G. clarum fungus formed the highest colonization, as revealed by trypan blue (TB) staining and for alkaline phosphatase (ALP) activity in onion plants, when it was inoculated in the sterile soil and intermediate in the presence of indigenous AM fungi, but it did not produced highest mycorrhizal responsiveness (MR) of wheat. The lowest colonization and non significant increases in plants growth were found after inoculation with A. laevis. Thus, neither strain was efficient at developing in the soil and improving plant growth. Inoculation with G. claroideum or A. longula led to intermediate colonization of onion when the fungus was inoculated in the sterile soil and in wheat in the native fertilized soil. Although fertilization depressed AM development, colonization and arbuscules were highest for plants inoculated with G. claroideum and A. longula. With nil P, colonization was highest for plants inoculated with G. claroideum followed by G. clarum and non inoculated (native mycorrhizal) plants. Shoot dry matter of wheat was really increased upon P fertilization, but it was only sporadically increased by inoculation – for shoots and roots only for G. claroideum at low P and for shoots for A. longula at high P-. Mycorrhizal responsiveness was higher with nil P in comparison with added P. Inoculation with G. claroideum and A. longula plus fertilization caused the greatest MR in shoot and grain dry matter and in P uptake compared to non-inoculated plants. This is the first report on the soil receptivity to non indigenous AM fungi in the Argentinean soil. Further research must confirm if the inoculation of field agricultural soils from Argentinean Pampas without indigenous A. longula or G. claroideum strains and moderate P fertilization, could enhance the development of an effective AM symbiosis for wheat crops.