PLANT GROWTH RESPONSES IN NATURAL ACIDIC SOIL AS AFFECTED BY ARBUSCULAR MYCORRHIZAL INOCULATION AND PHOSPHORUS SOURCES

ABSTRACT

The effect of arbuscular-mycorrhizal (AM) fungus Glomus etunicatum inoculation in interaction with two sources of phosphorus (P) [soluble P and partially acidulated phosphate rock (pa-PR)] at three rates (17, 43, and 86 kg P ha⁻¹) was studied in an acidic natural soil using wheat (Triticum aestivum L.) as host plant. Shoot and root dry biomass, AM colonized root length, macro-micronutrients content and soil phosphatase (P-ase) activity were determined after six months of plant growth. The inoculated G. etunicatum fungus, a fungal strain adapted to the prevailing soil conditions, enhanced plant growth (shoot and root biomass) and mineral acquisition of some elements when plants were fertilized with pa-PR but not with soluble P. The nutrient acquisition by AM inoculated plants varied with the source and amount of applied P. When pa-PR was supplied, the inoculated AM fungus enhanced P, potassium (K), aluminum (Al), and manganese (Mn) plant acquisition in comparison with indigenous endophytes alone. Shoot zinc (Zn) and copper (Cu) uptakes were also enhanced by G. etunicatum inoculation only at the intermediate assayed pa-PR level (43 kg ha⁻¹). AM root colonization in the efficient pa-PR treatments, did not relate well to the plant growth and nutrient acquisition in most cases. Nutrients (Ca and Mg) that increased in AM inoculated plants were not those commonly deficient in acidic soils. Nevertheless, some nutrients, which often become limiting under low pH conditions such as P and K were increased by G. etunicatum inoculation plus pa-PR. Changes in rhizospheric soil pH under pa-PR application may be involved in these mycorrhizal effects on nutrient acquisition. The increases in plant biomass as a result of mycorrhizal inoculation do not seem to account for all the changes observed in mineral acquisition. The highest soil P-ase activity was observed at the lowest pa-PR dose showing a negative relationship with P-availability. The inoculation of G. etunicatum was effective in this natural acidic soil in overcoming factors that restrict plant growth and nutrition when pa-PR was applied.

ACKNOWLEDGMENTS

We would like to thank the CONICYT for supporting this research through Research Grant Fondecyt 1990756 and also for providing funds to R. Azcón for travel and lodging in Chile through C.I. 7990086 Grant.