Enhanced uptake of phosphorus by mycorrhizal sorghum plants as influenced by forms of nitrogen

The effects of NH4+-N and NO3--N applications on rhizosphere pH and P uptake were investigated with and without mycorrhizal inoculation. Sorghum (Sorghum bicolor L.) plants were grown in split pots and in different soils with different initial soil pH values and P contents. In both soils, ammonium treatment resulted in higher plant dry weight and P content than nitrate treatment at all P levels. Mycorrhizal inoculation enhanced the differences. Sorghum plants acidified the rhizosphere at low soil P status with both nitrogen forms, and with or without mycorrhiza. When N was supplied as NO3--N, rhizosphere pH increased gradually with increasing P addition. The experiments showed that, although the zone of strong acidification coincided with the main zone of P depletion, the contribution of Arbuscular Mycorrhizal Fungi (AMF) to P uptake extended beyond the effect of rhizosphere pH change. There were no differences in pH change between mycorrhizal and non-mycorrhizal inoculated soils with the same N form, and it appears that pH change is an independent factor affecting P uptake regardless of whether the plant root is infected or not. The differences in soil pH change with N form do not however explain sufficiently the differences in dry matter increase and P uptake between mycorrhizal and non-mycorrhizal plants.