Addition of combined nitrogen decreases vesicular-arbuscular mycorrhizal development in young T. subterraneum plants. Ammonium ions, added as (NH4)2SO4, are more effective than nitrate ions, added as NaNO3. Addition of Na2SO4 as a control treatment also decreases infection, suggesting a less specific ‘salt effect’ underlying the responses to combined nitrogen. Inhibition of nodulation and nitrogenase activity occurs (as is normal) in the presence of combined nitrogen.
These effects have been studied as a function of time (10 to 30 days), using a range of external concentrations. Different methods of application (surface addition versus mixing throughout the soil) have been tested. Effects on the levels of N, K+ and Na+ in the plants have also been determined.
The results are discussed in relation to the effects of ammonium nitrogen and nitrate nitrogen on the mineral nutrition of plants, especially within the context of the influence of vesicular-arbuscular mycorrhizas on plant growth.