Ammonium versus Nitrate Nutrition of Plants Stimulates Microbial Activity in the Rhizosphere.

Using an alkaline calcareous soil, pot experiments were conducted to elucidate the effects of NHvs. NOnutrition (50 or 100 mg kg−1 soil) of wheat and maize on microbial activity in the rhizosphere and bulk soils. Dicyandiamide was used as nitrification inhibitor to maintain NHas the predominant N source for plants grown in NH-treated soil. While maize grew equally well on both N sources, root and shoot growth of wheat was higher under NHthan under NOnutrition. Bacterial population density on roots, but not in the rhizosphere soil, was higher under NHthan under NO supplied at 150 mg N kg−1 soil; whereas at both N levels applied, NHcompared to NOnutrition of wheat and maize significantly increased microbial biomass in the rhizosphere soil. Under both plant species, NH vs. NOnutrition also increased aerobic and anaerobic respiration, and dehydrogenase activity in the rhizosphere. As microbial activity in the planted bulk and unplanted soils was hardly affected by the N-source, we hypothesize that the stimulation by NHof the rhizosphere microbial activity was probably due to higher availability of root exudates under NH than under NOnutrition. [ABSTRACT FROM AUTHOR]