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 NH4+ vs. NO3-) nutrition ( 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 NH4+ as the predominant N source for plants grown in NH4+-treated soil. While maize grew equally well on both N sources, root and shoot growth of wheat was higher under NH4+ than under NO3- nutrition. Bacterial population density on roots, but not in the rhizosphere soil, was higher under NH4+ than under NO3- supplied at 150 mg N kg(-1) soil; whereas at both N levels applied, NH4+ compared to NO3- nutrition of wheat and maize significantly increased microbial biomass in the rhizosphere soil. Under both plant species, NH4+ vs. NO3- nutrition 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 NH4+ of the rhizosphere microbial activity was probably due to higher availability of root exudates under NH4+ than under NO3- nutrition.