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

Abstract

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.