Effect of Nitrogen Form and Counterion on Establishment of theRhizobium trifolii-Trifolium repensSymbiosis

The effect of level of different counterion forms of $NH4+$ or $NO3-$ or both, on establishment of the Rhizobium trifolii-Trifolium repens symbiosis was evaluated in plants cultured under bacteriologically controlled conditions. Ammonium had little effect on nodule formation, and even after $NH4+$ exhaustion there was little compensatory nodule formation. Plant growth, nodule mass and acetylene reduction activity all declined with increasing levels of $NH4+$. Except for $HCO3-$, different $NH4+$ counterion forms had little effect on plant growth, nodule number and mass, and acetylene reduction activity. Ammonium markedly decreased pH, the extent being dependent upon $NH4+$ level and counterion present. Nitrate inhibited nodulation while levels remained in excess of plant uptake. Compensatory nodulation followed exhaustion of $NH4+$ or $NO3-$ at lower $NH4+$ or $NO3-$ levels. Nodule mass and acetylene reduction activity decreased, but plant growth increased with increasing $NH4+$ or $NO3-$ levels. Nitrate raised the pH of the nutrient solution by one unit or less. Different $NH4+$ or $NO3-$ counterion forms had little effect on the symbiosis or plant growth. Ammonium nitrate severely inhibited nodulation but on exhaustion marked compensatory nodulation occurred. The patterns of nodule mass, acetylene reduction activity and plant growth with increasing NH₄NO₃ levels were similar to $NH4+$ or $NO3-$ alone. Plants provided with NH₄NO₃ or no nitrogen were similar in only slightly decreasing pH.