Arbuscular mycorrhiza maintains nodule function during external NH 4+ supply in Phaseolus vulgaris (L.)

The synergistic benefits of the dual inoculation of legumes with nodule bacteria and arbuscular mycorrhizae (AM) are well established, but the effect of an external NH4+ supply on this tripartite relationship is less clear. This effect of NH4+ supply was investigated with regards to the growth and function of the legume host and both symbionts. Nodulated Phaseolus vulgaris seedlings with and without AM, were grown in a sand medium with either 0 N, 1 mM or 3 mM NH4+. Plants were harvested at 30 days after emergence and measurements were taken for biomass, N2 fixation, photosynthesis, asparagine concentration, construction costs and N nutrition. The addition of NH4+ led to a decline in the percentage AM colonization and nodule dry weights, although AM colonization was affected to a lesser extent. NH4+ supply also resulted in a decrease in the reliance on biological nitrogen fixation (BNF); however, the AM roots maintained higher levels of NH4+ uptake than their non-AM counterparts. Furthermore, the non-AM plants had a higher production of asparagine than the AM plants. The inhibitory effects of NH4+ on nodule function can be reduced by the presence of AM at moderate levels of NH4+ (1 mM), via improving nodule growth or relieving the asparagine-induced inhibition of BNF.