Arbuscular mycorrhizae affect the N and C economy of nodulated Phaseolus vulgaris (L.) during NH4+ nutrition

In the symbiosis between nodulated legume roots and arbuscular mycorrhizal (AM) fungi, the C and N economy can be influenced by the source of N-supply from either AM-derived NH4+ uptake or nodule-derived biological nitrogen fixation (BNF). This relationship was investigated in terms of NH4+ supply and BNF by the two symbionts. Nodulated Phaseolus vulgaris seedlings with and without AM, were hydroponically grown with either 0 N or 1 mM NH4+ supply. Plants were harvested at 30 days after emergence and measurements were taken for biomass, N2 fixation, photosynthesis, CO2 and O2 root respiration, calculated C and N economy. AM roots had higher NH4+ uptake and this was associated with the suppression of BNF and nodule growth. The higher NH4+ uptake in AM roots occurred with lower root maintenance respiration, compared to when N was derived from BNF. There was also an increase in the below-ground sink strength of NH4+ fed AM roots compared to NH4+ fed non-AM roots, as evidenced by the increases in root CO2 and O2 respiration and photosynthetic stimulation. These results indicate that although the AM root had higher total below-ground respiratory costs during NH4+ nutrition, there were lower respiratory C costs associated with N derived from AM symbionts in comparison to N from BNF.