[Effects of nitrogen addition on <i>Arachis hypogaea</i> "Qicai"-rhizobia symbiosis and biomass allocation].

To reveal how rhizobia affects biomass allocation of peanuts under different nitrogen concentrations, we conducted a pot experiment by treatments of <i>Arachis hypogae</i>a "Qicai" with and without <i>Bradyrhizobium</i> inoculation to investigate the characteristics of plant biomass allocation and symbiotic nodulation at the nitrogen addition level of 0, 8, 16, 32, 64 and 128 mmol·L^-1. The results showed that: 1) Under non-inoculation, the addition of low-level nitrogen (8-32 mmol·L^-1) had limited impact on plant biomass. When nitrogen addition level reached 64 mmol·L^-1, total plant biomass, leaf biomass, leaf area, and total net photosynthetic rate increased significantly by 82.1%, 116.6%, 116.1% and 122.1% respectively in compared with those without nitrogen addition (0 mmol·L^-1). 2) Under the condition of inoculation, total plant biomass, leaf biomass, leaf area, and total net photosynthetic rate increased under the nitrogen addition level of 16 mmol·L^-1 by 65.3%, 97.5%, 91.7%, and 112.8%. The nodulation amount of plants and the total amount of leghemoglobin first increased and then decreased with the increases of nitrogen addition level, reaching their maximum values at 49.00 mg·plant^-1 and 0.12 mg·plant^-1 respectively at the nitrogen addition level of 16 mmol·L^-1. When the nitrogen addition level reached 64 mmol·L^-1, they decreased significantly. There was no nodulation of roots when the nitrogen addition was 128 mmol·L^-1. 3) Rhizobia inoculation significantly increased leaf biomass, aboveground biomass, leaf area, and total net photosynthetic rate when nitrogen addition level ranged from 8 to 64 mmol·L^-1, with an overall increase of 43.3%, 37.6%, 34.5%, and 53.8% respectively. However, rhizobia inoculation did not affect those indices when the nitrogen addition level was 0 or 128 mmol·L^-1. Overall, rhizobia inoculation significantly increased the allometric growth constants of leaf-root and leaf-total biomass, and decreased the allometric growth constants of root-stem and root-total biomass. In conclusion, peanuts actively adjust resource allocations among different organs with a trade-off between environmental nitrogen absorption and symbiotic nitrogen fixation, which would maximize the benefit of resource investments. Among the N addition levels involved in this study, 16 mmol·L^-1 is optimal for the symbiotic nodulation of <i>A. hypogae</i>a "Qicai" and <i>Bradyrhizobium</i>.