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

Abstract

To reveal how rhizobia affects biomass allocation of peanuts under different nitrogen concentrations, we conducted a pot experiment by treatments of Arachis hypogaea "Qicai" with and without Bradyrhizobium 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 A. hypogaea "Qicai" and Bradyrhizobium.