EFFECT OF RHIZOBIUM INOCULATION AND APPLICATION OF N, P, K FERTILIZER ON THE GROWTH AND NITROGEN-FIXATION OF FIELD-GROWN CHINESE MILK VETCH

A South Korean variety of Chinese milk vetch (Astragalus sinicus L.) was grown with or without the inoculation of Rhizobium (R. sp., NOKO-703 strain) and basal application of N, P, K, Mg fertilizer. Plant dry matter yield ranged from 4.3 to 6.0 tonnes/ha at 192 d after cultivation. With Rhizobium inoculation, the dry matter and N, P, K accumulation increased by 31.6, 28.9, 31.5, and 4.2% respectively. The application of P, K, and Mg (80 kg P2O5, 70 kg K2O, 10 kg Mg/ha) to the inoculated plants promoted N accumulation significantly but failed to increase both the dry matter yield and P or K accumulation. Additional application of N (50 kg N/ha) to the above treatment also did not increase the plant dry matter yield but increased the N accumulation by 13.2%. Nodule N2 fixation (ARA) was remarkably enhanced by both the Rhizobium inoculation and fertilizer application, particularly at the early stage of plant growth. The largest number of nodules and highest values of nodule dry weight and nodule ARA per plant were obtained in the combined treatment of Rhizobium inoculation and application of N, P, K, Mg fertilizer. The prevailing low temperature markedly inhibited plant growth and symbiotic N2 fixation but its effect on nodule formation and development was less significant than that on plant growth and nodule ARA. However, ARA values ranging from 0.15 to 0.51-mu-mol C2H4/(plant.h) were still recorded at 3-degrees-C. The concentrations of N and P in the nodules were much higher than those in the other organs and were positively correlated with the N2 fixation activity. The amount of symbiotically fixed nitrogen throughout the growth season which was estimated to range from 17.8 to 88.0 kg N/ha depending on the treatments, accounted for 13.4-48.5% of the total amount of accumulated nitrogen in the plants.