Abstract
The effects of nitrate supply on the growth and N_2-fixation of inoculated Chinese milk vetch (Astragalus sinicus L.), red clover (Trifolium repen L.), and soybean (Glycine max L.) were studied with soil pot experiment growing in the growth chamber for 42-56 days. Nitrate [Ca(^15No_3)_2] was applied as basal dressing at levels of 0, 50, 200 mg N/pot (10^<-4>a). The results obtained were as follows: 1) By the application of nitrate, red clover and soybean increased their total dry matter and N content, but milk vetch did not. 2) Among the plants, milk vetch had the highest N% in all organs, particulary in the nodules. 3) At the early stage, milk vetch and red clover showed almost the same ^15N concentrations, which were higher than that of soybean. However, at the later stage, milk vetch had the lowest ^15N concentration compared with the other two plants, reflecting its high nodule N_2-fixation activity. 4) The responses of nodule N% to nitrate supply were different among plant species. Milk vetch and red clover increased their nodule N% with increasing nitrate supply, but soybean lowered it. 5) At the early stage, the nodule formation and growth, ARA and SARA of all plants were strongly inhibited by the nitrate. At the final stage, conversely, higher SARA was found in all plants fed with nitrate although the dry matter of nodules and the ARA of these plants except red clover were still lower. 6) A high correlation (r=0.801) between SARA and nodule N% was observed in all plants.
