Abstract
To estimate the amount of fixed nitrogen (N), we compared the N absorption of cowpea with root nodules grown under 0 gN per pot conditions (0N cowpea) with that of corn grown under 0 gN (0N corn). We also compared the water use efficiency (WUE) of 0N cowpea with that of cowpea without nodules produced by growing with 5 gN per pot fertilization (5N cowpea). We noted that although the increase in total dry weight had stopped at the early growth period in 0N corn, it continued to increase in 0N cowpea that received a fixed N supply via the root nodules. Furthermore, we noted that growth was greatly inhibited in the 5N cowpea but WUE was kept high throughout the growth period. Although the WUE of 5N cowpea tended to remain nearly constant during the experimental period, that of 0N cowpea decreased with the increase in the amount of fixed nitrogen. From these results, we consider that the amount of fixed N can be estimated by comparing the WUE of cowpea without root nodules (no N fixation) with that of cowpea with root nodules (N fixation).
