Abstract
To investigate the possibility of estimating symbiotic nitrogen fixation through the measurements of dry matter production and evapotranspiration, we grew two isogenic lines of soybeans, non-nodulating line T201 and nodulating line T202 under three and five levels of nitrogen (N) application, respectively. The chlorophyll content (SPAD value) of T201 was lower than that of T202, and the difference between them expanded successively from the nil to high-N plot. As the difference in nodule dry weight (Ndw) and SPAD value became large between the two lines and among N application treatments, the difference in quantitative relationships between dry matter production (DM) and evapotranspiration (ET) became conspicuous. The value of ET (ETw=0) when DM was zero, which was determined from the regression lines, was higher in T202 than in T201, and ETw=0 found in T201 corresponded nearly to evaporation from the bare soil surface (E0). Both ETw=0 and Ndw in T202, on the other hand, varied with the amount of N application, and they were connected closely with each other. The difference in DM between T201 and T202 (δW) calculated from the difference in ETw=0 (δETw=0) was correlated significantly and positively with the amount of fixed nitrogen estimated from the difference between the accumulated N in the two lines (P < 0.001). These results suggest that the amount of symbiotic nitrogen fixation can be estimated from DM and ET by paying attention to its energy cost.
