Estimation of the Amount of Symbiotic Nitrogen Fixation Based on the Relationship between Dry Matter Production and Evapotranspiration in Soybean

Abstract

The quantitative relationship between dry matter production (W) and evapotranspiration (ET) was examined in soybean cv. Tsurunoko cultivated in 8 L containers fertilized with a large amount of inorganic nitrogen (high N), where nodulation was poor, and with a small amount of N (low N), where nodulation was abundant. It was also examined in two isogenic lines; non-nodulating line T201 and nodulating line T202. The value of ET when W was zero (ET_W=0), which was determined from the regression lines, was higher in the plants fertilized with a low N than with a high N, and in T202 than in T201. The transpiration coefficient (ΔET/ΔW) was not influenced by N level and was nearly the same in T201 and T202. Difference in W between T201 and T202 (δW) calculated from the difference in ET_W=0 (δET_W=0) was correlated with the amout of fixed nitrogen estimated in N difference method. These results suggest that the difference in ET_W=0 between T201 and T202 represents the energy cost of nitrogen fixation.