The Relationship between the Amount of Evapotranspiration and Dry Mass in Mixed Planting, Especially in Relation to Nitrogen Fixation

Abstract

Nitrogen (N) fixing plants have a lower water-use efficiency (WUE) than non-fixing plants. Although both types produce the same amount of mass upon drying, nitrogen-fixing plants were found to have consumed more water than non-fixing plants. We investigated whether this phenomenon could be detected in mixed planting. We measured WUE values of non-nitrogen-fixing maize and sunflower under single-species conditions and total dry weight (W) under mixed-species conditions. Then we calculated the sum transpiration amount (∑T, =W/WUE) and sum soil evaporation amount (∑E₀). The sum of evaporation (∑ET) from a mixed pot containining non-nitrogen-fixing maize and sunflower species was consistent with the total of ∑T and ∑E₀. On the other hand, the measured ∑ET from the mixed pot containing maize and nodulating cowpea was larger than the calculated ∑ET; this latter value was calculated by the procedure described above, using the WUE of maize and that of non-nodulating cowpea grown in a 5 gNO₃-N pot^-1. The difference between the values was closely related to nitrogen fixation. These results suggest that, if the WUE for non-nodulating plants can be estimated, a method for predicting the quantity of symbiotically-fixed nitrogen through measurements of ∑ET and W can be applied to mixed planting.