Estimation of nitrogen fixation in Azolla—rice association using 15N isotope dilution

Abstract

Nitrogen fixation by Azolla and the availability of Azolla-N and urea-N to rice plants were determined using a 15N isotope dilution technique. Four treatments were evaluated: control (no nitrogen, no Azolla), urea labelled with 15N, labelled Azolla, and a mixture of urea-15N plus unlabelled Azolla. Urea and Azolla were equally effective for increasing the dry mass of shoots and roots. Azolla applied in combination with urea was a better treatment than either urea or Azolla-N applied singly. Azolla-N taken up by shoots and roots of rice was essentially identical to that taken up by labelled urea-N. There was no interaction between Azolla-N and urea-N. Combined treatment with Azolla and urea showed that the nitrogen derived from air was the major source of nitrogen taken up by the rice, followed by that derived from soil, and then that derived from fertilizer. The amounts of N2 fixed were correlated with both dry mass of shoots and roots of rice and total nitrogen taken up by rice. Source: CAB