Use of 15N isotope dilution method to quantify nitrogen fixation in legumes and its potential use for non-legumes

The N-15 isotope dilution method is potentially accurate for measuring N-2 fixation in plants. In using the method one has to ensure that the isotope ratio in the plant reflects the integrated ratio in the soil, which can be variable in time and with soil depth. The errors due to the use of an inappropriate reference plant are much higher at low levels of fixation, and these decrease with increasing rates of fixation. The N-15 isotope dilution method may be suitable for quantifying associative N-2 fixation, especially if the experimental error is low, or where N-2 fixation is high. Various aspects affecting the accuracy of the method will be discussed, especially potential errors of measurements of nitrogen fixation in non-legumes by associative or endophytic diazotrophs.