To develop further the methods for estimation of NOx absorption by plants supplied with 15N-labelled fertilizer, we proposed a new calculation method, total N fixed method (TNF), and compared with the 15N dilution method and the classical mass balance method (MB).

Hydroponically grown soybean plants were supplied with 15N-labelled nitrate and exposed to 200–250 nl l−1 NO2 for 7 d. The proportions of the N derived from NO2 to total N in exposed plants were estimated by the three methods.

The reported rates of NO2 absorption by several plant species, estimated by the 15N dilution method, were recalculated using the TNF method. The results of the two methods were compared and showed that: (1) The 15N dilution method overestimated the content of NO2-N in exposed plants compared with the MB method whilst the TNF method produced estimations of NO2-N closer to those by the MB method when the plants were supplied with 5 mM nitrate. (2) The differences in estimations between the MB method and either the 15N dilution method or the TNF method increased with decreasing supply of 15N-labelled nitrate to roots.