Improvement of the 15N dilution method for estimation of absorption of NOx by plants supplied with 15N-labelled fertilizer

To develop further the methods for estimation of NOx absorption by plants supplied with N-15-labelled fertilizer, we proposed a new calculation method, total N fixed method (TNF), and compared with the N-15 dilution method and the classical mass balance method (MB). Hydroponically grown soybean plants were supplied with N-15-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 N-15 dilution method, were recalculated using the TNF method. The results of the two methods were compared and showed that: (1) The N-15 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 N-15 dilution method or the TNF method increased with decreasing supply of N-15-labelled nitrate to roots.