Poplar plants are cultivated as woody crops, which are often fertilized by addition of ammonium (NH4(+)) and/or nitrate (NO3(-)) to improve yields. However, little is known about net NH4(+)/NO3(-) fluxes and their relation with H(+) fluxes in poplar roots. In this study, net NH4(+)/NO3(-) fluxes in association with H(+) fluxes were measured non-invasively using scanning ion-selective electrode technique in fine roots of Populus popularis. Spatial variability of NH4(+) and NO3(-) fluxes was found along root tips of P. popularis. The maximal net uptake of NH4(+) and NO3(-) occurred, respectively, at 10 and 15 mm from poplar root tips. Net NH4(+) uptake was induced by ca. 48 % with provision of NO3(-) together, but net NO3(-) uptake was inhibited by ca. 39 % with the presence of NH4(+) in poplar roots. Furthermore, inactivation of plasma membrane (PM) H(+)-ATPases by orthovanadate markedly inhibited net NH4(+)/NO3(-) uptake and even led to net NH4(+) release with NO3(-) co-provision. Linear correlations were observed between net NH4(+)/NO3(-) and H(+) fluxes in poplar roots except that no correlation was found between net NH4(+) and H(+) fluxes in roots exposed to NH4Cl and 0 mM vanadate. These results indicate that root tips play a key role in NH4(+)/NO3(-) uptake and that net NH4(+)/NO3(-) fluxes and the interaction of net fluxes of both ions are tightly associated with H(+) fluxes in poplar roots.