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Natural N isotopic composition of nine plant species with different sensitivity to NH4+ nutrition. Natural isotopic signatures (δ15N, ‰) of the shoots (A) and roots (B) of several plant species cultured under hydroponic conditions with different concentrations of NH4+ (●) or NO3- (○) as the sole N source. The following numbers indicate the species that correspond to each point: (1) Lactuca sativa L., (2) Spinacia oleracea L., (3) Solanum lycopersicum L., (4) Lolium perenne L., (5) Pisum sativum L., (6) Lupinus albus L., (7) Trifolium repens L., (8) Ceratonia siliqua sp., and (9) Acacia aneura sp. Each point is the average of several biological replicates (at least n = 3, depending on the species; see Methods). δ15N of the N sources: NO3- = +0.3 and -1.514 and NH4+ = +0.029, +0.5 and +2.31 ‰.