The impacts of various nitrogen sources, i.e. NO3- NH4+ or NH4NO3 in combination with gaseous NH3, on nitrogen-, carbon- and water-use efficiency and C-13 discrimination (delta(13)C) by plants of the C-3 species Triticum aestivum L. (wheat) and the C-4 species Zea mays L. (maize) were studied. Triticum aestivum and Z. mays were hydroponically grown with 2 mol m(-3) of N supplied as NO3-, NH4+ or NH4NO3 for 21 and 18 d, respectively and thereafter exposed to gaseous NH3 at 320 mu g m(-3) or to ambient air for 7 d. In T. aestivum and Z. mays over a 7-d growth period, nitrogen-use efficiency (NUE) values were influenced by N-sources in the decreasing order NH4NO3-N > NO3-N > NH4+-N and NO3--N > NH4NO3-N > NH4+-N, respectively. Fumigation with NH3 decreased the NUE values of plants grown with any of the N-forms. During 28- and 7-d growth periods, N-sources affected water-use efficiency (WUE) values in the decreasing order of NH4+-N > NO3--N approximate to NH4NO3-N in non-fumigated T. aestivum, while fumigation with NH3 increased the WUE of NO3- -grown plants. There were insignificant effects of N-sources on WUE values of Z. mays over 25-and 7-d growth periods. Furthermore, delta(13)C values in plant tissues (leaves, stubble and roots) were higher (less negative) in NH4+-grown plants of T. aestivum and Z. mays than in those supplied with NH4NO3 or NO3-. Regardless of the N-form supplied to the roots of the plant species, exposure to NH3 caused more-positive delta(13)C values in the plant tissues. These results indicate that the variations in N-source were associated with small but significant variations in delta(13)C values in plants of T. aestivum and Z. mays. These differences in delta(13)C values are in the direction expected from differences in WUE values over long or short growth periods and with differences in the extent of non-Rubisco (ribulose-1,5-bisphosphate carboxylase-oxygenase, EC 4.1.1.39) carboxylate contribution to net C acquisition, as a function of N-source.
