It is becoming increasingly apparent that soil amino acids are a principal source of nitrogen (N) for certain plants, and especially those of N-limited environments. This study of temperate upland grasslands used glycine-2-C-13-N-15 and ((15)NH4)(2)SO4 labelling techniques to test the hypothesis that plant species which dominate 'unimproved' semi-natural grasslands (Festuca-Agrostis-Galium) are able to utilise amino acid N for growth, whereas those plants which dominate 'improved' grasslands (Lolium-Cynosurus), that receive regular applications of inorganic fertiliser, use inorganic N forms as their main N source. Data from field experiments confirmed that 'free' amino acids were more abundant in 'unimproved' than 'improved' grassland and that glycine was the dominant amino acid type (up to 42% of total). Secondly, the injection of representative amounts of glycine-2-C-13-N-15 (4.76 and 42.86 mM) into intact soil cores from the two grassland types provided evidence of direct uptake of glycine by plants, with both N-15 and C-13 being detected in plant material of both grasslands. Finally, a microcosm experiment demonstrated no preferential uptake of amino acid N by the grasses which dominate the grassland types, namely Holcus lanatus, Festuca rubra, Agrostis capillaris from the 'unimproved' grassland, and Lolium perenne from the 'improved' grassland. Again, both C-13 and N-15 were detected in all grass species suggesting uptake of intact glycine by these plants. Copyright (C) 2000 John Wiley & Sons, Ltd.