Spatial and physical heterogeneity of N supply from soil does not influence N capture by two grass species

1. Plant responses after addition of nitrogen-rich complex organic material (Lolium perenne L. shoots), differing in physical (coarsely cut or finely ground) and spatial (uniform dispersion or discrete patches) heterogeneity, were examined. 2. The organic material was added to microcosm units containing L. perenne and Poa pratensis L. as monocultures or mixed swards in order to examine ho w species composition affected exploitation of the added N-15-labelled N so urce. 3. Capture of N (as N-15) from the organic material was followed by harvest ing individual shoots. Poa pratensis captured more N from the added materia l when in mixtures with L. perenne than when in monoculture, particularly f rom the milled material. Capture of N by L. perenne shoots was not affected by either sward composition or method of organic material placement. 4. At final harvest (70 days after addition of the organic material), roots from P. pratensis monocultures had greater dry weights, lengths and N cont ents than those from L. perenne monocultures; P. pratensis shoot dry weight s were also greater. 5. Capture by swards of N from the added organic material was the same (app roximate to 26% of that originally available) regardless of sward compositi on or organic material placement. However, within the mixed swards P. prate nsis captured more N than L. perenne (approximate to 18 and 9%, respectivel y).