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 how species composition affected exploitation of the added 15N-labelled N source.
3. Capture of N (as 15N) from the organic material was followed by harvesting individual shoots. Poa pratensis captured more N from the added material when in mixtures with L. perenne than when in monoculture, particularly from 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 contents than those from L. perenne monocultures; P. pratensis shoot dry weights were also greater.
5. Capture by swards of N from the added organic material was the same (≈26% of that originally available) regardless of sward composition or organic material placement. However, within the mixed swards P. pratensis captured more N than L. perenne (≈18 and 9%, respectively).