Effect of nitrogen supply and defoliation on loss of organic compounds from roots of Festuca rubra.

The aim of this study was to determine the effects of N-supply and defoliation on rhizodeposition from Festuca rubra, in the context of whole-plant C- partitioning and root morphology. Plants were grown for 36 d in axenic sand microcosms continuously percolated with nutrient solutions of either high or low N concentration (2 mM or 0.01 mM NH(4)NO(3), respectively). The effects of partial defoliation at weekly intervals were determined at high and low N. At low N, dry matter accumulation in roots and shoots was reduced significantly (P<0.001), with proportionately increased partitioning to roots, in comparison with the high N treatment. Root morphology was also affected by N-treatment; at low N, lower biomass production was offset by increased specific root length (P<0.001), reducing the magnitude of the significant (P=0.002) increase in total root length at high N. Cumulative release of organic C from roots of F: rubra over the experimental period was not altered significantly by N-treatment. However, as a proportion of net C-assimilation, rhizodeposition was significantly (P<0.001) greater at low N than at high N. Defoliation transiently (3-5 d) increased the release of soluble organic compounds from roots at each N-supply rate, and increased significantly (P<0.001) cumulative rhizodeposition over the experimental period. These effects of N-supply and defoliation on rhizodeposition are of importance in understanding interactions between plant and microbial productivity in grazed grasslands, and in interpretation of concurrent effects on microbially driven nutrient cycling processes in these systems.