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

Abstract

The aim of this study was to determine the effects of N-supply anddefoliation on rhizodeposition from Festuca rubra, in the contextof whole-plant C- partitioning and root morphology. Plants were grownfor 36 d in axenic sand microcosms continuously percolated with nutrientsolutions of either high or low N concentration (2 mM or 0.01 mMNH(4)NO(3), respectively). The effects of partial defoliation atweekly intervals were determined at high and low N. At low N, drymatter 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 alsoaffected by N-treatment; at low N, lower biomass production was offsetby increased specific root length (P<0.001), reducing the magnitudeof the significant (P=0.002) increase in total root length at highN. Cumulative release of organic C from roots of F: rubra over theexperimental period was not altered significantly by N-treatment.However, as a proportion of net C-assimilation, rhizodeposition wassignificantly (P<0.001) greater at low N than at high N. Defoliationtransiently (3-5 d) increased the release of soluble organic compoundsfrom roots at each N-supply rate, and increased significantly (P<0.001)cumulative rhizodeposition over the experimental period. These effectsof N-supply and defoliation on rhizodeposition are of importancein understanding interactions between plant and microbial productivityin grazed grasslands, and in interpretation of concurrent effectson microbially driven nutrient cycling processes in these systems.