Species strategy and N fluxes in grassland soil - A question of root litter quality or rhizosphere activity?

A clear understanding of the feedback between plant communities and soil nutrient cycles is necessary to predict the dynamics of grassland ecosystems. Nitrogen fluxes may depend on specific functional plant traits in relation to dead and living roots, which determine a litter and a rhizosphere effect of the plant community. Two species of nutrient-rich grasslands that demonstrate either a more conservative (Dactylis) or more competitive strategy (Lolium) for nitrogen were grown at a non-optimal level of N availability and at optimal water, P and K supply. In a factorial experiment, monocultures of both species were sown on soil monoliths containing the intact dead root litter system of either species. The N fluxes were measured for two years in these microcosms. The presence of a growing stand increased the N mineralisation rate of the root litter. Both the type of root litter and that of the rhizosphere significantly affected the N fluxes. A greater N harvest was recorded in the Dactylis stand and on the Lolium litter; while greater N loss by leaching occurred under the Lolium stand and from the Lolium litter. Lolium induced a greater N turnover in SOM, while Dactylis caused a faster decay rate of litter N. Dactylis favoured the short N cycle between stand and root litter, while Lolium favoured the complete N cycle between stand and SOM. The chemical composition of the root litter was similar, but the species effects on the N cycles could be accounted for by differences in morphological root traits. A set of functional root processes is proposed that differentiates species strategies towards the nitrogen cycle. Dynamic consequences for a mixture of the two species are discussed. (C) 2004 Elsevier B.V. All rights reserved.