Nitrogen fluxes in a perennial energetic crop, Miscanthus x giganteus : experimental study and modelling elements

Using dedicated plants in order to produce bioenergy is often presented as one of the solutions in order to limit global warming and to contribute to replacing fossil energy. The use of biofuels will be acceptable only if it allows reducing negative impacts of agriculture on a global (GHG emissions) and a local (nitrate leaching, water consumption) scale. Energy crops have to answer to these requirements in allying high yields per hectare, in order to limit food and non food productions competition. We decided to quantify the role of Miscanthus x giganteus storage organs in nitrogen nutrition of the crop and to analyse internal nitrogen cycle in the crop during this Phd work, with the aim of modelling these processes on the long term. The experimental approach was done on a three year experiment, crossing two nitrogen fertilisation rates and two harvest dates. In a first part, we show that aboveground biomass production and nitrogen content in belowground organs were different between treatments. Belowground biomass nitrogen stocks play a key role on nitrogen and carbon accumulation in aboveground organs during plant growth during the subsequent year of growth. We determined strong linear relationships between nitrogen fluxes and nitrogen accumulated in source organs during nitrogen remobilisation in spring and autumn. In a second part, we show that differences in aboveground biomass production were mainly due to differences of belowground biomass nitrogen stocks before regrowth due to their effect on radiation use efficiency (RUE) of the crop. In a third part, we underline the low nitrogen fertiliser recovery in the crop thanks to 15N tracing. We also show that rhizomes absorbed nitrogen during nitrogen remobilisation in spring. M. giganteus is able to maintain a conservative nitrogen cycle: the amount of remobilised nitrogen in autumn and spring are in the same order of magnitude. Finally, we draw some conclusions on knowledge and questions on the role of nitrogen remobilisation in nitrogen nutrition of M. giganteus. This plays a key role, via spring and autumn remobilisation processes that have direct impacts on biomass production by the crop. Taking into account belowground biomass nitrogen stocks is essential in order to succeed in an operational model which allows us to simulate the biomass production by the crop and the nitrogen balance on the long term.