Internal nitrogen dynamics in the graminoid Molinia caerulea under higher N supply and elevated CO2 concentrations

Nutrient resorption from senescing leaves is an important aspect of internal plant nutrient cycling. Global environmental change very likely affects this process. In an 8-month experiment, we investigated the effect of increased nitrogen (N) availability and CO2 concentration on the contribution of leaf N resorption to the internal nitrogen dynamics of the perennial deciduous graminoid Molinia caerulea (L.) Moench. Plants were grown in a factorial combination of two levels of N (65 and 265 N ha(-1)year(-1)) and CO2 (380 and 700 mu L L-1) in a greenhouse. Both N and CO2 addition increased the total biomass and the total N pools of mature Molinia plants considerably, without a significant interaction. Nitrogen-resorption efficiency from senescing leaves (% of the mature leaf N pool that is resorbed) was neither affected by the N- nor by the CO2 treatments. When averaged over the treatments, the N-resorption efficiency was 85% +/- 1 (SE). The final N concentration in the litter (N-resorption proficiency) was also not affected by the treatments and was on average 3.6 mg N g(-1) +/- 0.25 (SE). The contribution of resorbed N from senescing leaves to the late seasonal N requirements (seed and stem production and storage of N for next year's growth) of M. caerulea plants was (negatively) affected by the N treatment only, and no interaction effects with CO2 were found. Resorption from stems and/or direct reserve and seed formation during growth became relatively more important. Thus, internal N cycling processes in Molinia caerulea are only affected when N availability is increased, but not under elevated CO2 concentrations. Under high N conditions, this species shifts from a N recycling strategy to reserve formation during growth.