Interactive effects of elevated CO2 concentration and nitrogen supply on partitioning of newly fixed C-13 and N-15 between shoot and roots of pedunculate oak seedlings (Quercus robur)

Pedunculate oak (Quercus robur L.) seedlings were grown for 3 or 4 months ( second- and third-flush stages) in greenhouses at two atmospheric CO2 conce ntrations ([CO2]) (350 or 700 mu mol mol(-1)) and two nitrogen fertilizatio n regimes (6.1 or 0.61 mmol N l(-1) nutrient solution). Combined effects of [CO2] and nitrogen fertilization on partitioning of newly acquired carbon (C) and nitrogen (N) were assessed by dual C-13 and N-15 short-term labelin g of seedlings at the second- or third-flush stage of development. In the l ow-N treatment, root growth, but not shoot growth, was stimulated by elevat ed [CO2], with the result that shoot/root biomass ratio declined. At the se cond-flush stage, overall seedling biomass growth was increased (13%) by el evated [CO2] regardless of N fertilization. At the third-flush stage, eleva ted [CO2] increased growth sharply (139%) in the high-N but not the low-N t reatment. Root/shoot biomass ratios were threefold higher in the low-N trea tment relative to the high-N treatment. At the second-flush stage, leaf are a was 45-51% greater in the high-N treatment than in the low-N treatment. A t the-third flush stage, there was a positive interaction between the effec ts of N fertilization and [CO2] on leaf area, which was 93% greater in the high-N/elevated [CO2] treatment than in the low-N/ambient [CO2] treatment. Specific leaf area was reduced (17-25%) by elevated [CO2], whereas C and N concentrations of seedlings increased significantly in response to either e levated [CO2] or high-N fertilization. At the third-flush stage, acquisitio n of C and N per unit dry mass of leaf and fine root was 51 and 77% greater , respectively, in the elevated [CO2]/high-N fertilization treatment than i n the ambient [CO2]/low-N fertilization treatment. However, there was dilut ion of leaf N in response to elevated [CO2]. Partitioning of newly acquired C and N between shoot and roots was altered by N fertilization but not [CO 2]. More newly acquired C and N were partitioned to roots in the low-N trea tment than in the high-N treatment.