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Nitrogen use efficiency determination of different poplar species under FACE vs. control

Growth of trees is largely regulated by nitrogen availability. When plants are exposed to elevated atmospheric CO2 concentrations, they often exhibit enhanced growth with increased biomass accumulation which in the long-term can result in an increased nitrogen uptake from the soil. In fact, nitrogen and carbon cycles are strictly related within plants, and nitrogen metabolism is regulated by signals that are derived from carbon metabolism.

Scientific achievements from EuroFACE experiment are as follows:

- At the end of the rotation cycle of a short rotation poplar plantation, we estimated N use by trees of three different poplar species exposed for three years to free air-CO2 enrichment (FACE) and we analysed whether this might influence the future N availability for the plantation.

- Destructive harvest at the end of the three year-rotation cycle was used to determine N concentration and N content of the long-lived (woody) tissues, whereas N uptake of fast-turnover tissues (fine roots and litter) was measured throughout the whole rotation cycle. These observations were related with previously published variations of soil N content during the same period. Moreover, we estimated retranslocation from green canopy and some processes determining N mobilization and N immobilization.

- Elevated CO2 treatment significantly increased Nitrogen Use Efficiency (NUE), i.e. NPP per unit of annual N uptake, in all species; decreased N concentration in all plant tissues, but did not change significantly the cumulative N uptake by trees at the end of the rotation cycle. Total soil N was depleted more under elevated CO2 than ambient CO2, although not significantly for all layers. Despite the differences among species were sometimes relevant, the response to elevated CO2 was similar for all species for most of the parameters analysed.

- The high rate of retranslocation under elevated CO2 in this experiment caused lower N concentration in leaf litter compared to ambient CO2. Once in the litter layer, it appeared that elevated CO2 treatment affected decay rate of such litter in two opposite modes, at least during the first stage of litter decay. While litter produced under elevated CO2 decomposed slower than litter in ambient CO2, leaf litter incubated in elevated CO2 decomposed faster. This mechanism could be also true for fine roots whose C/N was increased under elevated CO2 similarly to leaf litter.

Despite the lower N concentration, litter grown in elevated CO2 did not immobilize N differently from ambient CO2, during the first 250 days of field incubation. This appears to be a result of N limitation in soils under elevated CO2, rather than a decreased N demand by the litter.

- In conclusion, despite the higher productivity, N uptake by poplar trees did not change under elevated CO2 as compared to the ambient. As a consequence, NUE increased under elevated CO2, excluding so far the idea that additional nitrogen would be required to maintain increased yields of carbon uptake under elevated CO2 in such plantations. However, a stronger decrease of soil N was observed under elevated CO2 at the end of the first rotation cycle in comparison with ambient CO2, probably due to a decreased input by leaf litter and decreased decomposition rate.

Moreover we observed some processes indicating a trend, although not significant, of increased N immobilization under elevated CO2 which might lead to limiting conditions of N availability over a longer term period possibly influencing the productivity of such plantations over multiple rotation cycles.

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