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The goal of this study was to investigate whether increased nitrogen use efficiency found in Populus nigra L. (Jean Pourtet) under elevated CO2 would correlate with changes in the production of carbon-based secondary compounds (CBSCs). Using Free-Air CO2 Enrichment (FACE) technology, a poplar plantation was exposed to either ambient (about 370 µmol mol−1 CO2) or elevated (about 550 µmol mol−1 CO2) [CO2] for 5 years. After three growing seasons, the plantation was coppiced and half of the experimental plots were fertilized with nitrogen. CBSCs, total nitrogen and lignin-bound nitrogen were quantified in secondary sprouts in seasons of active growth and dormancy during 2 years after coppicing. Neither elevated CO2 nor nitrogen fertilisation alone or in combination influenced lignin concentrations in wood. Soluble phenolics and soluble proteins in wood decreased slightly in response to elevated CO2. Higher nitrogen supply stimulated formation of CBSCs and increased protein concentrations. Lignin-bound nitrogen in wood ranged from 0.37–1.01 mg N g−1 dry mass accounting for 17–26% of total nitrogen in wood, thus, forming a sizeable nitrogen fraction resistant to chemical degradation. The concentration of this nitrogen fraction was significantly decreased by elevated CO2, increased in response to nitrogen fertilisation and showed a significant CO2 × fertilisation interaction. Seasonal changes markedly affected the internal nitrogen pools. Soluble proteins in wood were 52–143% higher in the dormant than in the growth season. Positive correlations existed between the biosynthesis of proteins and CBSCs. The limited responses to elevated CO2 and nitrogen fertilisation indicate that growth and defence are well orchestrated in P. nigra and that changes in the balance of both resources—nitrogen and C—have only marginal effects on wood chemistry.

Carbon-based secondary metabolites and internal nitrogen pools in Populus nigra under Free Air CO2 Enrichment (FACE) and nitrogen fertilisation