Nitrogen enrichment lowers Betula pendula green and yellow leaf stoichiometry irrespective of effects of elevated carbon dioxide

Elevated carbon dioxide (CO2) and increased nitrogen (N) availability generally increase deciduous tree biomass and alter green and yellow leaf stoichiometry. This paper investigates whether this also applies to Betula pendula (Birch). The effects of elevated atmospheric CO2 (600 ppmv) and increased N availability (50 and 100 kg N ha(-1) year(-1)) of differential inorganic nitrogen type (ammonium, nitrate and ammonium nitrate) on net primary production (NPP) and green/yellow leaf C:N of Betula pendula saplings were studied for 3 years. The combination of both factors raised NPP, but elevated CO2 alone did not. In green leaves, increased N availability raised N concentrations, outweighing decreases caused by elevated CO2. After senescence, increased N concentrations were found at 100 kg N ha(-1) year(-1), also leading to a lower C:N ratio. Although a lower C:N ratio may increase the decomposition rate during early decomposition, it may have the opposite effect during later decomposition stages. This, in combination with increased biomass production and possibly lower soil pH values, might increase soil C storage. However, due to the complexity of soil C formation and related processes this remains unsure.