Forest productivity under elevated CO₂ and O₃: positive feedbacks to soil N cycling sustain decade-long net primary productivity enhancement by CO₂.

The accumulation of anthropogenic CO₂ in the Earth's atmosphere, and hence the rate of climate warming, is sensitive to stimulation of plant growth by higher concentrations of atmospheric CO₂. Here, we synthesise data from a field experiment in which three developing northern forest communities have been exposed to factorial combinations of elevated CO₂ and O₃. Enhanced net primary productivity (NPP) (c. 26% increase) under elevated CO₂ was sustained by greater root exploration of soil for growth-limiting N, as well as more rapid rates of litter decomposition and microbial N release during decay. Despite initial declines in forest productivity under elevated O₃, compensatory growth of O₃ -tolerant individuals resulted in equivalent NPP under ambient and elevated O₃. After a decade, NPP has remained enhanced under elevated CO₂ and has recovered under elevated O₃ by mechanisms that remain un-calibrated or not considered in coupled climate-biogeochemical models simulating interactions between the global C cycle and climate warming.