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 Earths 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 climatebiogeochemical models simulating interactions between the global C cycle and climate warming.