Decade-long soil nitrogen constraint on the CO2 fertilization of plant biomass

The stimulation of plant growth by elevated CO2 concentration has been widely observed. Such fertilization, and associated carbon storage, could dampen future increases in atmospheric CO2 levels and associated climate warming1. However, the CO2 fertilization of plant biomass may be sensitive to nitrogen supply2–4. Herein we show that in the latest decade of a long-term perennial grassland experiment, low ambient soil nitrogen availability constrained the positive response of plant biomass to elevated CO2, a result not seen in the first years (1998–2000) of the study. From 2001 to 2010, elevated CO2 stimulated plant biomass half as much under ambient as under enriched nitrogen supply, an effect mirrored over this period by more positive effects of elevated CO2 on soil nitrogen supply (net nitrogen mineralization) and plant nitrogen status under enriched than ambient nitrogen supply. The results did not strongly support either the progressive nitrogen limitation hypothesis, or the alternative hypothesis of priming of soil nitrogen release by elevated CO2. As nitrogen limitation to productivity is widespread, persistent nitrogen constraints on terrestrial responses to rising CO2 are probably pervasive. Further incorporation of such interactions into Earth system models is recommended to better predict future CO2 fertilization effects and impacts on the global carbon cycle. Continued emissions of CO2 from fossil-fuel combustion and deforestation are likely to further increase atmospheric CO2 concentrations ([CO2]) and the temperature of the Earth1. However, the stimulation of plant biomass accumulation by elevated [CO2]—the CO2 fertilization effect—could increase ecosystem carbon (C) storage and thus dampen the future rate of increase in [CO2] and associated climate warming1. Uncertainty regarding the magnitude of this biomass fertilization makes it the largest unknown for terrestrial feedbacks to the C cycle–climate system1. One important aspect of this uncertainty is whether limited soil nitrogen (N) availability restricts CO2-induced biomass enhancement and related C sequestration2–19. Several published studies on plant biomass and productivity under contrasting CO2 and N supply suggest an N-limitation constraint on CO2 fertilization3–6, as noted in the last Intergovernmental Panel on Climate Change report1. Nevertheless, nutrient limitationswere not incorporated into the coupled climate–C cycle models used in that report. This may have been due in part to the very low number and limited range of ecologically realistic long-term experiments of atmospheric CO2 and N supply together. Herein we report on long-term (13 year) results of one such experiment, which is one of only three published, well-replicated long-term (>5 years) open-air CO2×N experiments4,5,19.