Nitrogen limitation constrains sustainability of ecosystem response to CO2

Future trends in atmospheric CO2 concentrations, and thus future climate, will depend in part on the degree to which terrestrial ecosystems can accumulate CO2 emissions from human activities. A six-year grassland field study has found that the biomass enhancement due to elevated CO2 declines over time under ambient nitrogen supply, but not under nitrogen enrichment, suggesting that both natural variation among soils and variation in nitrogen deposition rates are likely to influence plant biomass accumulation responses to elevated CO2. Nitrogen is a limiting factor in many managed and unmanaged soils, so soil nitrogen may be an important constraint on the global response to elevated CO2. Enhanced plant biomass accumulation in response to elevated atmospheric CO2 concentration could dampen the future rate of increase in CO2 levels and associated climate warming. However, it is unknown whether CO2-induced stimulation of plant growth and biomass accumulation will be sustained or whether limited nitrogen (N) availability constrains greater plant growth in a CO2-enriched world1,2,3,4,5,6,7,8,9. Here we show, after a six-year field study of perennial grassland species grown under ambient and elevated levels of CO2 and N, that low availability of N progressively suppresses the positive response of plant biomass to elevated CO2. Initially, the stimulation of total plant biomass by elevated CO2 was no greater at enriched than at ambient N supply. After four to six years, however, elevated CO2 stimulated plant biomass much less under ambient than enriched N supply. This response was consistent with the temporally divergent effects of elevated CO2 on soil and plant N dynamics at differing levels of N supply. Our results indicate that variability in availability of soil N and deposition of atmospheric N are both likely to influence the response of plant biomass accumulation to elevated atmospheric CO2. Given that limitations to productivity resulting from the insufficient availability of N are widespread in both unmanaged and managed vegetation5,7,8,9, soil N supply is probably an important constraint on global terrestrial responses to elevated CO2.