Nitrogen limitation constrains sustainability of ecosystem response to CO₂

Enhanced plant biomass accumulation in response to elevated atmospheric CO₂ concentration could dampen the future rate of increase in CO ₂ 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 world ^1-9. 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, 7-9, soil N supply is probably an important constraint on global terrestrial responses to elevated CO₂.