CO ₂ Elicits Long-Term Decline in Nitrogen Fixation

Rising atmospheric carbon dioxide (Ca), a product of fossil fuel burning, land-use change, and cement manufacture, is expected to cause a large carbon sink in land ecosys-tems, partly mitigating human-driven climate change (1). Increasing biological nitrogen fixation with rising Ca has been invoked as a means to provide the N necessary to support C accumulation (2). As in many short-term experiments (3), we found that Ca enrichment increased N fixation during the first year of treatment in an oak woodland. However, the effect declined and disappeared by the third year. Ca enrichment consistently depressed N fixation during the 5th, 6th, and 7th years of