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Attempts to understand the ecological effect of increasing atmospheric CO₂ concentration, [CO₂], usually involve exposing today's ecosystems to expected future [CO₂] levels^,. However, a major assumption of these approaches has not been tested—that exposing ecosystems to a single-step increase in [CO₂] will yield similar responses to those of a gradual increase over several decades. We tested this assumption on a mycorrhizal fungal community over a period of six years. [CO₂] was either increased abruptly, as is typical of most [CO₂] experiments, or more gradually over 21 generations. The two approaches resulted in different structural and functional community responses to increased [CO₂]. Some fungi were sensitive to the carbon pulse of the abrupt [CO₂] treatment. This resulted in an immediate decline in fungal species richness and a significant change in mycorrhizal functioning. The magnitude of …