Abrupt rise in atmospheric CO2 overestimates community response in a model plant-soil system

Attempts to understand the ecological effect of increasing atmospheric CO2 concentration, [CO2], usually involve exposing today's ecosystems to expected future [CO2] levels(1,2). 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(3). 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 changes in fungal diversity and functioning in response to gradually increasing [ CO2] was smaller, and not significantly different to those with ambient [ CO2]. Our results suggest that studies may overestimate some community responses to increasing [ CO2] because biota may be sensitive to ecosystem changes that occur as a result of abrupt increases.