Offspring of plants exposed to elevated or ambient CO₂ differ in their impacts on soil nitrification in a common garden experiment

Long-term exposure to elevated carbon dioxide (CO₂) may select for different plant phenotypes with consequences for microbial associations and functions. We collected seeds from two grasses that had grown in a Free Air Carbon Dioxide Enrichment (FACE) experiment for 5 and 11 years, and grew them in a common garden to assess how elevated CO₂ affected the rate of nitrification in rhizosphere soil. The comparison was made in a ‘common-garden’ so that differences could be ascribed to inherent changes in the plant populations. We found that seed of both species from the ambient populations produced plants that had similar rates of nitrification; however, seed from the elevated CO₂ population produced ryegrass plants with a significantly greater nitrification rate and browntop plants with a significantly lower rate than the ambient population. These differences were not related to changes in root mass so represent a fundamental shift in plant/microbe interactions under elevated CO₂.