Soil development under elevated CO2 affects plant growth responses to CO2 enrichment

The rising concentration Of CO2 in the atmosphere has the potential to alter many aspects of plant growth that in turn could result in changes in soil properties, such as storage of carbon and nitrogen. Through feed-backs between soils and plants, any changes in soil properties could potentially modify how plants respond to elevated CO2. To test for possible interactions, we measured plant growth responses of the perennial herb Plantago lanceolata L. to elevated CO2 using sites exposed long-term (>20 years) to low (372 ppm) or high (574 ppm) atmospheric CO2 in the area surrounding a natural mineral spring. The soil developed long-term at high CO2 had greater organic carbon, total nitrogen and net mineral-N mineralisation than the soil developed long-term at low CO2. Seedlings were transplanted into soils at each site and into soils swapped between the low and high CO2 sites and growth was recorded over 246 days. At the final harvest, plants grown in a soil developed at low CO2 but exposed to a high CO2 atmosphere (i.e. conventional CO2 enrichment experiment) had greater root and leaf mass and higher seed production than plants grown in soils developed at and exposed to a low CO2 atmosphere. However, a more realistic comparison of current (low CO2 soil-low CO2 atmosphere) and future (high CO2 soil-high CO2 atmosphere) environments showed no difference in leaf and root biomass but reduced seed production in the high CO2 soil-high CO2 atmosphere treatment compared to the low CO2 soil-low CO2 atmosphere treatment. The results indicate that the properties of soils developed long-term at elevated CO2 may affect how plants respond to an elevated CO2 atmosphere. Caution is therefore needed in how we interpret plant responses to elevated CO2 that are based on experiments using step changes in atmospheric CO2 and which are of short duration.