The Old-Field Community Climate and Atmosphere Manipulation (OCCAM) experiment tested single and interactive effects of [CO₂], warming, and altered precipitation regime on old-field ecosystems. The experiment consisted of constructed plant communities in open-top chambers. Plant species differed in their responses to the climate change factors, resulting in changes in plant community composition. Plant responses could have resulted from changes in physiology or from altered competitive interactions; however, changes in soil properties could also have contributed. After the OCCAM shut-down, we collected soils from the chambers to establish a follow-up experiment testing plant-soil feedbacks. We hypothesized that climate change effects on soil properties influence plant performance, and hence, must have contributed to the net climate change effects on plant community composition as observed in the OCCAM experiment. We selected three plant species, and tested growth and biomass production under common conditions, both in monocultures and in multi-species combinations. To separate between abiotic and biotic effects, we grew plants both on living and sterilized soils. In addition, we explicitly tested for effects of changes in the soil microbial community by growing plants on sterilized soils supplemented with microbial inocula.

Results/Conclusions

Preliminary results show that both root and shoot biomass production of Festuca pratense (grass) and Plantago lanceolata (non-leguminous forb) was generally higher on formerly dry than on formerly wet soils. Interestingly, the effect was much stronger in living soils than in sterilized soils, suggesting that the observed effects primarily should be contributed to differences in the biotic component of the formerly dry and formerly wet soils. This idea was supported by the plant responses on sterilized soils to which we added microbial inoculaŻ the biomass patterns of Festuca and Plantago reflected those observed in the living soils. The N-fixing sub-shrub Lespedeza cuneata was not responsive to the soil treatments, indicating that plant-competitive interactions may shift in response to water-mediated changes in soil properties. In to contrast the water treatment, former [CO₂] and temperature treatments had no effect on plant biomass production for any of the species. We conclude that to better understand and predict plant community responses to climate change, especially alteration of the precipitation regime, more emphasis should be put on climate change-induced changes in soil properties.