Elevated ozone decreases the multifunctionality of belowground ecosystems.

Elevated tropospheric ozone (O 3 ) affects the allocation of biomass aboveground and belowground and influences terrestrial ecosystem functions. However, how belowground functions respond to elevated O 3 concentrations ([O 3 ]) remains unclear at the global scale. Here, we conducted a detailed synthesis of belowground functioning responses to elevated [O 3 ] by performing a meta-analysis of 2395 paired observations from 222 publications. We found that elevated [O 3 ] significantly reduced the primary productivity of roots by 19.8%, 16.3%, and 26.9% for crops, trees and grasses, respectively. Elevated [O 3 ] strongly decreased the root/shoot ratio by 11.3% for crops and by 4.9% for trees, which indicated that roots were highly sensitive to O 3 . Elevated [O 3 ] impacted carbon and nitrogen cycling in croplands, as evidenced by decreased dissolved organic carbon, microbial biomass carbon, total soil nitrogen, ammonium nitrogen, microbial biomass nitrogen, and nitrification rates in association with increased nitrate nitrogen and denitrification rates. Elevated [O 3 ] significantly decreased fungal phospholipid fatty acids in croplands, which suggested that O 3 altered the microbial community and composition. The responses of belowground functions to elevated [O 3 ] were modified by experimental methods, root environments, and additional global change factors. Therefore, these factors should be considered to avoid the underestimation or overestimation of the impacts of elevated [O 3 ] on belowground functioning. The significant negative relationships between O 3 -treated intensity and the multifunctionality index for croplands, forests, and grasslands implied that elevated [O 3 ] decreases belowground ecosystem multifunctionality.