Interactions between plant growth and soil nutrient cycling under elevated CO₂: a meta‐analysis

Abstract

Abstract free air carbon dioxide enrichment (FACE) and open top chamber (OTC) studies are valuable tools for evaluating the impact of elevated atmospheric CO 2 on nutrient cycling in terrestrial ecosystems. Using meta‐analytic techniques, we summarized the results of 117 studies on plant biomass production, soil organic matter dynamics and biological N 2 fixation in FACE and OTC experiments. The objective of the analysis was to determine whether elevated CO 2 alters nutrient cycling between plants and soil and if so, what the implications are for soil carbon (C) sequestration. Elevated CO 2 stimulated gross N immobilization by 22%, whereas gross and net N mineralization rates remained unaffected. In addition, the soil C : N ratio and microbial N contents increased under elevated CO 2 by 3.8% and 5.8%, respectively. Microbial C contents and soil respiration increased by 7.1% and 17.7%, respectively. Despite the stimulation of microbial activity, soil C input still caused soil C contents to increase by 1.2% yr −1 . Namely, elevated CO 2 stimulated overall above‐ and belowground plant biomass by 21.5% and 28.3%, respectively, thereby outweighing the increase in CO 2 respiration. In addition, when comparing experiments under both low and high N availability, soil C contents (+2.2% yr −1 ) and above‐ and belowground plant growth (+20.1% and+33.7%) only increased under elevated CO 2 in experiments receiving the high N treatments. Under low N availability, above‐ and belowground plant growth increased by only 8.8% and 14.6%, and soil C contents did not increase. Nitrogen fixation was stimulated by elevated CO 2 only when additional nutrients were supplied. These results suggest that the main driver of soil C sequestration is soil C input through plant growth, which is strongly controlled by nutrient availability. In unfertilized ecosystems, microbial N immobilization enhances acclimation of plant growth to elevated CO 2 in the long‐term. Therefore, increased soil C input and soil C sequestration under elevated CO 2 can only be sustained in the long‐term when additional nutrients are supplied.