Effects of elevated CO₂ and N fertilization on plant and soil carbon pools of managed grasslands: a meta-analysis

Abstract. Elevated atmospheric CO₂ levels and increasing nitrogen deposition both stimulate plant production in terrestrial ecosystems. Moreover, nitrogen deposition could alleviate an increasing nitrogen limitation experienced by plants exposed to elevated CO₂ concentrations. However, an increased rate of C flux through the soil compartment as a consequence of elevated CO₂ concentrations has been suggested to limit C sequestration in terrestrial ecosystems, questioning the potential for terrestrial C uptake to mitigate increasing atmospheric CO₂ concentrations. Our study used data from 77 published studies applying elevated CO₂ and/or N fertilization treatment to monitor carbon storage potential in grasslands, and considered the influence of management practices involving biomass removal or irrigation on the elevated CO₂ effects. Our results confirmed a positive effect of elevated CO₂ levels and nitrogen fertilization on plant growth, but revealed that N availability is essential for the increased C influx under elevated CO₂ to propagate into belowground C pools. However, moderate nutrient additions also promoted decomposition processes in elevated CO₂, reducing the potential for increased soil C storage. An important role was attributed to the CO₂ response of root biomass in soil carbon responses to elevated CO₂, since there was a lower potential for increases in soil C content when root biomass increased. Future elevated CO₂ concentrations and increasing N deposition might thus increase C storage in plant biomass, but the potential for increased soil C storage is limited.