Enhanced CO2 uptake is marginally offset by altered fluxes of non-CO2 greenhouse gases in global forests and grasslands under N deposition

Abstract

Despite the increasing impact of atmospheric nitrogen (N) deposition on terrestrial greenhouse gas (GHG) budget, through driving both the net atmospheric CO₂ exchange and the emission or uptake of non-CO₂ GHGs (CH₄ and N₂ O), few studies have assessed the climatic impact of forests and grasslands under N deposition globally based on different bottom-up approaches. Here, we quantify the effects of N deposition on biomass C increment, soil organic C (SOC), CH₄ and N₂ O fluxes and, ultimately, the net ecosystem GHG balance of forests and grasslands using a global comprehensive dataset. We showed that N addition significantly increased plant C uptake (net primary production) in forests and grasslands, to a larger extent for the aboveground C (aboveground net primary production), whereas it only caused a small or insignificant enhancement of SOC pool in both upland systems. Nitrogen addition had no significant effect on soil heterotrophic respiration (R_H ) in both forests and grasslands, while a significant N-induced increase in soil CO₂ fluxes (R_S , soil respiration) was observed in grasslands. Nitrogen addition significantly stimulated soil N₂ O fluxes in forests (76%), to a larger extent in grasslands (87%), but showed a consistent trend to decrease soil uptake of CH₄ , suggesting a declined sink capacity of forests and grasslands for atmospheric CH₄ under N enrichment. Overall, the net GHG balance estimated by the net ecosystem production-based method (forest, 1.28 Pg CO₂ -eq year^-1 vs. grassland, 0.58 Pg CO₂ -eq year^-1 ) was greater than those estimated using the SOC-based method (forest, 0.32 Pg CO₂ -eq year^-1 vs. grassland, 0.18 Pg CO₂ -eq year^-1 ) caused by N addition. Our findings revealed that the enhanced soil C sequestration by N addition in global forests and grasslands could be only marginally offset (1.5%-4.8%) by the combined effects of its stimulation of N₂ O emissions together with the reduced soil uptake of CH₄ .

Keywords: carbon pool; climate change; greenhouse gas; nitrogen deposition; nitrogen pool.