Enhanced <scp>CO<sub>2</sub></scp> uptake is marginally offset by altered fluxes of non‐<scp>CO<sub>2</sub></scp> greenhouse gases in global forests and grasslands under N deposition
Shuqi Xiao, Chao Wang, Kai Yu, Genyuan Liu, Shuang Wu, Jinyang Wang, Shuli Niu, Jianwen Zou, Shuwei Liu
Abstract
Abstract Despite the increasing impact of atmospheric nitrogen (N) deposition on terrestrial greenhouse gas (GHG) budget, through driving both the net atmospheric CO 2 exchange and the emission or uptake of non‐CO 2 GHGs (CH 4 and N 2 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), CH 4 and N 2 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 CO 2 fluxes ( R S , soil respiration) was observed in grasslands. Nitrogen addition significantly stimulated soil N 2 O fluxes in forests (76%), to a larger extent in grasslands (87%), but showed a consistent trend to decrease soil uptake of CH 4 , suggesting a declined sink capacity of forests and grasslands for atmospheric CH 4 under N enrichment. Overall, the net GHG balance estimated by the net ecosystem production‐based method (forest, 1.28 Pg CO 2 ‐eq year −1 vs. grassland, 0.58 Pg CO 2 ‐eq year −1 ) was greater than those estimated using the SOC‐based method (forest, 0.32 Pg CO 2 ‐eq year −1 vs. grassland, 0.18 Pg CO 2 ‐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 N 2 O emissions together with the reduced soil uptake of CH 4 .