Enhanced CO2 uptake is marginally offset by altered fluxes of non-CO2 greenhouse gases in global forests and grasslands under N deposition |
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| Authors: | Shuqi Xiao Chao Wang Kai Yu Genyuan Liu Shuang Wu Jinyang Wang Shuli Niu Jianwen Zou Shuwei Liu |
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| Institution: | 1. Jiangsu Key Laboratory of Low Carbon Agriculture and GHGs Mitigation, Nanjing, China;2. Jiangsu Key Laboratory of Low Carbon Agriculture and GHGs Mitigation, Nanjing, China
Key Laboratory of Low-carbon and Green Agriculture in Southeastern China, Ministry of Agriculture and Rural Affairs, College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing, China;3. Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China |
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| 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 N2O), 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 N2O 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 (RH) in both forests and grasslands, while a significant N-induced increase in soil CO2 fluxes (RS, soil respiration) was observed in grasslands. Nitrogen addition significantly stimulated soil N2O 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 N2O emissions together with the reduced soil uptake of CH4. |
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| Keywords: | carbon pool climate change greenhouse gas nitrogen deposition nitrogen pool |
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