2023/5/22 15:58:33 阅读：201 发布者：
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论文内容
研究背景：
氮沉降对陆地生态系统土壤有机碳(C)和温室气体(GHG)排放的影响是全球气候变化下影响温室气体收支的主要驱动因素。尽管在这方面已经进行了很多研究，但对于全球范围内氮沉降如何影响土壤碳库和温室气体收支，我们仍然知之甚少。
为了澄清这些有争议和不确定的问题，我们分析了一个比以前的研究更广泛的全球网络，在2008年之前和之后的275个地点进行了氮素富集实验(图1)，并使用土壤碳库，二氧化碳排放，甲烷的吸收和排放四个响应变量来解决以下问题：(A)土壤碳库和土壤温室气体通量对不同生态系统类型(如森林、草地、农田、湿地)的氮富集有何反应？(B)哪些关键因素影响土壤碳库和土壤温室气体通量对氮富集的响应？(C)氮沉降如何影响全球尺度上土壤碳库和土壤温室气体通量的净变化？
研究内容:
我们从Web of Science、Google Scholar和中国知识资源综合数据库检索并选择了大部分2018年前发表的同行评议文章，使用的研究术语包括：N添加/施肥/富集、土壤C、土壤C通量、土壤呼吸、土壤C循环、CH4吸收、CH4排放、N2O排放和土壤温室气体收支。为了避免任何选择偏差，我们从我们的研究中提取了符合以下标准的论文：(A)只包括控制点和处理点位于相同气候、土壤和植被条件下的研究，研究必须包括至少一个我们选择的变量；(B)只包括现场进行的、不受其他管理措施(如灌溉、干旱、变暖、火灾或磷钾肥)影响的田间试验；(B)只包括现场进行的、不受其他管理措施(如灌溉、干旱、变暖、火灾或磷钾肥)影响的研究；(C)只有重复次数足够且成对或按时间顺序排列的地点具有相同土壤类型的研究才包括在内；(D)每年测量的土壤温室气体通量或作为生长季平均值的土壤温室气体通量以不同方式表示；(E)对照和处理组变量的平均值、标准偏差/误差和重复数据可直接从文字、表格或数字化图表中提取。
此外，还直接从论文中获得了地理位置(海拔和经度)、气候条件(MAP，年均降水量；MAT，年均温度)、生态系统类型(包括林地，即热带森林、温带森林和北方森林；草地；农田和湿地)、N富集速率(kg ha−1年−1)、N形态(尿素、NH4NO3、NH+4和NO−3)和N富集实验持续时间(年)等环境变量。还收集了控制组和处理组中报告的容重(BD)和土壤深度的数据。
最后，在考虑了3000多份出版物后，我们的数据集包括272项与氮肥富集有关的研究(数据集S1)，其中165项研究是自2008年以来发表的。所有272项研究都符合我们的荟萃分析选择标准。275个测点共1074次观测(图1)，其中土壤有机C浓度126次，土壤CO2 Em i s s I o n s A n d 1 3 7 o s I l C H 4通量539次。共观测到68次CH4吸收，69次CH4排放，272次土壤N2O排放。
F I G U R E 1 Distribution of sampling sites used for the global meta-analysis. The land use cover data came from the Global Land Cover Characteristics Database (GLCCGBE20; https ://earth explo rer.usgs.gov/) that was derived from the National Remote Sensing Center of China (NRSCC, 2015). To distinguish the differences between this study and a previous study by Liu and Greaver (2009), we divided the dataset into two parts: before and after 2008. 2008a indicates the data published before 2008, and 2008b indicates the data published after 2008 [Colour figure can be viewed at wileyonlinelibrary.com].
研究结论：
结果表明，增施氮肥后，土壤有机碳浓度和CO2、CH4、N2O排放量分别平均增加3.7%、0.3%、24.3%和91.3%，土壤对CH4的吸收减少6.0%。此外，N2O排放量增加的百分比(91.3%)比Liu和Greaver(生态学快报，2009，12：1103-1117)报告的215%低两倍。对土壤碳库的刺激作用(15.70kgC ha−1年−1/kg N ha−1年−1)也比以前报道的全球氮沉降下更大。全球氮沉降结果表明，农田是最大的温室气体来源(按CO2当量计算)，其次是湿地。然而，森林和草原是两个重要的温室气体汇。在全球范围内，氮沉降使陆地土壤的碳汇增加了6.34pgCO2/年。它还增加了10.20pgCO2-Geq(二氧化碳当量)/年的土壤温室气体净排放量。因此，根据全球变暖潜能法的计算，氮沉降不仅增加了土壤碳库的大小，而且增加了全球温室气体排放。
F I G U R E 3 Effects of N enrichment on soil organic C concentration (SOC; a), CO2 emissions (b), CH4 uptake (c), CH4 emissions (d) and N2O emissions (e) for different ecosystem types. The values in the figure are the response ratios (calculated by Equation 1). Error bars represent 95% confidence intervals (CI). The values next to the bars are the corresponding number of observations. The number of studies for SOC in wetlands (a), CH4 uptake in BF (c) and CH4 emissions in TF (d) were 1, 3 and 2 respectively. TRF, tropical forest; TF, temperate forest and BF, boreal forest. *Indicates significant mean responses byeach ecosystem type to N enrichment, that is, their 95% CI did not overlap zero [Colour figure can be viewed at wileyonlinelibrary.com]
TA B L E 2 Summary of N-induced emissions/uptake factors (F) for soil CO2 uptake and emissions (kg CO2-C ha−1 year−1 per kg N ha−1 year−1), CH4 uptake and emissions (kg CH4-C ha−1 year−1 per kg N ha−1 year−1), N2O emissions (kg N2O-N ha−1 year−1 per kg N ha−1 year−1) and C pools (kg C ha−1 year−1 per kg N ha−1 year−1) at the global scale
F I G U R E 4 Mechanisms that alleviate global warming under N enrichment (deposition). The current analysis indicated that first, N enrichment (deposition) enhanced the soil C pool and soil CO2 emissions by stimulating plants to fix more CO2 from the atmosphere. It can also be concluded that the soil C pool was increased by N enrichment (deposition). The plants input more C into the soil, causing the ecosystem to become a net CO2 sink, which negatively contributed to global warming. Second, N enrichment (deposition) suppressed the CH4 uptake but stimulated CH4 emissions, which increased atmospheric CH4 and positively contributed to global warming. Third, N enrichment (deposition) increased N2O emissions, which positively contributed to global warming. ‘+’, positive contribution or stimulation (red solid line); ‘−’, negative contribution or suppression (green solid line). Both ‘+’ and ‘−’, N enrichment (deposition) could increase or decrease greenhouse gas (GHG; grey dotted line) in terrestrial ecosystems. ‘?’, unresolved by this study and requiring further research in the future. The data are the global warming potentials in units of CO2 equivalent emissions (Pg CO2/year) caused by the effects of global N deposition on the soil carbon pool and GHG fluxes [Colour figure can be viewed at wileyonlinelibrary.com]
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