氮沉降对毛竹林GHGs排放和净碳汇效应的影响及其机制研究

Nitrogen (N) deposition tends to increase the carbon sequestration of forest ecosystem via improving the productivity of forest, but it also arouses the emissions of greenhouse gases (GHGs), including CO2, CH4 and N2O, which partly offsets the carbon benefits of N deposition. The magnitude of these effects is still uncertain. Being a key bamboo species, Moso bamboo (Phyllostachys pubescens) forests in China cover 70% of the country's bamboo forest area and 80% of the total global area of Moso bamboo. With rapid growth and strong carbon sequestration capacity, Moso bamboo forest are receiving increasing attention on the large potential in global carbon uptake and the unique role in mitigating climate change. However, the net carbon uptake effect of Moso bamboo forest at the ecosystem scale under rapidly growing N deposition conditions remains unclear. In this project, we intend to investigate the change in carbon storage and GHGs flux in Moso bamboo forest ecosystem under different level of simulated N deposition, and demonstrate the potential effect of N deposition on net carbon uptake of Moso bamboo forest. The forest productivity, litter dynamics, fine root turnover and soil respiration will be observed to reveal the process mechanism of change in biomass carbon of Moso bamboo forest and soil organic carbon under N deposition. The N contents and C:N ratios in different parts of Moso bamboo will be measured to elucidate the possible internal mechanism of productivity change of Moso bamboo forest. The abiotic factors, including water-soluble organic carbon (WSOC), NH4+-N and NO3- - N, C:N ratio and pH in soil, and biotic factors, including the composition and structure of soil microbes, will be also analyzed to reveal the possible mechanism of N deposition effects on GHGs flux in soil. Finally, the relationship model about N deposition level and net carbon uptake effect of Moso bamboo forest ecosystem will be set up to forecast the potential role of Moso bamboo forest ecosystem in mitigating climate change under the scenario of increasing N deposition in the future. This study will contribute to the comprehensive understand on carbon benefits of N deposition and climate policy-making in our country.

氮沉降既可增加森林的碳汇能力，又可激发温室气体(GHGs)排放，部分抵消了其增汇的功能。毛竹林在全球碳吸存中的巨大潜力及在减缓气候变化中的独特作用日益受到关注，但在日趋严重的氮沉降影响下其净碳汇能力仍不清楚。本项目拟通过观测不同强度氮沉降条件下毛竹林生态系统碳贮量和GHGs通量的变化，阐明氮沉降对毛竹林净碳汇能力的潜在影响，并从植物生产力、凋落物动态、细根周转和土壤呼吸变化的角度揭示氮沉降影响毛竹林生物量碳和土壤有机碳变化的过程机制，从毛竹各部位N含量、C/N比的化学计量学特征角度揭示毛竹林生产力变化的内在可能机理，从土壤WSOC、硝态氮和铵态氮、C/N比、pH值变化等非生物因素和土壤微生物的组成、结构变化等生物因素角度揭示氮沉降作用于GHGs通量的内在可能机理，构建毛竹林生态系统净碳汇效应与氮沉降水平的关系模型，预估未来氮沉降增加情景下我国毛竹林在减缓气候变化中的地位和作用。

本项目研究发现集约经营极显著增加了毛竹林的叶凋落量；低氮显著增加了毛竹林叶凋落量，而高氮则显著减少了叶凋落量，经营方式和氮沉降对凋落物量没有显著的交互作用。低氮显著提高了毛竹凋落叶和细根的年分解率，而高氮显著降低了凋落叶和细根的年分解分率。集约经营显著放大了低氮和高氮对凋落物分解的积极/消极作用，减弱了氮沉降对氮流失的作用，但增加了氮沉降对磷流失的作用。氮沉降能显著提高土壤微生物量碳，减少细菌多样性。但是，当氮的添加超过60 kg∙N∙ha−1∙yr−1时，将会限制土壤微生物量碳的增加。与氮沉降相比，经营管理措施对毛竹林土壤微生物量碳的管理和微生物的多样性影响更显著。氮沉降显著抑制了蔗糖酶、纤维素酶、硝酸还原酶的活性，显著增加了粗放经营毛竹林土壤的脲酶活性，对过氧化氢酶活性的影响不显著.氮沉降与经营方式的复合作用显著影响了土壤酶活性。集约经营毛竹林中氮沉降会增加竹叶的氮磷浓度，但当氮沉降超过60 kg N ha-1yr-1时,这种积极的影响就会减弱。粗放经营毛竹林中氮沉降会增加竹叶氮浓度，但加剧了磷缺乏。氮沉降对竹叶化学计量的积极影响受经营方式和毛竹生长阶段的影响。氮沉降显著影响了毛竹地上器官的NSC含量及其分配，且随竹龄而显著变化。氮沉降显著降低了粗放经营毛竹林土壤的DOC和DON含量. 高氮沉降(≥90 kg•ha–1•yr–1)显著增加了集约经营下土壤DOC和DON含量。氮沉降没有显著影响到20-40cm土壤DOC含量.说明集约经营大大减缓了氮沉降对毛竹林土壤DOC和DON的抑制作用，甚至将氮沉降的消极作用转变为积极影响。研究结果为全球氮沉降增强背景下合理经营毛竹林提供了科学依据，为我们全面、准确认识氮沉降在减缓全球气候变化中的净碳汇效应提供了基础数据，为国家制定“增汇减排”相关的重大战略决策提供了科学参考。