大气氮沉降量和形态对北方针叶林土壤温室气体排放的影响机制

Greenhouse gas (GHG) emissions from boreal forests act as an important indicator for changes of carbon and nitrogen sources and sinks in the terrestrial ecosystem. However, as a sensitive and fragile ecosystem, the boreal forest is vulnerable to global changes. The increase in the quantity and change in the form (NH4+-N or NO3--N) of atmospheric nitrogen deposition are likely to induce changes of soil carbon and nitrogen cycling, thus influencing GHG emissions. Therefore, the investigation on the mechanisms of how nitrogen deposition affects soil microbial community structure and regulates carbon and nitrogen transformations to drive GHG emissions from a boreal forest soil is of great significance for maintaining the stability of the boreal forest as an important carbon sink. Coniferous forests in northern China will be selected as the study sites in the proposed research. The first objective of the research is to monitor the seasonal and interannual variations of GHG emissions from soils and explore the key controlling factors under simulated nitrogen deposition with different quantity and forms through long-term in situ observation of GHG fluxes; the second objective is to investigate the associations of soil carbon and nitrogen transformations and microbial characteristics with GHG emissions in simulated nitrogen deposition with different quantity and forms by the 13C and 15N isotope labeling technique and the microbial method in a cultivation experiment. The research can provide a scientific basis to fully understand responses of boreal forest to global change and to further promote researches on boreal forest ecology in the global change scenarios.

北方针叶林温室气体排放是陆地生态系统碳氮源汇变化的重要表征，但北方针叶林作为敏感且脆弱的生态系统，极易受全球变化影响。大气氮沉降量的增加或形态改变必然引起土壤碳氮循环过程发生改变，从而影响温室气体排放。因此，探讨氮沉降通过影响土壤微生物群落结构和调控碳氮转化过程来驱动北方针叶林土壤温室气体排放的作用机理，对于维持北方针叶林作为重要碳汇的稳定性具有重要意义。本项目拟以我国漠河北方针叶林为研究对象，通过“野外长期连续观测”温室气体排放通量，阐明不同量和不同形态氮沉降模拟下土壤温室气体排放的季节和年际变化规律及其关键控制因素；利用13C和15N同位素标记技术、室内培养试验和相关微生物学手段，揭示在不同量和不同形态氮沉降影响下土壤碳氮转化过程及微生物特征与温室气体排放的关联。研究结果将为深入了解北方针叶林对全球变化的响应规律，进一步推动全球变化情景下北方针叶林生态学的深入研究提供科学依据。

氮沉降量和形态对北方针叶林土壤温室气体排放的影响存在较大分歧，氮沉降量增加和形态改变影响土壤微生物群落结构和碳氮循环过程进而影响温室气体排放的机制不甚明确。本项目以大兴安岭地区典型北方针叶林为研究对象，通过对三个生长季（5-9月）土壤温室气体排放的监测发现，北方针叶林土壤温室气体排放存在明显季节差异，年际差异不显著；还原态氮沉降（Nred）对北方针叶林土壤CO2、N2O和CH4排放通量没有影响，而氧化态氮沉降（Noxi）显著提高了土壤CO2和N2O排放通量，但对CH4排放没有影响；低量氮沉降（Nlow）显著提高了北方针叶林土壤N2O排放通量，而对CO2和CH4排放通量没有影响，高量氮沉降（Nhigh）显著提高了土壤CO2和N2O排放通量，而对CH4排放通量无显著影响。土壤CO2排放通量与土壤总碳、可溶性有机碳、可溶性氮、微生物量碳氮浓度以及地下5 cm处土壤温度呈正相关；土壤CH4排放通量与所测土壤性质之间没有显著关系；土壤N2O排放通量与土壤可溶性氮、微生物量氮、铵态氮、硝态氮浓度以及地下5 cm处土壤温度呈正相关。北方针叶林土壤微生物量和微生物群落结构对不同量、不同形态氮沉降响应存在差异。Nred显著降低了微生物量碳和土壤细菌Chao1指数和shannon指数，而对微生物量氮没有影响，Noxi不但显著增加了微生物量碳氮，而且显著增加了土壤细菌Chao1指数和shannon指数，表明还原态氮沉降和氧化态氮沉降对土壤微生物具有截然相反的影响。Nred显著抑制了Alphaproteo bacteria细菌群落，而对Acido bacteria细菌群落有促进作用，而Noxi显著促进了Alphaproteo bacteria细菌群落，而对Acido bacteria细菌群落没有影响。研究结果可以加深缺氮北方针叶林土壤对大气氮沉降响应特征的了解、提高在全球变化条件下我国北方针叶林温室气体排放清单的准确性以及为进一步推动气候变化情景下北方针叶林生态学的深入研究提供基础资料和理论依据。