寒温带针叶林土壤N2O排放对氮沉降增加的非线性响应研究

The mechanisms involved in the responses of nitrous oxide (N2O)emission from boreal forest soils to increased atmospheric nitrogen deposition are so far unclear. In this project, a cold-temperate coniferous forest in the Great Xin'an Mountain region will be selected to conduct a multi-level N addition experiment.Various methods, such as long-term field monitoring,nitrogen and oxygen isotope tracing, laboratory incubation experiments,and molecular biological techniques will be adopted to address the following topics. First, the nonlinear responses of soil N2O emissions to nitrogen addition will be explored,so as to determine a critical level of atmospheric nitrogen deposition leading to a rapid shift of N2O emission from the cold-temperate coniferous forest soil. Second, the effects of N addition on rates of soil gross nitrogen mineralization, gross nitrification,ammonium immobilization, and nitrate consumption will be examined,it will help to clarify the linkage between soil gross nitrogen transformation rates and soil N2O fluxes. Third, in order to clarify the main production processes of soil N2O flux in the cold-temperate coniferous forest and their responses to nitrogen additio,we will quantify the relative contributions of soil nitrification,nitrifier denitrification and denitrification to soil N2O fluxes. Finally, the short and long-term effects of nitrogen addition on the abundance and composition of soil nitrifying bacteria and denitrifying bacteria communities will be examined, because it is necessary to reveal the coupling between the key functional microbial communities and soil N2O emission.Above studies are expected to clarify the microbilogy mechanisms responsible for the responses of N2O emissions from the cold-temperate coniferous forest soil to nitrogen addition. Our study will help to reveal the mechanisms responsible for the regulatory of nitrogen availability on N2O emission from forest soils, and to effectively reduce the uncertainty of the estimation of regional forest soil N2O emission. Moreover,it has important theoretical and practical values for controlling and reducing N2O emissions from boreal coniferous soil as well as improving the ecosystem nitrogen use efficiency.

北方森林土壤氧化亚氮（N2O）排放对大气氮沉降增加的响应机制尚不清楚。本项目以大兴安岭寒温带针叶林为研究对象，设置多水平的增氮控制试验和室内培养实验，利用原位监测、氮氧同位素示踪和微生物分子生态学方法，研究土壤N2O排放对增氮的非线性响应，明确导致寒温带针叶林土壤N2O排放急剧转变的大气氮沉降临界水平；研究增氮对土壤氮素总矿化、总硝化、NH4+固持和NO3-消耗的影响，解析土壤氮总转化过程与土壤N2O排放之间的耦合关系；量化土壤硝化、硝化细菌反硝化和反硝化作用对土壤N2O排放的贡献，阐明寒温带针叶林土壤N2O的主要产生过程及其对增氮响应的差异；研究增氮对土壤硝化细菌和反硝化细菌群落丰度和组成的影响，揭示土壤功能微生物群落与N2O排放的耦合机制。研究结果在降低区域土壤N2O排放估算的不确定性，控制或降低寒温带针叶林土壤N2O排放，提高生态系统氮素利用率等方面均具有重要的理论意义和实践价值。

北方森林土壤氧化亚氮（N2O）排放对大气氮沉降增加的响应机制尚不清楚。本项目以大兴安岭寒温带针叶林为研究对象，设置多水平的增氮控制试验和室内培养实验，利用原位监测、氮氧同位素示踪和微生物分子生态学方法，研究土壤N2O排放对增氮的非线性响应，明确导致寒温带针叶林土壤N2O排放急剧转变的大气氮沉降临界水平；研究增氮对土壤氮素总矿化、总硝化、NH4+固持和NO3-消耗的影响，解析土壤氮总转化过程与土壤N2O排放之间的耦合关系；量化土壤硝化、硝化细菌反硝化和反硝化作用对土壤N2O排放的贡献，阐明寒温带针叶林土壤N2O的主要产生过程及其对增氮响应的差异；研究增氮对土壤硝化细菌和反硝化细菌群落丰度和组成的影响，揭示土壤功能微生物群落与N2O排放的耦合机制。研究结果表明：①施氮4年没有显著改变有机层和矿质层土壤NH4+-N和DON含量，但显著导致有机层和矿质层NO3--N和总溶解性氮（TDN）积累，土壤酸化明显。②施氮显著促进土壤N2O排放，高剂量施氮促进效应显著，增加比例介于194.2~333.5%，土壤N2O对增氮的响应可用指数增加方程来描述。③土壤N2O通量主要受温度驱动，其次是NO3--N含量和水分；氮素富集条件下土壤N2O通量的变化量与水分、NO3--N含量变化量正相关，暗示，硝化过程是控制土壤N2O产生的主要过程。④施氮显著增加氨氧化古菌(AOA)amoA基因丰度，不改变AOB基因丰度，AOA对外源性氮素输入响应更加敏感，对土壤氮素转化和N2O排放过程贡献更大。此外，AOA和AOB丰度均与矿质层土壤水分含量极显著正相关，土壤水分能够解释其变异的50%；其次DOC对AOB也有显著影响，说明DOC是AOB的有效底物。⑤土壤NH4+-N的产生以有机氮矿化为主，土壤NO3--N的产生以自养硝化为主，施氮短期内促进了活性有机氮矿化和自养硝化，对其它过程影响不显著。氮素富集条件下，土壤总NH4+-N的产生速率均显著低于总NH4+-N的消耗速率，相反，土壤NO3--N的总产生速率显著高于总消耗速率，土壤N2O通量与总硝化、自养硝化速率正相关。研究结果在降低区域土壤N2O排放估算的不确定性，有助于提高生态系统氮素利用率。