中亚热带典型森林土壤氮气释放及微生物学机制

Dinitrogen (N2) emission has been considered to one of the most uncertain steps in the soil nitrogen losses. It is critical to understand how and to what extent soil N2 emission affects the nitrogen availability and the accurate assessment of nitrogen budget in forest ecosystems. However, little is known about the magnitude and microbial mechanism on soil N2 emission in forest ecosystem. In this proposed project, we will set two typical subtropical forests in southern China. In each forest, we will collect the soil samples from different depths (0-10 cm, 10-20 cm, 20-30 cm, 30-40 cm, and 40-50 cm) and seasons. The 15N and 13C tracer incubation experiments with different labelled substrates (ammonium, nitrate, nitrite and methane) will be conducted in all the soil samples. The gas samples containing labelled N2 and CO2 will be respectively determined by a continuous flow isotope ratio mass spectrometry (CF-IRMS) and a gas chromatographic mass spectrometry (GC-MS). The rates of N2 emission will be calculated, meanwhile microbial processes (such as denitrification, anaerobic ammonia oxidation, codenitrification and nitrite-dependent anaerobic methane oxidation) will be differentiated using isotope pairing technique. In addition, Microbial functional communities, diversities and gene abundance in soil samples will be determined by microbial molecular biological technique, such as qPCR and clone sequencing. Combined the above results, we will quantify the rate and explore the dynamic characteristic of N2 emission in soil vertical section and seasonal change; Finally, clarify the inner linking between the function microbial structures and gene abundance coupling with influence factors and N2 emission. Overall, the implementation of this project will be to deepen the mechanism knowledge of nitrogen losses in Mid-subtropical forest soils in southern China and provide important insights into nitrogen cycle processes and characteristic for forest ecosystem.

氮气（N2）释放是土壤氮损失研究中不确定性最大的一个环节，但目前对森林土壤N2释放及其驱动机制的认识仍十分有限。土壤N2释放速率的明确及机制的揭示是准确评估森林生态系统氮素利用和氮素收支平衡的关键所在。本项目拟以我国中亚热带典型常绿阔叶林和杉木人工林为研究对象，季节性采集不同深度土壤样品，利用15N和13C同位素标记室内培养技术，量化土壤N2释放速率，区分和明确反硝化、厌氧氨氧化、共反硝化和亚硝酸盐型厌氧甲烷氧化作用对土壤总N2产生的贡献；利用实时荧光定量PCR和克隆文库构建与测序等微生物分子生物学技术，研究产N2功能微生物群落组成、多样性和功能基因丰度，阐明土壤N2产生的微生物学机理。研究结果有望揭示我国中亚热带森林土壤氮损失的机制，拓展对森林生态系统氮循环过程和特征的理解。

本项目对我国亚热带森林（常绿阔叶林和杉木人工林）土壤进行稳定同位素标记培养实验，耦合荧光定量PCR等分子生物学技术，研究土壤氮气（N2）释放速率及其产生途径的贡献，同时探讨驱动土壤氮气释放的影响机制。结果表明：土壤N2释放速率为0.049-2.34 nmol N g-1h-1，常绿阔叶林显著高于杉木人工林（除0-10 cm外）。杉木人工林土壤N2释放沿剖面分布呈先增后降趋势，而常绿阔叶林则呈显著的倒“V”型分布。厌氧氨氧化和共反硝化过程主要发生在常绿阔叶林表层和杉木人工林深层土壤中，其共同的N2释放速率为0.0025-0.095 nmol N g-1h-1，贡献了总N2释放的0.11%-57.7%，杉木人工林显著高于常绿阔叶林。两个森林土壤剖面中均检测到反硝化nosZ基因和厌氧氨氧化hzsB基因。土壤反硝化nosZ基因丰度在常绿阔叶林显著高于杉木人工林，而厌氧氨氧化hzsB基因丰度则刚好相反，与其N2释放速率相吻合。杉木人工林中深层土壤hzsB/nosZ比值约为12.7，说明其深层土壤具有较高的厌氧氨氧化潜力。土壤水分、微生物生物量氮和速效磷含量与土壤反硝化、厌氧氨氧化N2释放速率及功能基因丰度呈显著正相关关系，可能是引起不同森林土壤N2释放存在差异的重要因素。本研究表明，厌氧氨氧化和共反硝化作用能在亚热带森林土壤中发生，其对土壤氮损失的贡献程度因森林类型不同而不同，反硝化作用依然是土壤主要的N2产生途径。未来需要加强亚热带区域不同站点森林土壤气态氮损失的研究。