真菌异养硝化在亚热带森林土壤N2O排放中的作用

Nitrous oxide (N2O) is a potent greenhouse gas and dominant anthropogenic stratospheric ozone-depleting emission. Soil is a major source of the N2O, accounting for nearly 70% of the anthropogenic atmospheric loading of N2O. Due to the complexity of soil N cycle and in-depth understanding of N2O production processes, how to accurately predict the N2O flux from soil and develop appropriate management practice for N2O mitigation is still a challenge. Previous researches suggested that fungal heterotrophic nitrification is a potential important process releasing N2O. However, reliable methods to quantify fungal origin of N2O, precise information about the contribution of fungal heterotrophic nitrification to N2O emissions and controlling factors to regulate this process are still lacking. As we known, N2O contains both bulk (δ15N and δ18O) and site specific isotopic information. Site preference (SP) is defined as the difference in δ15N between the central (α) and outer (β) N atoms in N2O. Recently, SP has emerged as a potential conservative tracer of microbial N2O production. Although many studies have provided specific SP values of several different N2O production processes, the characteristic SP values of N2O produced by fungal/bacterial heterotrophic nitrification have never been reported. In this study, they would be determined by a new stable isotope method (Isotopomer signatures of N2O). Then, the contribution of fungal heterotrophic nitrificaiton to soil N2O emission in subtrophic forest soil with different soil properties, for example vegetation type, soil pH, organic matter, soil temperature, and so on, would be well investigated. The results about likely contribution of fungal heterotrophic nitrification to N2O emissions would be cross-validated through a parallel SIR experiments combined with 15N tracer (Substrate induced respiration inhibition methods). The key factors affecting soil N2O emission from fungal heterotrophic nitrification would be clarified. In general, this study is of important significances to understand soil N2O emissions, especially from fungal dominated heterotrophic nitrification, and to accurately predict the soil N2O flux and effectively establish mitigation strategies for N2O emission.

氧化亚氮（N2O）是一种重要的温室气体，土壤是最重要的N2O排放源之一。由于对N2O的产生过程尚未明晰，有关土壤N2O排放量的估算仍存在很大不确定性。现有结果表明，在湿润的亚热带和热带地区，真菌异养硝化可能是N2O的重要产生途径，因缺乏合适的研究方法，对真菌异氧硝化的研究并未深入。本项目以我国亚热带森林土壤为研究对象，以N2O同位素异位体法为研究手段，通过系列试验确定该地区土壤真菌/细菌异养硝化产生的N2O的SP（site preference）特征值，以填补文献中这一数值的空缺。运用基于SP-δ15N值的Monte Carlo计算法定量分析真菌异养硝化对土壤N2O的排放贡献，通过15N标记结合抑制剂法（SIR）验证结果准确性，以期阐明亚热带酸性森林土壤真菌异养硝化的主要特点及其主要影响因素。为深入认识土壤N2O产生途径，特别是真菌主导的异养硝化过程，准确估算土壤N2O排放量提供科学依据。

氧化亚氮（N2O）是一种重要的温室气体，土壤是最重要的N2O排放源之一。在湿润的亚热带地区，真菌异养硝化可能是N2O的重要产生途径，因缺乏合适的研究方法，对真菌异氧硝化的研究并未深入。本项目以我国亚热带森林土壤为研究对象，优化并建立了N2O同位素异位体的SP值的质谱测定方法及酸性森林土壤中NH4+、NO3-的15N自然丰度测定方法；通过纯培养和回接土壤试验确定该地区土壤真菌异养硝化产生的N2O的SP（site preference）特征值（SP=20~28‰），填补了文献中这一数值的空缺；运用基于SP-δ15N值的Monte Carlo计算法，定量分析亚热带地区林地、农田、茶园、竹林四种不同土地利用类型土壤N2O的排放贡献，发现农田、茶园、竹林三种土壤的N2O排放主要来源于自养硝化和反硝化过程，而林地土壤的N2O排放主要来源于异养硝化过程；土壤pH值和C/N比是影响亚热带森林土壤真菌异养硝化的两个重要因素，N2O的SP值随pH值升高而增大，真菌异养硝化对N2O的排放贡献减弱，而随土壤C/N比降低N2O的SP值明显升高，自养硝化作用对N2O排放贡献有所加强。以上结果可为深入认识土壤N2O产生途径，特别是真菌主导的异养硝化过程，准确估算亚热带地区土壤N2O排放量提供科学依据。