干湿交替条件下土壤厌氧氨氧化反应的发生机制与过程效应

For decades, denitrification by heterotrophic bacteria was assumed to be the only pathway for loss of fixed nitrogen to the atmosphere. The discovery of anaerobic ammonium oxidation (anammox) mediated by autotrophic anammox bacteria oxidizing ammonia directly to nitrogen gas (N2) without emission of nitrous oxide (N2O) challenged this view. So far, extensive large-scale occurrence of anammox has been detected responsible for at least 30-50% of the total N turnover in marine ecosystems. However, the significance of anammox process and their roles in terrestrial ecosystems has remained uncertain.. Financially supported by the NSFC funding, our research team has investigated the extensive occurrence and biogeochemical features of anammox in inland waters. We found that anammox was ubiquitous in inland waters, and the biogeochemical and biogeographical parameters such as substrate, temperature and pH were not the limiting factors for anammox occurrence. In our previous studies, polymerase chain reaction (PCR) screening showed negative results from dry soils in the riparian zone. Moreover, anammox has not yet been detected in terrestrial upland soils so far, not only by our team but also by other researchers. More interestingly, we found that after flooding for few months anammox bacteria were detected in each sample of the previous sampling sites in the riparian zone. These results promoted us to apply this project and investigate whether and how the anammox process occurs in upland soils. Based on these results and the principal knowledge on microbial growth factors, we hypothesize that the key limiting factor of the occurrence of anammox is water content in soils.. The project aims to explore the mechanism, distribution and role of anammox in soil under alternate drying-wetting condition by intermittently flooding, and finds out if the large-scale anammox occur in terrestrial upland soil. The results of this project would supplement and revise the theory system of nitrogen cycle in terrestrial ecosystem, therefore has significant influence on global nitrogen cycle balance and flux.

厌氧氨氧化作为一种新型氮循环模式在自然界的发现，显著改变了人们对地球氮循环的认识。目前具有全球效应的厌氧氨氧化仅发现于海洋，其产氮气量占海洋总产量的30-50%；在陆地系统，厌氧氨氧化的发生及贡献量均鲜有报道。在自然基金连续资助下，本研究团队证实厌氧氨氧化在全球尺度的水生态系统广泛存在，且底物、温度、pH值等均非反应发生的限制性因子。而本团队和其他学者在干旱土壤均未检测到该反应发生。为什么旱地土壤不发生厌氧氨氧化值得深入探讨。申请者前期研究发现经一定时间水淹的水陆交错带土壤，均能检测到大量厌氧氨氧化菌的增长。根据微生物生长要素和前期研究，猜测土壤系统厌氧氨氧化反应发生的关键限制因子很可能是土壤中的水含量。本项目计划研究干湿交替条件下土壤厌氧氨氧化反应规律性发生的机制、活性区域和过程效应，探索陆地表层土壤是否存在广泛的厌氧氨氧化过程。研究结果将对陆地系统氮循环模式和通量模型计算有重要影响。

项目组前期研究发现厌氧氨氧化广泛发生在陆地水生态系统，水陆交错带是厌氧氨氧化热区。但厌氧氨氧化在陆地生态系统的分布特征仍然是未知的，陆地土壤亚表层随地下水位变化形成干湿交替区域，猜测此区域可能是厌氧氨氧化的另一个热区，因此项目组通过已构建的同位素示踪和分子生物学技术，综合研究厌氧氨氧化在陆地生态系统的发生机制、过程效应与关键影响因素。.（1）白洋淀湖泊岸边带陆向土壤厌氧氨氧化研究表明，表层土壤未检测到厌氧氨氧化活性，地下水位线以下的含水层土壤中发现较高丰度的厌氧氨氧化细菌，活性范围为0.62-8.78 mmol-Nm-2 d-1，对氮损失的贡献达52.3%，并显著减排N2O。.（2）在高原生态系统中发现零星存在厌氧氨氧化过程，厌氧氨氧化菌群丰度及活性均较低，厌氧氨氧化对氮损失贡献为5.5%-17.4%，反硝化仍是主要的氮损失过程（82.6%-94.5%）。菌群结构解析表明，Candidatus Brocadia , Candidatus Jettenia 和 Candidatus Kuenenia是高原生态系统厌氧氨氧化的主要菌属。.（3）在全球陆地生态系统中发现亚表层饱和土壤存在广泛的厌氧氨氧化热层，地下水位线以下的饱和土壤发生显著的厌氧氨氧化反应，对氮损失贡献为36.8%-79.5%。.（4）厌氧氨氧化活性分布与反应底物浓度、环境因子等多种因素有关，研究发现水是激活厌氧氨氧化菌的关键因素，加水3个月可复苏厌氧氨氧化菌，水诱导硝酸盐还原，为厌氧氨氧化提供了电子受体与能量。另外，水的干湿交替驱动了土壤氧化还原电位变化，促进了厌氧氨氧化的发生。.（5）嘉兴石臼漾人工湿地验证了干湿交替界面引起的氮循环热区效应，小沟岸边带系统不仅是厌氧氨氧化的热区，还是各个氮循环过程的热区，同时达到温室气体N2O减排的效果。.陆地生态系统厌氧氨氧化活性区域的新发现，以亚硝酸盐为枢纽的氮循环内在机制的解析，扩展了人们对厌氧氨氧化作用的认识；亚表层饱和土壤中厌氧氨氧化热层的发现将显著影响氮通量计算模式，为治理地下水氮污染、调控N2O释放途径、维持氮素动态平衡提供了新思路。

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