典型农田土壤氧化亚氮（N2O）还原细菌及其与N2O源汇关系研究

The denitrification loss was much higher in rice-wheat rotation agro-ecosystems of Taihu Lake regions than that in maize-wheat rotation agro-ecosystems of North China Plain, whilst the nitrous oxide (N2O) emissions from these two agro-ecosystems were comparative. The soil microbial mechanisms of the phenomenon are poorly understood. Various microbial guilds have involved in N2O production and emission, and the only known sink is N2O reduction to N2 by N2O-reducing bacteria that possess nosZ genes to encode nitrous oxide reductase. N2O-reducing bacteria have two groups, typical denitrify bacteria (CladeⅠ) and recently reported non-typical denitrifying bacteria (CladeⅡ). Little information is known about the community composition and abundance of soil N2O-reducing bacteria, and their relation with N2O sink effect in these two agro-ecosystems. This project will select typical maize-wheat rotation and rice-wheat rotation agro-ecosystems, and investigation the influences of the different rotations and soil types on community composition and abundance of soil N2O-reducing bacteria under field plot conditions. The community composition and abundance of active N2O-reducing bacteria will be illustrated, and the ecological factors that determine the community composition and abundance will be explored through soil microcosm experiments. The key ecological factors that govern the community composition and abundance of N2O-reducing bacteria will be further summarized with incorporation of field plot experiments. Finally, the community composition and abundance of N2O-reducing bacteria will be linked to N2/N2O emission patterns, and the relationship will be revealed between the community composition and abundance of N2O-reducing bacteria and N2O sink effects. At last, the soil microbial mechanisms for great difference in denitrification loss while comparative N2O emissions from these two agro-ecosystems will be fully understood.

太湖地区稻麦轮作相对于华北平原玉米小麦轮作土壤反硝化损失差异较大、但氧化亚氮（N2O）排放量却相当，相应的微生物机制尚不明确。N2O还原细菌是目前已知的唯一的N2O汇，包括经典的CladeⅠ类群和新发现的CladeⅡ类群。这两种典型生态系统中N2O还原细菌群落组成和丰度知之甚少，土壤N2O还原细菌与N2O汇效应的关系仍不清楚。选取典型玉米小麦轮作和稻麦轮作系统，研究不同轮作系统和土壤类型对N2O还原细菌群落组成和丰度的影响。通过室内模拟试验研究不同轮作系统和土壤类型中活跃的N2O还原细菌群落组成和丰度，明确驱动这些N2O还原细菌群落组成和丰度的生态因子。结合田间原位试验结果，阐明驱动土壤N2O还原细菌群落组成和丰度的关键生态因子。最后，将土壤N2O还原细菌群落组成和丰度与N2和N2O排放特征相结合，揭示N2O还原细菌群落组成和丰度与N2O汇效应的关系，最终阐明上述现象的土壤微生物机制。

太湖地区稻麦轮作土壤和华北平原玉米-小麦轮作土壤N2O产生的微生物过程不清，N2O还原细菌在典型农田土壤汇效应中的作用尚不明确。通过室内模拟试验研究了添加15N标记的尿素对30N2, 44N2O和46N2O排放的影响，明确了施肥等外源加入的无机氮大部分并没有直接用于生成N2O，而是刺激了土壤氮库的周转和其中的氮化合物转化为N2O；研究了不同pH和铜浓度对华北平原玉米-小麦轮作土壤和太湖地区稻麦轮作N2O排放的影响，明确了两种典型农田土壤N2O排放特征明显不同，玉米-小麦轮作土壤N2O排放量显著高于太湖地区稻麦轮作土壤，自然pH条件下农田土壤N2O排放量最高，中性pH则显著降低，可能pH条件有利于N2O还原细菌将N2O还原为N2；添加外源铜明显减少了稻麦轮作土壤N2O排放，但对小麦-玉米轮作土壤没有明显效果。通过连续2年的田间原位试验，明确了太湖地区三种利用方式（自然湿地、稻麦轮作和设施栽培蔬菜）土壤的N2O排放规律，即淹水还原状态有利于N2O还原为N2，例如自然湿地和稻麦轮作系统的水稻季N2O排放量均明显较低。进一步选取珠江三角洲典型双季稻田，通过连续2年的田间原位试验证明，无论是施用化肥还是有机肥，土壤N2O排放量很低甚至没有排放，验证了淹水水稻有利于土壤N2O汇的形成。接着从太湖地区稻麦轮作土壤和珠三角双季稻田土壤分离到数百株反硝化细菌，部分接种到有机肥和土壤可以减少N2O排放量，意味着可通过调控N2O还原细菌增强土壤N2O汇效应。本项目明确了外源氮添加与土壤N2O产生、还原的关系，阐明了pH和铜浓度对华北平原玉米-小麦轮作土壤和太湖地区稻麦轮作N2O排放的影响和规律，揭示了淹水水稻种植是典型农田土壤N2O汇效应形成的机理，初步结果显示N2O还原细菌Clade Ⅰ类群的Herbaspirillum、Azospirillum和Bradyrhizobium属与Clade Ⅱ类群的Geobacter和Anaeromyxobacter属的细菌与土壤N2O汇效应关系密切。这些结果为认识农田土壤N2O汇的形成规律和精准调控土壤N2O汇效应提供了关键科学基础。