冻融对旱地黑土氧化亚氮排放的影响及其机制研究

Agricultural soil is an important source for atmospheric nitrous oxide (N2O) which is a potent greenhouse gas. The cultivated black soil located in Northeast China is vital for the national crop production. It has been demonstrated that N2O emissions from the black soil upland are low in normal precipitation years. However, extreme rainfall and snowfall events can induce large N2O emissions during the spring thaw period. Climate extremes are predicted to intensify accompanying with global warming, especially in the mid-high latitude of the North Hemisphere, including Northeast China. Therefore, it is likely that black soil will be a “hotspot” for N2O emission under future climate change. However, currently, the effects of freeze-thaw on N2O production processes, supplies of available carbon (C) and nitrogen (N) substrates, and the underlying microbial mechanisms are remaining unclear. In the present study, soil samples will be collected from typical black soil upland to conduct freeze-thaw simulation experiments under different moisture conditions. Soil extractable N, dissolved organic C (DOC), carbohydrate, etc. will be measured to explore the key factors affecting freeze-thaw N2O flux. Soil will be separated into different size aggregates through wet-sieving, the molecular structure of soil organic C and DOC will be assessed by nuclear magnetic resonance spectroscopy analysis, and soil amino sugars will also be analyzed to understand the effects of freeze-thaw on soil available organic C supplies and the underlying mechanisms. The gross rates of N transformation processes and the contributions of different processes to N2O production for soils exposed to freeze-thaw under various moisture conditions will be quantified in a 15N tracing study. The N-cycling genes will be targeted using quantitative real time PCR and high-throughput sequencing methods to evaluate the impacts of freeze-thaw on sizes and community structures of soil nitrifying and denitrifying microorganisms. The key microbial populations related to freeze-thaw induced N2O production will be identified. The objectives of this study are to understand the effects of freeze-thaw on black soil N2O emissions, C and N cycling processes, and soil microbial communities involved in N2O production so as to reveal the mechanisms leading to N2O emissions induced by freeze-thaw. This project will provide a scientific basis for evaluation and mitigation of N2O emissions from the cultivated black soils in Northeast China under future climate change.

东北地区黑土是我国重要的粮食生产基地。前期研究发现，在正常降水年份旱地黑土N2O排放较弱，但极端降水导致春季冻融期N2O大量排放。在极端气候事件不断加剧情景下，黑土可能成为N2O排放“热点”。然而，目前冻融诱导土壤N2O排放的机制尚不清楚。项目拟选取东北地区的典型旱地黑土，建立冻融模拟试验，研究冻融对不同水分含量黑土N2O排放的影响，量化冻融土壤含水量与N2O排放量之间的关系；利用团聚体分级、固体13C核磁共振和生物标志物技术，研究冻融对土壤有机质组分的影响，解析冻融过程中活性碳底物的含量变化和产生机制；建立15N同位素双标记的冻融模拟试验，研究土壤氮素转化过程，利用数值计算模型，明确土壤氮素初级转化速率和N2O产生途径；采用定量PCR和高通量测序等技术，研究硝化和反硝化微生物数量和群落结构，揭示冻融影响N2O产生的微生物机制，为评估和预测气候变化情景下黑土N2O排放及其减排措施的提出提供科学依据。

土壤是温室气体氧化亚氮的重要来源，位于中高纬度的季节性结冰黑土农田在冻融过程可能产生大量的氧化亚氮，但是其产生机制尚不明确。为研究黑土冻融期氧化亚氮的排放强度、产生机制以及减排措施，本研究通过三年原位试验，阐明了黑土春融期氧化亚氮排放的强度以及导致年际变化的主要因素；利用室内模拟试验，结合微生物磷脂脂肪酸分析、高通量测序、荧光定量PCR和氧化亚氮-氮-氧同位素方法探究了氧化亚氮产生的微生物过程；利用原位试验，评估了硝化抑制剂对黑土冻融氧化亚氮的减排潜势。结果发现，在丰雪年份春融期，氧化亚氮出现爆发式排放，背景排放量和化肥处理排放量是其他年份的172和62倍，春融期占全年总量的90%和76%。春融期氧化亚氮高峰排放出现在表层土壤融化时，积雪融化提高土壤含水量是导致氧化亚氮大量排放的直接诱因。室内模拟试验发现，冻融促进黑土微生物矿化作用释放更多铵态氮和活性有机碳底物，微生物的养分胁迫缓解；与之伴随的是，氨氧化微生物特别是氨氧化细菌AOB数量显著增加，但是反硝化细菌narG、nirK和nirS基因丰度先下降继而恢复。氧化亚氮同位素异构体技术证实融化后高水分黑土氧化亚氮的剧烈排放主要来自细菌反硝化过程。冻融促进微生物矿化-硝化-反硝化的密切偶联，导致高水分条件下氧化亚氮的剧烈排放，硝化作用是黑土冻融氧化亚氮通过反硝化作用产生的前提。两年原位试验表明，春融期一氧化氮与氧化亚氮排放量比值远小于1，进一步说明氧化亚氮主要产自反硝化过程。硝化抑制剂不能显著降低春融期氧化亚氮排放，在春融期氧化亚氮大量排放的年份，硝化抑制剂不能显著抑制全年氧化亚氮排放。因此，在冬季气候变化不断发展的情势下，黑土冻融氧化亚氮排放与削减仍需更多研究。