增温和积雪对天山高寒草原N2O排放的影响及其机理研究

Nitrous oxide (N2O) is one of atmospheric greenhouse gases, which is a research hotspot in field of atmospheric and environmental science with global change and human activity. Although most research about N2O emission during growth season were reported in grassland, there is less study outside growth season, especially in the soil freeze-thaw cycle. While no pulse emissions of N2O were found at grazing management sites and nitrogen addition sites during the soil freeze-thaw cycle in the alpine grassland of Tianshan mountains, low temperature and soil water content may suppress the N2O emissions. Soil temperature and soil water content could effect on the N2O emissions with the increasing cold-season warming and snow depth change in semi-arid regions of central Asia. However, N2O emissions response to warming and snowfall are less understood especially in the alpine ecosystem. This project was conducted in the alpine grassland of Tianshan Mountains in China. Temperature free-air controlled enhancement system and snowfall controlled experiments were conducted, and monitoring edaphic factors, microbial factors, and atmospheric temperature, snowfall and rain. Explaining and confirming the response of N2O emissions to warming and snowfall change in alpine grassland. This study is important to assess N2O emissions in grassland ecosystem under global change and forecast greenhouse gas emissions in the alpine ecosystem.

N2O是温室气体的重要成分之一，其对全球变化和人为活动的响应已成为大气和环境科学研究的热点议题。草原生态系统生长季节的N2O排放已有较多报道，然而非生长季尤其是春季冻融交替阶段的研究相对较少。新疆天山高寒草原春季冻融交替阶段的N2O排放未发现显著增加，可能受到相对低温和土壤水分的共同限制。伴随中亚干旱区冷季增温迅速和积雪变化，土壤温度和水分改变可能影响N2O排放。本研究拟在新疆天山巴音布鲁克高寒草原开展增温和积雪控制实验，长期定位观测N2O排放，确定年际动态、季节动态（春季冻融交替、生长季、秋冬交替、冬季阶段）和日动态排放的响应规律。从土壤环境、微生物和气候要素思考，解析高寒草原N2O排放对增温和积雪的响应规律以及生物和非生物因素的调控机理。本研究有助于评价全球变化背景下草原生态系统N2O排放的不确定性，对预测高寒地区温室气体排放具有重要意义。

高寒干旱生态系统对气候变暖和降水变化具有较高的敏感性，以往研究较少关注同时受温度和降水胁迫的天山高寒草原这一脆弱生态系统，对气候变化的响应及其机理。本研究以长期围封的紫花针茅（Stipa purpurata）群落为研究对象，开展了远红外模拟增温和降雨变化（生长季雨水增加、非生长季积雪变化）控制试验，分析了气候变化对新疆天山高寒草原的N2O等主要温室气体通量的影响。结果如下：（1）气候变暖和雨水增加共同作用抑制了N2O排放，年累积排放量为0.13 kg N ha-1，而单独增加降水或增加温度都不会显著影响N2O排放。增加降水均显著抑制了CH4吸收，而气候变暖影响不显著。未来的暖湿环境显著增加了23.18%的年累积生态系统呼吸量。非生长季加倍增雪显著促进CO2排放和CH4吸收，促进效应分别达34.3%和15.9%，但对N2O排放促进作用不显著。通过分析净GWP发现，降雨增加和气候变暖共同作用显著促进了净GWP，对气候变暖表现出正反馈作用。（2）相比冻融季（FTP），非生长季期间的非冷冻季（NFS）有较长的持续天数，同时NFS相比冷冻季（FS）有较高的排放速率，均高于非生长季其他两个时期的比例。N2O非生长季累积排放量具有较大的年际变异性，可能与N2O本身排放量较低有关，非生长季的低温更降低了N2O排放，而且其本身具有较大的不稳定性。非生长季N2O排放占比，以冷冻季最高，其次是非冷冻季，冻融季占比依然同另外两种温室气体一致，均较低。（3）结构方程模型分析发现土壤湿度是影响生态系统呼吸和甲烷吸收的主要因素，解释率分别达73%和55%。另外，降雨增加对与碳循环相关的纤维素酶活性的影响与其对生态系统呼吸的影响具有显著的正相关关系，而对N2O排放的作用同对木质素酶的作用存在显著的正相关关系。本研究初步明晰了气候变暖、降水变化等气候变化因子对温室气体排放的影响及其影响机理。在全球变化背景下，开展长期、多水平驱动因子对天山高寒草原温室气体排放影响的试验，可为深入解析天山高寒草原温室气体排放的响应规律和机理提供科学依据。