Supplementary original data

The  ongoing climate change is predicted to induce more weather extremes  such as frequent drought and high-intensity precipitation events,  causing more severe drying-rewetting cycles in soil. However, it remains  largely unknown how these changes will affect soil nitrogen (N)-cycling  microbes and the emissions of potent greenhouse gas nitrous oxide (N₂O).  Utilizing a field precipitation manipulation in a semi-arid grassland  on the Loess Plateau, we examined how precipitation reduction (ca. −30%)  influenced soil N₂O and carbon dioxide (CO₂)  emissions in field, and in a complementary lab-incubation with simulated  drying-rewetting cycles. Results obtained showed that precipitation  reduction stimulated plant root turnover and N-cycling processes,  enhancing soil N₂O and CO₂ emissions in field, particularly after each rainfall event. Also, high-resolution isotopic analyses revealed that field soil N₂O  emissions primarily originated from nitrification process. The  incubation experiment further showed that in field soils under  precipitation reduction, drying-rewetting stimulated N mineralization  and ammonia-oxidizing bacteria in favor of genera <em>Nitrosospira</em> and <em>Nitrosovibrio</em>, increasing nitrification and N₂O  emissions. These findings suggest that moderate precipitation  reduction, accompanied with changes in drying-rewetting cycles under  future precipitation scenarios, may enhance N cycling processes and soil  N₂O emissions in semi-arid ecosystems, feeding positively back to the ongoing climate change.

当前气候变化预计将诱发更多极端天气事件，例如频发的干旱与高强度降水过程，进而加剧土壤干湿循环的剧烈程度。然而，目前学界尚未明确此类变化将如何影响土壤氮（N）循环微生物，以及强效温室气体一氧化二氮（nitrous oxide, N₂O）的排放。本研究依托黄土高原（Loess Plateau）半干旱草原（semi-arid grassland）的野外降水调控实验（field precipitation manipulation），探究了约30%降水减量对野外土壤一氧化二氮与二氧化碳（carbon dioxide, CO₂）排放的影响，并辅以模拟干湿循环（simulated drying-rewetting cycles）的室内培养实验（lab-incubation）开展补充验证。研究结果显示，降水减量促进了植物根系周转（plant root turnover）与氮循环过程（N-cycling processes），进而提升了野外土壤的一氧化二氮与二氧化碳排放，尤其在每次降雨事件后更为显著。此外，高分辨率同位素分析（high-resolution isotopic analyses）表明，野外土壤的一氧化二氮排放主要源自硝化过程（nitrification process）。室内培养实验进一步发现，在经降水减量处理的野外土壤中，干湿循环刺激了氮矿化（N mineralization）过程，同时使氨氧化细菌（ammonia-oxidizing bacteria）的群落结构偏向亚硝化螺菌属（Nitrosospira）与亚硝化弧菌属（Nitrosovibrio），进而强化了硝化作用与一氧化二氮排放。上述研究结果表明，未来降水格局变化下，适度的降水减量配合干湿循环的改变，可能会强化半干旱生态系统（semi-arid ecosystems）的氮循环过程与土壤一氧化二氮排放，进而对当前气候变化形成正反馈（positive feedback）效应。