Short-lived pulses of soil nitrous oxide (N2O) emissions during freeze-thaw periods can dominate annual cumulative N2O fluxes from temperate managed and natural soils. However, the effects of freeze thaw cycles (FTCs) on dinitrogen (N2) emissions, i.e., the dominant terminal product of the denitrification process, and ratios of N2/N2O emissions have remained largely unknown because methodological difficulties were so far hampering detailed studies. Here, we quantified both N2 and N2O emissions of montane grassland soils exposed to three subsequent FTCs under two different soil moisture levels (40 and 80% WFPS) and under manure addition at 80% WFPS. In addition, we also quantified abundance and expression of functional genes involved in nitrification and denitrification to better understand microbial drivers of gaseous N losses. Our study shows that each freeze thaw cycle was associated with pulse emissions of both N2O and N2, with soil N2 emissions exceeding N2O emissions by a factor of 5–30. Increasing soil moisture from 40 to 80% WFPS and addition of cow slurry increased the cumulative FTC N2 emissions by 102% and 77%, respectively. For N2O, increasing soil moisture from 40 to 80% WFPS and addition of slurry increased the cumulative emissions by 147% and 42%, respectively. Denitrification gene cnorB and nosZ clade I transcript levels showed high explanatory power for N2O and N2 emissions, thereby reflecting both N gas flux dynamics due to FTC and effects of different water availability and fertilizer addition. In agreement with several other studies for various ecosystems, we show here for mountainous grassland soils that pulse emissions of N2O were observed during freeze-thaw. More importantly, this study shows that the freeze-thaw N2 pulse emissions strongly exceeded those of N2O in magnitude, which indicates that N2 emissions during FTCs could represent an important N loss pathway within the grassland N mass balances. However, their actual significance needs to be assessed under field conditions using intact plant-soil systems.

中文翻译：

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山地草原土壤冻融循环过程中的二氮 (N2) 脉冲排放

冻融期间土壤一氧化二氮 (N2O) 排放的短寿命脉冲可以支配来自温带管理和天然土壤的年度累积 N2O 通量。然而，冻融循环 (FTC) 对二氮 (N2) 排放（即反硝化过程的主要终端产品）和 N2/N2O 排放比率的影响在很大程度上仍然未知，因为迄今为止方法学上的困难阻碍了详细研究。在这里，我们量化了在两种不同土壤湿度水平（40% 和 80% WFPS）下以及在 80% WFPS 的肥料添加下暴露于三个后续 FTC 的山地草原土壤的 N2 和 N2O 排放。此外，我们还量化了参与硝化和反硝化的功能基因的丰度和表达，以更好地了解气态 N 损失的微生物驱动因素。我们的研究表明，每个冻融循环都与 N2O 和 N2 的脉冲排放相关，土壤 N2 排放量超过 N2O 排放量的 5-30 倍。将土壤水分从 40% 的 WFPS 增加到 80% 以及添加牛粪使 FTC N2 的累积排放量分别增加了 102% 和 77%。对于 N2O，将土壤水分从 40% WFPS 增加到 80% WFPS 和添加泥浆使累积排放量分别增加了 147% 和 42%。反硝化基因 cnorB 和 nosZ 进化枝 I 转录水平显示出对 N2O 和 N2 排放的高解释力，从而反映了由于 FTC 引起的 N 气体通量动态以及不同可用水量和肥料添加的影响。与针对各种生态系统的其他几项研究一致，我们在这里展示了在冻融过程中观察到 N2O 脉冲排放的山地草原土壤。更重要的是，这项研究表明，冻融 N2 脉冲排放量在数量上大大超过 N2O，这表明 FTC 期间的 N2 排放可能代表草原 N 质量平衡内的重要 N 损失途径。然而，它们的实际意义需要在田间条件下使用完整的植物-土壤系统进行评估。