Inclusion of clover in grasslands increases functional diversity, N yield and forage quality and has been advocated for mitigating nitrous oxide (N₂O) emissions. However, boreal grass-clover leys often show poor winter survival with considerable aboveground losses of nitrogen (N) and carbon (C). Little is known about how these losses affect off-season N₂O emissions. Here we report field experiments over two winters, conducted at two coastal locations in Western and Northern Norway. N₂O emissions were measured in plots with 0, 30 and 100% red (T. pretense) and white clover (T. repens) in a timothy - meadow fescue mixture. Overwinter N loss from the sward was quantified by comparing N contents in roots, stubble and herbage in autumn and spring. Additional treatments were removal of above-ground biomass in autumn and soil compaction. Off-season N₂O emissions correlated positively with estimated overwinter N loss from herbage, which in turn depended on the fraction of clover in the ley. Pure grass leys emitted less N₂O than leys that contained clover. Corrected for background emissions from pure grass, up to 13% of the above-ground N loss was emitted as N₂O–N when clover was grown in pure stand. This fraction was much smaller, however, when clover was grown in mixture with grass (1.9 ± 0.9%), suggesting reassimilation of inorganic N. Indeed, we found significant increases in root and stubble N in mixtures throughout winter. Removal of above-ground biomass in autumn appeared to reduce the sward's ability to retain N throughout winter, and hence had no or a stimulating effect on N₂O emissions. Soil compaction increased off-season N₂O emissions 1.3–1.6-fold. Our results show that boreal grass-clover leys can be a significant source of N₂O during winter, intricately controlled by loss and reassimilation of N. This underscores the importance of off-season plant-soil management for reducing the greenhouse gas (GHG) footprint of animal production in high latitude ecosystems.

在草原中种植三叶草可提高功能多样性、氮产量和草料质量，并被提倡用于减少一氧化二氮 (N ₂ O) 排放。然而，北方的三叶草通常表现出较差的冬季存活率，地上的氮 (N) 和碳 (C) 损失相当大。人们对这些损失如何影响淡季 N ₂ O 排放知之甚少。在这里，我们报告了在挪威西部和北部两个沿海地区进行的两个冬天的现场实验。N ₂ O 排放量在具有0、30和 100% 红色（假装的 T. pretense）和白三叶草（T. repens) 在蒂莫西 - 草甸羊茅混合物中。通过比较秋季和春季根、茬和牧草中的 N 含量，量化草地越冬 N 损失。额外的处理是在秋季去除地上生物量和土壤压实。淡季 N ₂ O 排放量与估计的越冬 N 损失呈正相关，而后者又取决于麦秆中三叶草的比例。与含有三叶草的麦草相比，纯草麦草排放的 N ₂ O更少。校正纯草的背景排放，高达 13% 的地上 N 损失以 N _{2 形式}排放当三叶草在纯林中生长时，O-N。然而，当三叶草与草混合生长时，这一比例要小得多 (1.9 ± 0.9%)，表明无机 N 的重新同化。 事实上，我们发现整个冬季混合物中根和残茬 N 的显着增加。秋季去除地上生物量似乎会降低草地在整个冬季保留 N 的能力，因此对 N ₂ O 排放没有影响或有刺激作用。土壤压实使淡季 N ₂ O 排放量增加了1.3-1.6 倍。我们的结果表明，北方的三叶草是 N ₂的重要来源冬季的 O 受到 N 的损失和再同化的复杂控制。这强调了淡季植物土壤管理对于减少高纬度生态系统动物生产温室气体 (GHG) 足迹的重要性。