Atmospheric nitrogen (N) deposition has a significant impact on terrestrial N cycling. However, it has not been well understood how external inorganic N addition affects the dynamics of soil organic N (SON) that supports long-term plant productivity and carbon (C) sequestration. Here, we added six levels of ammonium (NH-N) or nitrate (NO-N) to a temperate grassland soil to reveal the mechanisms of SON responses to inorganic N addition. Results showed that the SON content decreased by 7% ± 2%–15% ± 1%, which corresponded with the increase in the abundance of functional gene related to SON mineralization (i.e., ) by both NH-N and NO-N. The inorganic N addition stimulated gaseous losses from the native soil N, leading to at least 6% ± 2%–12% ± 1% of the initial SON lost at different N addition levels (i.e., N-priming). However, the responses of nitrification- and denitrification-related functional genes to NH-N and NO-N treatments varied remarkably. The positive responses of nitrification- and denitrification-related functional genes (i.e., , , and ) to NO-N addition contributed to gaseous N losses. The negative responses of some denitrification-related functional genes (i.e., , , and ) to NH-N addition resulted in less gaseous N losses by NH-N compared with that by NO-N addition. Additionally, the release of H by NH-N addition may liberate organic matter from associated minerals, showing a decrease in mineral-associated organic matter (MAOM) but an increase in extractable Fe. These findings provide direct evidence that the addition of inorganic N results in a net native SON loss through microbe-mediated mineralization and gaseous emissions. The study also emphasizes the different mechanisms of reduced and oxidized N additions in affecting SON mineralization and gaseous N losses.

中文翻译：

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由于微生物矿化以及添加铵和硝酸盐引起的气体排放，导致与天然矿物相关的有机氮损失

大气氮（N）沉降对陆地氮循环有重大影响。然而，人们尚未充分了解外部无机氮添加如何影响支持植物长期生产力和碳（C）固存的土壤有机氮（SON）动态。在这里，我们在温带草原土壤中添加了六种水平的铵 (NH-N) 或硝酸盐 (NO-N)，以揭示 SON 对无机氮添加的响应机制。结果表明，SON含量下降了7%±2%~15%±1%，这与NH-N和NO-N导致SON矿化相关的功能基因（即 ）丰度增加相对应。无机氮的添加刺激了原生土壤氮的气态损失，导致在不同的氮添加水平（即氮引发）下至​​少有 6% ± 2%–12% ± 1% 的初始 SON 损失。然而，硝化和反硝化相关功能基因对NH-N和NO-N处理的反应存在显着差异。硝化和反硝化相关功能基因（即 、 和 ）对 NO-N 添加的积极反应导致气态氮损失。一些反硝化相关功能基因（即 、 和 ）对 NH-N 添加的负面反应导致 NH-N 导致的气态 N 损失少于 NO-N 添加的气态 N 损失。此外，通过添加 NH-N 释放的 H 可能会从伴生矿物中释放出有机物，这表明矿物伴生有机物 (MAOM) 减少，但可萃取铁增加。这些发现提供了直接证据，表明添加无机氮会通过微生物介导的矿化和气体排放导致天然 SON 净损失。该研究还强调了还原氮和氧化氮添加影响 SON 矿化和气态氮损失的不同机制。