Global nitrogen (N) deposition greatly impacts soil carbon sequestration. A two‐year multiple N addition (0, 10, 20, 40, 80 and 160 kg N⋅ha^‐1⋅yr^‐1) experiment was conducted in alpine grassland to illustrate the mechanisms underlying the observed soil organic matter (SOM) dynamics on the Qinghai‐Tibet Plateau (QTP). Labile fraction SOM (LF‐SOM) fingerprints were characterized by pyrolysis‐gas chromatography/tandem‐mass spectrometry (Py‐GC‐MS/MS), and microbial functional genes (GeoChip 4.6) were analyzed in conjunction with LF‐SOM fingerprints to decipher the responses of LF‐SOM transformation to N additions. The significant correlations between LF‐SOM and microbial biomass, between organic compounds in LF‐SOM and compound degradation‐related genes, as well as between LF‐SOM and net ecosystem exchange (NEE) implied LF‐SOM was the main fraction utilized by microorganisms and the most sensitive fraction to N additions. The LF‐SOM increased at the lowest N addition levels (10 and 20 kg N⋅ha^‐1⋅yr^‐1) and decreased at higher N addition levels (40 to 160 kg N⋅ha^‐1⋅yr^‐1), but the decrease of LF‐SOM was weakened at 160 kg N⋅ha^‐1⋅yr^‐1 addition. The non‐linear response of LF‐SOM to N additions was due to the mass balance between plant inputs and microbial degradation. Plant‐derived compounds in LF‐SOM were more sensitive to N addition than microbial‐derived and aromatic compounds. It is predicted that when the N deposition rate increased by 10 kg N⋅ha^‐1⋅yr^‐1 on the QTP, carbon sequestration in the labile fraction may increase by nearly 170% compared with that under the current N deposition rate. These findings provide insight into future N deposition impacts on LF‐SOM preservation on the QTP.

中文翻译：

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多级氮添加通过对植被和微生物的影响，改变了不稳定组分土壤有机物的化学组成和转换

全球氮（N）的沉积极大地影响了土壤碳固存。两年多次添加N（0、10、20、40、80和160 kg N·ha ^‐1· yr ^-1）实验是在高寒草原上进行的，目的是说明在青藏高原（QTP）上观测到的土壤有机质（SOM）动态的潜在机制。通过热解-气相色谱/串联质谱（Py-GC-MS / MS）表征不稳定级分SOM（LF-SOM）指纹，并结合LF-SOM指纹分析微生物功能基因（GeoChip 4.6）进行解密LF-SOM转换对N个添加的响应。LF-SOM和微生物生物量之间，LF-SOM中的有机化合物与化合物降解相关基因之间以及LF-SOM和净生态系统交换（NEE）之间的显着相关性暗示LF-SOM是微生物利用的主要部分对N的添加最敏感的分数。LF-SOM以最低的氮添加量增加（10和20 kg N·ha^-1 ⋅yr ^-1），并在高N的添加水平（40至160公斤N⋅ha降低^-1 ⋅yr ^-1），但LF-SOM的降低在160千克N⋅ha减弱^-1 ⋅yr ^{- 1}个。LF-SOM对添加N的非线性响应是由于植物投入和微生物降解之间的质量平衡所致。LF-SOM中植物衍生的化合物比微生物衍生的和芳香族化合物对氮的添加更敏感。据预测，当氮沉积速率增加10 kg N·ha ^-1· yr ^-1时在QTP上，与当前氮沉降速率下相比，不稳定部分的碳固存可能增加近170％。这些发现提供了对未来氮沉积对QTP LF-SOM保存的影响的见解。