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Soils in the Qinghai‐Tibet Plateau are young in terms of development and form a thin, skeletal soil layer because the gravel parent materials are glaciofluvial deposit, eluvium, and fluvial sediment, which are extremely sensitive to global climate change. The lack of understanding of soil organic matter (SOM) transformation processes in this region hinders the prediction of SOM stocks under future climate conditions. In this study, SOM transformation processes were investigated by density groupings and pyrolysis‐gas chromatography/tandem‐mass spectrometry (Py‐GC–MS/MS) in five alpine ecosystems: alpine desert, alpine grassland, alpine meadow, alpine wetland, and alpine forest. The roles of microbial community in different density fractions were elucidated. Soil samples were separated into three fractions with NaI solution: labile fraction (FL; ρ ≤ 1.6 g/cm3), moderate fraction (FM; 1.6 < ρ < 2.25 g/cm3), and recalcitrant fraction (FR; ρ ≥ 2.25 g/cm3). The following results were obtained. (1) The ratios of different fractions (FM/FL, FR/FM, and FR/FL) indicated the transformation processes of SOM, and ratios less than 1 represented degradation. The transformation processes were similar in alpine grassland and meadow and were similar in alpine desert, wetland, and forest but differed from each other. (2) Fungi preferred to degrade plant detritus, which mainly affected FL, whereas bacteria chiefly affected FM. (3) Precipitation altered vegetation type and soil pH, thus affecting communities and microorganism activities and resulting in the above differences. These findings highlight the importance of understanding the limited transformation processes of SOM in ecosystems to predict the impact of climate change on SOM preservation.

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Global nitrogen (N) deposition greatly impacts soil carbon sequestration. A 2-yr 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, 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, and between LF-SOM and net ecosystem exchange implied LF-SOM were 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 nonlinear 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. Key Points:• The LF-SOM quantity increased at the lowest N additions (N10 and N20) and decreased from N40 to N160, but the decrease was weakened at the highest N addition (N160) • Plant-derived compounds in LF-SOM were more sensitive to N addition than microbial-derived and aromatic compounds • The organic compounds in LF-SOM were significantly correlated with compound degradation-related genesSupporting Information:• Supporting Information S1 , et al. (2020). Multilevel nitrogen additions alter chemical composition and turnover of the labile fraction soil organic matter via effects on vegetation and microorganisms.

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