Multilevel Nitrogen Additions Alter Chemical Composition and Turnover of the Labile Fraction Soil Organic Matter via Effects on Vegetation and Microorganisms

Abstract

AbstractGlobal 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.