Warming and increased precipitation indirectly affect the composition and turnover of labile-fraction soil organic matter by directly affecting vegetation and microorganisms.

Global warming accompanied by precipitation changes impacts soil carbon sequestration. A three-year field manipulation experiment with warming (+2 °C above ambient temperature) and increased precipitation (+15% and +30% above ambient precipitation) was conducted in an alpine grassland to investigate the response of soil organic matter (SOM) to future climate change 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 organic compounds in LF-SOM were used as indicators to quantify the contributions of vegetation input and microbial degradation to LF-SOM transformation. Increased precipitation promoted LF-SOM accumulation, which were mainly due to the positive effect of increased precipitation on vegetation productivity. Plant-derived compounds in LF-SOM (including lignin, long-chain alkyl compounds, polysaccharides and phenols) were more sensitive to increasing soil moisture than microbial-derived (including short-chain alkyl compounds, N compounds and chitin) and aromatic-derived compounds (including aromatics and polyaromatics). In contrast, warming alone intensified the effect of drought on the alpine grassland, which had negative effects on both vegetation and microorganisms and reduced LF-SOM. Warming plus increased precipitation not only alleviated the water loss caused by warming but also increased soil temperature, which was more favorable for the growth of microorganisms. This was reflected in the increase in microbial-derived compounds in LF-SOM with increasing soil temperature, which contributed to LF-SOM degradation. Aromatic-derived compounds, as refractory compounds in soil, showed no significant response to either warming or increased precipitation treatments. Acidobacteria (approximately 25%) and Actinobacteria (approximately 20%), as the dominant soil bacterial communities in the alpine grassland, were significantly correlated with plant-derived compounds. At the same time, there were significant correlations between Proteobacteria and microbial-derived compounds, as well as between Firmicutes and aromatic-derived compounds (relative abundance). Under future climate change, microbial activity will increase as temperature increases, which will promote LF-SOM degradation only if precipitation also increases.