Effects of soil organic matter chemical quantity on carbon dioxide emissions in alpine grassland soils in Northern Tibet

The soil organic matter (SOM) represents the main terrestrial carbon pool and plays a key role in the carbon biogeochemical cycle. However, it is challenging to characterize the SOM at the molecule scale, due to its heterogeneous composition. To isolate the effects of the SOM composition on carbon dioxide emission, correlations were made between the SOM compositions and cumulative CO₂ emissions in two alpine grassland soils. The SOM compositions were investigated using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The results showed that: 1) the N-compounds were the main chemical components in the two types soils, with values of 38.0% and 52.5%; 2) there was a significant difference in the relative abundance of polysaccharides and terpenes between the two grassland types (P < 0.05); 3) polysaccharides affect the soil carbon dioxide emissions, the equation for which was Y = 86.76X + 344.87 (R² = 0.63, P < 0.05) (Y is the cumulative CO₂ emission, X is the relative abundance of polysaccharides); 4) the relative abundance of polysaccharides, N-compounds, and aromatics are significantly (P < 0.05) correlated with the bacteria, fungi, actinomycetes, gram-negative bacteria, and gram-positive bacteria. These results indicate that the SOM chemical composition of alpine grasslands is directly related to the soil microbial community and cumulative CO₂ emission, and the relative abundance of some compounds can be used to analyze the soil respiration under specific incubation conditions, which is important for understanding the characteristics of soil CO₂ emissions under different environmental conditions.