How does soil organic matter stabilize with soil and environmental variables along a black soil belt in Northeast China? An explanation using FTIR spectroscopy data

Chemical composition of soil organic matter (SOM) is an important determinant of SOM stabilization and soil function. Better understanding of the chemical composition of SOM is vital for assessing soil formation processes and its controlling factors. Fourier transform infrared (FTIR) spectroscopy was applied to characterize the chemical compositions of SOM for 234 surface soils collected from a representative black soil belt in Northeastern China. The relationships of SOM chemical composition with soil and environmental variables were assessed. Soil samples were divided into eight regional groups and four soil organic carbon (SOC) range groups according to the administrative division and SOC contents, respectively. Generally, SOC contents varied from 8.2 to 34.5 g kg−1, together with the relative abundances of aliphatic-C increased from the south to the north along the black soil belt, while the relative abundance of aromatic-C decreased. Total nitrogen (TN), relative abundance of polysaccharide-C, and minerals showed no obvious zonal changes. In four SOC range groups, averaged SOC, TN, and aliphatic-C increased significantly with increasing SOC contents, while the aromatic-C decreased (P < 0.05). The polysaccharide-C and minerals also increased. Correlation analysis showed that SOC was positively related with aliphatic-C, and negatively related with aromatic-C regardless of regions and SOC contents (P < 0.05). Principal component analysis demonstrated obvious separations among soils divided into four SOC ranges compared to those divided into eight regions. SOC, TN, and aliphatic-C pointing to SOC > 20.0 g kg−1 groups, and aromatic-C pointing to SOC < 15.0 g kg−1 groups were the main driving factors via PC1. The aromatic-C/aliphatic-C ratio representing SOM decomposition degree, mean air temperature, and precipitation pointing to SOC < 10.0 g kg−1 groups, and minerals were the main driving factors via PC2. Our results contribute to a further understanding of spatial variations in SOM chemical compositions regarding soil formation and SOC stabilization caused by environmental conditions and agricultural managements.