Chemometric assessment of soil organic matter storage and quality from humic acid infrared spectra

The knowledge of biogeochemical mechanisms involved in soil organic carbon (SOC) storage is crucial to control its release to the atmosphere. In particular, the chemical composition of soil organic matter (SOM) plays an important role in the performance of the C storage and resilience in soils. The structural information provided by infrared spectroscopy (IR) of soil humic acid (HA) was used in the assessment of the C storage potential of 35 Spanish soils. Partial least squares (PLS) regression using the intensities of the points of the IR spectra of the HAs (4000–400 cm−1) as descriptors shows that a relationship exists between IR spectral pattern and the SOC content. This was also the case for E4 (humification index based on HA optical density at 465 nm). In addition, the chemical characteristics of the HAs correlated with the SOC levels were identified from digital data treatments of the IR spectra. Additional application of principal component analysis (PCA) and multidimensional scaling (MDS) suggested that bands assigned to carboxyl and amide structures were characteristic in HAs from soils with low C content, whereas HA spectra from soils with high C levels showed a conspicuous band pattern suggesting structural units of lignin from slightly transformed plant residues. The spectral profiles were analyzed in detail by an approach based on digital subtraction of IR spectra obtained by averaging those from HAs extracted from soils in the upper and lower quartiles of the SOC distribution. The results showed that significant relationships exist between the molecular composition of HAs and SOC levels and E4 values in a way in which aromatic, carboxyl and amide groups were predominant in HAs from soils with low SOC content, whereas lignin-derived structures were more characteristic of HAs from soils with high SOC content.