Soil organic matter qualification with FTIR spectroscopy under different soil types in Estonia

Soil organic matter (SOM) is a heterogeneous material consisting of different functional groups depending on soil texture and moisture regime. The lability of SOM (rapidity of turnover rate) is also an important indicator in soil quality evaluation. In this study, SOM lability (p-WEOC) was assessed based on the proportion of total soil organic carbon (SOC) represented by water-extractable organic carbon (WEOC) for a wide range of Estonian arable soils. We applied Fourier-transform infrared spectroscopy (FTIR) to examine the SOM relationships between qualitative (functional groups, SOM lability (p-WEOC), hydrophobicity (HI), degree of degradation (DDI), and water repellency (W)) and quantitative (total SOC and WEOC concentration) parameters depending on soil texture and moisture regime. Gleysols had the highest SOC and WEOC concentrations and p-WEOC was typically highest in automorphic sandy soils and lowest in sandy Gleysols. Overall, p-WEOC values for Gleysols were lower than those for automorphic soils. SOC and WEOC concentrations were positively correlated with all measured absorbance peaks and p-WEOC was negatively correlated with some peaks but positively correlated with DDI. HI and W were highest in Gleysols; however, sandy and automorphic soils had higher DDI. In general, automorphic soils had higher lability and the SOM was more degraded than wet soils. In summary, it is possible to assess and compare SOM composition in different soil types with FTIR spectroscopy, although eliminating inorganic material from soil will provide more accurate results.