Use of specific peaks obtained by diffuse reflectance Fourier transform mid‐infrared spectroscopy to study the composition of organic matter in a Haplic Chernozem

This study assessed specific peaks obtained by diffuse reflectance Fourier transform mid‐infrared spectroscopy (DRIFTS) for characterizing the soil organic matter (SOM) composition of a Haplic Chernozem. Soils were collected from the Static Fertilization Experiment, Bad Lauchstädt, Germany, during 5 years from the farmyard manure (FYM), mineral fertilizer (NPK), combination (FYM + NPK) and no fertilizer (Control) treatments. Soils were extracted with hot water (HWE), and fractionated by size and density. Bulk soil and fractions were analysed by DRIFTS. Peak areas at 2930, 1620, 1530 and 1159 cm^?1 were selected as a range of organic functional groups (with limited mineral interference), integrated with a local baseline (corrected peak area) and each was divided by the summed area of the four peaks (relative peak area). Positive correlations between carbon (C) in fractions representing labile OM (<1.8 g cm^?3, 1.8–2.0 g cm^?3, C_HWE) and the corrected peak area at 2930 cm^?1 (3010–2800 cm^?1) in the bulk soil indicated that this aliphatic peak corresponded to the more labile C compounds. Negative correlations between the same fractions and the corrected area of the predominantly aromatic peak at 1620 cm^?1 (1660–1580 cm^?1) in the bulk soil suggested a relationship with more stable SOM compounds. All relative peak areas were significantly affected by fertilizer treatment, with an increasing relative peak area at 2930 cm^?1 in FYM compared with non‐FYM treatments. The ratio of the peaks at 1620 and 2930 cm^?1 was positively correlated with the ratio of stable C (sum of C in >1.8 g cm^?3 and clay fractions) to labile C (C content of <1.8 g cm^?3 fraction) and thus taken as an indicator of SOM stability. The DRIFTS peak area method reflected changes in SOM quality and composition under long‐term management as measured by size and density fractionation, indicating heterogeneous chemical composition of the latter. Further, the DRIFTS analysis of undiluted soil samples can be used to assess SOM composition in small sample sets if specular reflection and mineral interferences are considered.