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Soil organic matter (SOM) is a combination of materials having different origin and with different stabilization and decomposition processes. To determine the different SOM pools and their turnover rates, a silt loam-textured Luvisol from West Hungary was taken from the 0–20 cm soil depth and incubated for 163 days. Maize residues were added to the soil in order to obtain natural 13C enrichment. Four different SOM fractions—particulate organic matter (POM), sand and stable aggregate (S + A), silt- plus clay-sized (s + c) and chemically resistant soil organic carbon (rSOC) fractions—were separated and analyzed using FT-IR, δ13C, and 14C measurements. The mean residence time (MRT) of the new C and the proportion of maize-derived C in the fractions were calculated. The POM fraction was found to be the most labile C pool, as shown by the easily decomposable chemical structures (e.g., aliphatic, O-alkyl, and polysaccharides), the highest proportion (11.7 ± 2.5%) of maize-derived C, and an MRT of 3.6 years. The results revealed that the most stable fraction was the rSOC fraction which had the smallest proportion of maize-derived C (0.18 ± 2.5%) and the highest MRT (250 years), while it was the only fraction with a negative value of Δ14C (−75.0 ± 2.4‰). Overall, the study confirmed the hypothesis that the SOM associated with finer-sized soil particles decomposes the least, highlighting the significance of the fractionation process for more accurate determination of the decomposition processes of SOM pools.

The volume of soil organic matter (SOM) changes, owing to variations in tillage systems. Conservation tillage (CT) is a useful method for recovering the SOM content of crop fields. However, little is known about the SOM composition of silt- and clay-associated and aggregate-occluded organic matter (OM). The present study aimed at determining the SOM compositions of various SOM fractions in the same Luvisol in a native forest and under ploughing and CT. SOM fractions (silt and clay associated; sand and aggregates associated; restricted OM) were characterized using diffuse reflectance Fourier transform infrared (FTIR) spectroscopy. The size of both the aggregate-occluded and resistant SOM pools increased, owing to the shift in the tillage system to CT for 15 years. As a general trend, the soil organic carbon content was inversely proportional to aromaticity under both crop fields, which supported the preferential mineralization of aliphatic components in each fraction. The shift in the tillage system could trigger rapid qualitative changes even in the stable restricted carbon pools; nevertheless, it was difficult to distinguish between the role of OM and the mineral composition in the FTIR spectra. In particular, the clay-related organic-mineral complexes could trigger difficulties in the traditional interpretation methods.