Abstract
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.
Highlights
Soil organic matter (SOM) plays an important role in the functioning of land ecosystems
The total C and N concentrations in the different fractions are presented in Table 1, which shows that the particulate organic matter (POM) fraction had the highest and the resistant soil organic carbon (rSOC) fraction the smallest C and N concentrations in the native, amended, and control samples
The fact that C/N ratios decreased from the POM to the s + c fraction (Table 1) is consistent with the literature [42,43,44] showing that the C/N ratio decreased as the density of the fractions increased
Summary
Soil organic matter (SOM) plays an important role in the functioning of land ecosystems. It regulates many physical, chemical, and biological functions and processes in the soil. Chemical, and biological functions and processes in the soil It maintains the soil structure and porosity by aggregate formation and stabilization, increasing the water and air supply. At the same time SOM provides a habitat and nutrient source for microorganisms It enhances the buffer capacity of the soil over a wide range of pH values due to the wide variety of functional groups and organic compounds (such as carboxylic, hydroxy, amine, and amide) in SOM [1]. The residence time (the time the C atom spends in the soil system) of root exudates is a few hours, whereas plant residues decompose within a few weeks or months, while certain organic compounds may remain untouched for even ten thousand years [3,4,5]
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