Composition and mean residence time of molecular weight fractions of organic matter extracted from two soils under different forest species

The organic matter extracted from various mineral horizons of two forest soils, one under silver fir (Abies alba Mill.), the other under European beech (Fagus sylvatica L.), was fractionated by dialysis into three fractions, 100–1000, 1000–8000, and >8000 Da. On a C basis, in all horizons the recovered organic matter amounted to less than a half of the total and was mainly composed of molecules >8000 Da. The 100–1000 Da fraction had a principal elemental composition profoundly different from the other two fractions, which, instead differed from each other significantly only for the S content and the molar ratio of C with N. No significant difference in this regard was found between soils. The richness in O and some typical absorption bands in the FT-IR spectra indicated that the 100–1000 Da fraction had a lot of carboxyl moieties. The spectroscopic (13C NMR) investigation showed that the 1000–8000 and >8000 Da fractions had a prevalently aliphatic nature and signals attributable to polysaccharides (O-alkyl C) revealed overall a high presence of non-humic biopolymers. These latter were significantly more abundant, suggesting a lower degree of humification, in the >8000 Da fraction than in the 1000–8000 Da fraction. Comparing soils, that under beech appeared significantly richer in O-alkyl C than that under fir. The organics extracted from the A horizon of both soils had positive Δ14C values, indicating recent synthesis mainly due to the present forest cover. The mean residence time (MRT) of the combined 100–1000 Da and 1000–8000 Da fractions and the >8000 Da fraction increased with depth, even to about 5000 years in the more than 1-m deep BC horizons under beech. In some cases, and especially in the soil under fir, despite higher values of δ13C denoting stronger microbial decomposition, the 100–8000 Da fraction showed a higher MRT than that of the >8000 Da fraction, perhaps due to its ascertained lower content of non-humic biopolymers.