## 3 Results and Discussion
### Effects of Treatments on the Spectroscopic Properties of Each Soil Fraction  
Figure 2 shows the DRIFT spectra of different soil fractions under various tillage. The peak at about 3623 cm\({}^{-1}\) can be ascribed to the O-H vibration in the octahedral layers of 2:1 and/or 1:1 silicates [36]. The same peak can be highlighted by removing the SOM through an appropriate procedure [37]. This peak was observed in B, C, and D fractions regardless of the type of tillage, with a slight increase of the relative intensity as the fraction size decreased. This was in line with other papers [38; 39; 40; 41] showing that phyllosilicates (kaolinite, chlorite, smectite, illite) are the main components of the clay fraction of soils. The peak at 2927 cm\({}^{-1}\), ascribed to the stretching of the aliphatic C-H group, was evident only in the A fraction, and could be due to the signal of organic matter consisting of labile plant residues [42]. Additionally, in this case, this peak can be highlighted by removing minerals through HF treatment [43]. The peak at about 2517 cm\({}^{-1}\) can be attributed to the CO\({}_{3}\) stretching and calcite bending, as indicated by peaks at about 1450, 867, and 698 cm\({}^{-1}\)[15; 44]. These peaks, especially those at 1450 and 867 cm\({}^{-1}\), were more pronounced in the spectrum of the A fraction. High percentage of calcite in the sand-sized fraction of a Mediterranean soil was also found in a previous work [3] and was ascribed to its lithogenic origin. The peaks at about 1991, 1868, 1793, and 698 cm\({}^{-1}\) were related to Si-O bending of quartz minerals [15; 43; 44] as well as the peaks at about 791 cm\({}^{-1}\) related to the Si-O stretching of quartz minerals [15; 43]. Overall, these peaks were slightly more pronounced in all fractions B and C possibly because of the physical breakup of sand size quartz into silt dimension. In contrast, the reduced intensity of the same peaks in D fractions suggested a limited physical alteration of quartz minerals in the finest particles and, therefore, an intermediate stage of soil evolution [45]. The peak at about 1630 cm\({}^{-1}\) was ascribed to aromatic C=C skeletal vibrations, C=O stretching of quinone and amide groups, C=O of H-bonded conjugated ketones, and it was typical of the organic components [46]. As expected, this peak was absent in the A fraction, appeared in the B fraction, and became more pronounced from fraction C to D. \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Organic Carbon (mg kg\({}^{-1}\))**} \\ \hline Depth & *** \\ Tillage & n.s. \\ Fertilization & n.s. \\ Size & *** \\ Depth (cm) & \\
0–30 & 125.9 c (10.9) \\
30–60 & 89.8 b (7.8) \\
60–90 & 54.5 a (4.7) \\ Tillage & \\ NT & 94.9 a (8.2) \\ MT & 81.8 a (7.1) \\ CT & 93.5 a (8.1) \\ Fertilization & \\ No & 92.8 a (6.6) \\ Yes & 87.3 a (6.2) \\ Size & \\ A & 7.7 a (0.8) \\ B & 22.0 ab (2.2) \\ C & 54.8 b (5.5) \\ D & 275.7 c (27.6) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Analysis of variance and mean values of the OC in soil fractions, subdivided by soil depth, tillage, fertilization treatment, and size of fractions. The standard deviation is in parentheses. The presence of the peak linked to aromatic structures in the fraction D could be due to the presence of organic matter involved in the formation of organo-mineral associations and in the coating of the mineral surface by sorption or precipitation processes [47; 48; 49]. The broad band at about 1030 cm\({}^{-1}\) can be ascribed to the stretching of the carbohydrate and polysaccharides-like substances [50]. It was found mainly in the silt-clay and free fine silt plus clay fractions (fraction C and D). Many previous studies have reported high proportions of these compounds in the mineral-associated organic matter fraction [5; 51; 52; 53; 54]. Polysaccharides of microbial origin mainly bind clay particles by promoting the formation of microaggregates of \(<\)50 \(\upmu\)m [55].