## 3 Results and Discussion  
### Effects of Treatments on the Amount of Each Soil Fraction and on their Organic Carbon Content  
The physical fractionation recovered 98% of the mass and 99% of the OC from all samples. Such percentages were comparable to those previously reported by other authors [26; 27; 28], which indicates that the loss of material was very low and confirms the efficiency of the fractionation method adopted. Table 1 shows the amounts of soil dry matter obtained from each fraction (g kg\({}^{-1}\) soil), while Table 2 reports the corresponding statistical analyses, as influenced by treatments (soil depth, tillage, and fertilization). On average, 70% to 75% of the soil fractions consisted of fine silt and clay (<20 \(\upmu\)m). The A, B, and C fractions decreased with depth (Table 1), while the D fraction showed an inverse trend with the only exception being the deepest layer of NTF treatment. The quantity of the smallest fraction (<20 \(\upmu\)m) increased with depth and ranged from 500 to 867 mg kg\({}^{-1}\). \begin{table} \  
The fertilized plots resulted in the highest amount of B fraction and the lowest amount of D fraction with respect to the unfertilized ones. Since microaggregates are the result of microbial decomposition of SOM from the macroaggregates [9; 29], the highest amount of B fraction in fertilized soils could derive from the higher microbial activity promoted by the same fertilization. For example, [PERSON] et al. [30] found the higher fungal abundance in microaggregates (250-53 \(\upmu\)m) regardless of the type of fertilization. With regard to the tillage, the highest amount of D fraction and the lowest amount of B fractions were obtained from CT soils, which is likely due to the major physical disturbance and microbiological activity induced by the conventional tillage that increased macro-aggregates and micro-aggregates turnover [5]. Table 3 shows the analysis of variance and mean values of OC content of soil fractions, as affected by soil depth, tillage, fertilization, and size fraction. The interactions among these parameters were not significant (data not shown) except the one between the OC content of each soil fraction and soil depth (\(P\leq 0.001\)) since, as expected, the OC content of all fractions decreased with depth. The significant decrease of the OC content of all fractions from the upper layer (on average, 125.9 mg OC kg\({}^{-1}\) fraction) to the deepest one (54.5 mg OC kg\({}^{-1}\) fraction) resembles the common stratification of SOC along the profiles. The highest value of OC was found in the D fraction due to the entrapment of organic components in the finest fractions of soil, as reported by [PERSON] et al. [31]. This highlighted the role of clay particles in the OM stabilization due to their high specific surface area and charge. In fact, clay minerals are considered the most active constituents in the formation of organo-mineral complexes [32] and are responsible for long-term preservation of soil OM, even over millennia [33]. No kind of tillage significantly affected the OC content of each fraction likely due to the balance in the soil achieved because of the long-term experiment, as reported by [PERSON] et al. [34] who found no significant difference in OC fraction content among several 30-year-old land-use trials. In addition, the main OM input, which is the above-ground crop residues, has been removed from the field at the end of the crop cycles in all treatments. With regard to the latter topic, it has been demonstrated that the aboveground crop residues are important for building up soil fertility not only as input of OM, but also because they cover the soil during the hot weather, which conserves SOM [35].