## 4 Discussion  
Long-term experiments can be considered valuable research infrastructures, which enable the long-term study and monitoring of the effects of agricultural strategies or management scenarios. Among agricultural practices, soil tillage strategies are major determinants of soil status and quality. By directly acting on soil physical properties--modifying porosity (total pore space and pore size distribution), soil aggregate size and stability, hydraulic conductivity, soil tillage modifies air-water capacity relationshipships and thus induces changes in soil organic carbon dynamics, nutrient cycling and solute transport [60]. Significant effects can usually be observed when stable or near-stable conditions are established, after transition periods [61]. In this study, the long-term iteration (over about 13 years) of two soil management strategies--minimum tillage and no-tillage--considerably affected certain physical and chemical properties of the upper soil layer in the system investigated. Analysis of variance showed that significantly greater bulk density (BD) and relative field capacity (RFC) values and, consequently, lower air capacity (AC), were recorded under no-tillage soil management. Since RFC is the ratio between the water content at the field capacity and that at water saturation, relatively higher RFC values highlight a reduced availability for soil air. In accordance with previous results on fine-textured soils [62], soil physical quality was thus indicative of lower aeration conditions under no-tillage. The long-term no-tillage soil management also enhanced total organic and extractable carbon contents (TOC and TEC). The behavior observed is in agreement with several studies [19; 63; 64], since the reduced soil disturbance reduces the turnover of soil aggregates favoring the accumulation and stabilization of organic matter within micro- and macro-aggregates, thus leading to a net gain of soil carbon. Significantly different values of C and N of the microbial biomass, bulk density, hydraulic conductivity and average weight of soil aggregates were also observed by [PERSON] et al. [49] under conventional and no-tillage in a long-term field experiment. [PERSON] et al. [65], comparing the effect of different cropping systems (wheat/wheat and wheat/bean) and most used tillage managements (conventional, dual layer and no-tillage), in a long-term field experiment on soil organic C pools (total and extractable organic C, microbial biomass C, basal respiration), observed that tillage management affected the soil organic C stored in the first 15 cm of soil more than  
Figure 4: Plots of the component loadings (**a**) and scores (**b**) of the first two principal components extracted in the analysis carried out on the indicators selected from results of PCA, SDA, PLS. Variance explained: PC1 = 70.37%; PC2 = 18.97%. MT = minimum tillage; NT = no-tillage. TOC = total organic carbon; TEC = alkali-extractable carbon; WEN = water extractable nitrogen; P_Olsen = Olsen available phosphorus; P\({}_{\text{MAC}}\) = macroporosity; AC = air capacity; RFC = relative field capacity. cropping system. No-tillage caused a 3.6 Mg ha\({}^{-1}\) increase of C content in a wheat/faba rotation, and an increase of 5.6 Mg ha\({}^{-1}\) in wheat monoculture after 19 years [65]. A greater exchangeable K\({}^{+}\) content was also observed under no-tillage. PCA and SDA confirmed and summarized results of analysis of variance but also underlined the role of organic carbon fractions -huric and fulvic acids and WEOC-, and available P as main sources of variability in describing the data (PCA), and as the variables that most discriminated the treatments compared (SDA). In a study assessing the suitability of different labile C fractions as soil quality indicators, [PERSON] et al. [66] found that WEOC (dissolved organic C) was not sensitive to soil tillage, unlike the particulate organic C; in any case, WEOC content highly depends on environmental conditions and soil sampling time [66].