## 4 Discussion
In accordance with the present study, [PERSON] and [PERSON] [67] measured higher concentrations of exchangeable K\({}^{+}\) and available P under no tillage compared to minimum tillage, at a soil depth of 0-20 cm. Similarly, [PERSON] et al. [68] found greater concentrations of plant-available K and P in surface soil (0-20 cm) under no-tillage compared to minimum tillage system, but no difference was observed between the two soil management systems at higher soil depths. The accumulation of available P, K and other nutrients in surface soil layers under no-tillage is usually ascribed to the decomposition of organic matter (which is more abundant in no-tilled soils) and to the accumulation of mineral fertilizers in topsoil [69]. The tillage system also influences the relations occurring between plant roots, soil and microorganisms at the rhizosphere level. After long-term no tillage, a higher activity of alkaline phosphatase and acid phosphatase was measured by [PERSON] et al. [70]. These two enzymes, which can be released both by plant roots and soil microorganisms, are involved in the release of labile P from the organic pools. Moreover, the organic acids exuded by plant roots and/or released through organic matter decomposition could compete with P for the binding sites on soil particles, thus enhancing P availability [69]. Finally, PLSR, by considering simultaneously soil indicators and plant response (grain yield, protein content or both), selected as important variables the mineral nutrients (available P, and both total and water extractable N), particularly when grain protein content was used as dependent variable, together with soil physical quality indicators (P\({}_{\text{MAC}}\), AC, RFC), pH and exchangeable cations. To this regard, VIP statistics, being a weighted sum of the squares of PLS X-score coefficients for the retained components, with the weights calculated from the amount of dependent variable (Y) variance explained by each retained component [33; 37], were able to summarize the contribution of both predictors and response variables. Thus, the contribution of mineral nutrients in determining plant response was also highlighted. Results of both PCA and SDA, as well as of univariate analysis of variance, returned TOC and RFC among the most influential variables both on the set of chemical and physical indicators analyzed separately as well as on the whole dataset. Previous studies highlighted the role of RFC among soil indicators in assessing soil physical quality. This variable was able to summarize part of the information given by AC and P\({}_{\text{MAC}}\) and, supported also by BD and plant available water content, showed the highest discriminating capability of the soil and crop residues management strategies compared [15]. TOC selection is important because is a necessary input for soil structural quality indicators [71]. This variable is also indicative of the soil chemical quality, being positively related with soil CEC and nutrient retention capacity. Moreover, TOC can be also indicative of the soil biological quality, due to its key role in maintaining the soil trophic relations and stimulating both the plant and soil microbial activity. [PERSON] et al. [24], selecting key soil indicators by means of factor analysis, concluded that TOC was the most dominant measured soil attribute as soil quality indicator for the two soil depths investigated and suggested its use for monitoring soil quality changes [24; 34]. Overall, the inspection of VIP profiles, together with the results of all methods compared, underlined the role of physical soil indicators (P\({}_{\text{MAC}}\), AC and RFC), which were selected as important variables in all the analyses performed. This reinforces the idea that such soil capacitive indicators (e.g., RFC) can be suggested for several practical applicationsincluding, for example, the estimation of optimal rate of amendments in laboratory, before use in the field.