Studies have shown that AFS contributes to the maintenance and enhancement of soil attributes related to soil quality (Cezar et al. 2015; Cherubin et al. 2019; Froufe et al. 2020; Stöcker et al. 2020), since soil management, especially tillage intensity and crop diversity, promotes changes in soil structural attributes, which influences also the chemical and biological soil properties (Kim et al. 2020; Nunes et al. 2020; Silva et al. 2014).
AFS-R presented higher TP and lower BD at the superficial layer, an increased volume of pores with large diameter (Ma and Bp) through the soil profile, as well as higher MDW, large and medium macroaggregates (0.1–0.2 m) and a better overall VESS score, which are indicators of increased physical quality. The absence of tillage and the action of the different root systems and soil organisms provided by the diversity of plant species cultivated in the rows, as well as the plant residues added to ASF-R through the pruning practice, may have contributed to these results (Cherubin et al. 2019; Rodríguez et al. 2021), which also explain some of the physical attributes showing better values in AFS-R in comparison to the CT area tilled one time after fallow.
On the other hand, the vegetable cropping in agroforestry inter-rows increased soil BD (0-0.1 m) and reduced soil TP (0-0.1 m), Ma and Bp (0-0.2 m) in relation to the row with trees. The TP decrease observed in the surface layer from AFS-R to AFS-I (Table 2) can be attributed to the tillage employed to vegetable cultivation in the inter-rows. Tillage disrupts the bonds between particles in the large macroaggregates (responsible for soil structure stability), rearranging them inside the larger pores (ex. Macropores), with consequent reduction in TP and increase in BD (Cavalcanti et al. 2020). This hypothesis can be confirmed by the distribution of aggregate size classes, in which AFS-I presented a reduction in the proportion of macroaggregates and consequently an increased number of microaggregates (Fig. 2).
The values of CT soil attributes in general remained between the AFS treatments and many of the physical variables in CT were not statistically different from AFS-R, probably due to the short time since the conversion of the area from fallow to agriculture. Moreover, studies have shown that in the short term, tillage promotes increases in soil porosity and aeration. Along the years, however, tillage practices result in physical quality degradation (Haruna et al. 2018; Moraes et al. 2014; Vizioli et al. 2021) as has occurred in AFS-I.
Most of the significant differences between the areas occurred within the 0-0.2 m soil layer, probably due to this depth be also more prone to the rotary tiller action to prepare the soil horticultural beds. Rotary tiller operates with a series of blades that shred, mix and crumble the soil, breaking soil aggregates excessively and usually forming a subsoil pan at the depth the implement weight is supported (Laudicina et al. 2017). This supports our findings for MDW, aggregates size distribution, and VESS Sq among the studied areas, as well as the values of macroporosity below the ideal limits for gaseous exchange and root growth (0.1 m3 m-3) (Xu et al. 1992) at the 0.1–0.2 m soil layer in AFS-I and CT.
Soil organic carbon did not vary in the more superficial layers, independent of the soil management practices. Increases in SOC are usually observed in the long term, in systems with positive carbon balances, thus assessments of vegetable cultivation effects on agroforestry inter-rows should be carried out over longer periods. This is highlighted by the fact that in conventional tillage systems (i) SOC might be redistributed within the soil profile (Ferreira et al. 2018; McCarty et al. 1998) and (ii) the formation and stability of soil aggregates might be compromised by exposing the physically protected carbon to the soil microbial community (Hok et al. 2021). Soil microbial activity increases due to the aeration and the availability of substrates for their metabolisms and as a result SOC is released from the system as CO2 (Xiao et al. 2019).
The visual evaluation of soil structure (VESS) demonstrated that soil structure was best following the order AFS-R > AFS-I > CT. AFS-R presented roots and small aggregates in the whole block indicating homogeneity of the soil structure. Between AFS-I and CT, less uniformity was observed in the blocks, with all the sampled points showing two layers of soil with structural differences: a superficial layer visually structureless, consisting with loose soil particles with absence of aggregates, and another layer with firmer, less porous and with bigger aggregates. These differences in the soil profile are characteristic from anthropogenic management, in this case, mainly by the action of mechanization techniques as the rotary tiller, which tends to disrupt soil structure and result in higher Sq values (Cherubin et al. 2019; Guimarães et al. 2017).
The lower Sq found for CT might be an outcome from the poorer Sq attributed to its second soil layer, probably remaining from the previous soil management explored in the area before the fallow period. This finding reinforces the importance of plants root systems for mitigating compaction problems and building soil structure (Zhang and Peng 2021), considering the lower tillage mobilization in CT, but also the absence of plants with aggressive root systems cultivated in the recent years compared to AFS-I. Despite the differences, all managements where within the range of good soil quality based on the interpretation that Sq < 3 is adequate and means that no management interventions should take place (Ball et al. 2007; Guimarães et al. 2011).
The better structural attributes found in AFS-R may explain its highest AC and Kfs values (Table 3). The non-mobilization of the soil and the action of roots and edaphic fauna in the tree rows contributed to the formation of large pores (Ma and Bp), through which the movement of air and water mainly occurs (Borges et al. 2019; Millan et al. 2014). In AFS-I, pores size continuity may have been impaired due to soil tillage intensity, resulting in lower AC and Kfs values (Borges et al. 2019; Dal Ferro et al. 2014), similarly to the findings of Moraes et al. (2016) which observed doubled values for Kfs in no-till (NT) compared to CT and attributed the results to the higher number and continuity of biopores present in the NT area.
The Kfs in CT was statistically similar to the AFS row and inter-row, probably due to soil tillage practice with plowing and harrowing that, in the short term, reduces Bd, increases soil aeration (Haruna et al. 2018), and the volume of cracks (Lopez-Bellido et al. 2016), implying in higher saturated hydraulic conductivity in the CT area. Despite the lack of differences compared to AFS-R, it is important to remember that the CT area presented critical values (lesser than 0.1 m3 m-3) for Ma at the 0.1–0.2 layer (Xu et al. 1992) (Table 2), where water infiltration in the soil may be compromised.
PCA showed that the differences in soil management and crop diversity have separated the areas regarding soil physical and microbiological quality (Fig. 4). Besides, it was evident that soil structure affects soil microbial attributes. Soil aeration and water content are important parameters for plant growth and have also direct effects on soil microbial community (Hungria et al. 2009; Silva et al. 2014), which might explain the negative relationship between BD and Kfs, and MBC. The better structure in AFS-R soil together with the diversified vegetation and mulching may have contributed to the generation of a more beneficial environment for the soil microbial community, being the tree rows more linked to MB-C and qMic. Higher values of qMic suggest more favorable conditions for microbial biomass formation (Babujia et al., 2010; Bastida et al., 2008), with soil microorganisms increasing the proportion of carbon in its cells in relation to the SOC. On the other hand, AFS-I and CT were more associated with qCO2, indicating a lower metabolic efficiency, and more intense biological activity for carbon degradation than microbial biomass growth, which might occur in stressful conditions (Bini et al. 2014; Lopes et al. 2021).
These results agree with a range of studies that have shown that conservationist soil management systems provide higher MB-C values and lower RB values, resulting in higher qMic and lower qCO2 values (Bini et al. 2014; Hungria et al. 2009; Lopes et al. 2021; Silva et al. 2014; Thomazini et al. 2015b). In this sense, the absence of differences in BR might have a different interpretation for the areas, revealing a more stable community in AFS-R, and a more stressed community in AFS-I and CT.
In AFS-I, the intense tillage may have promoted a rapid access of the microorganisms to SOC through aggregates breakage and internal carbon exposition along the time, what ended up in the depletion of energy resources and microbial community starving periods (Fiedler et al. 2016). For CT, although SOC was the highest among the areas, the microorganisms were also in a stressful condition, which could be explained by the low organic inputs added to the area and the presence of SOC content in large aggregates (higher MWD values), protected from MB access.
Regarding microbiological attributes linked to N cycling, the higher AB in AFS-R can be an indicative that in the rows, where the input of organic residues is higher and the root system is persistent, ammonium production results from the mineralization of organic matter, and this process acts as an important source of nitrogen to the trees (Beule et al. 2020). On the other hand, BNF was higher in AFS-I what might be a surprising result if we take in account that many studies have shown that conservation soil practices improve BNF compared to tillage (Torabian et al. 2019). However, this finding could be explained by the possible differences in the diversity and the metabolism of the microbial communities in the areas. In environments with more diverse C inputs, as native forests and perhaps AFS-R, microbial communities are usually more diverse and formed by K-strategists, that are less metabolic active, grows slowly and are more adapted to use C sources (Zhou et al. 2018). However, in more simplified and disturbed environments like agricultural sites and in this case AFS-I, r-strategists may dominate, being less generalists in terms of C sources, more active, and growing at higher rates, especially when there is great availability of energy sources (Zhou et al. 2018).
Despite the absence of differences in NOB among the areas, PCA showed that this variable was more associated with AFS-I (Fig. 4), possibly due to tillage that mixes soil and disperse soil nitrogen and nitrifiers more uniformly, creating more substrate-rich microsites for NOB development (Liu et al. 2017). NOB was also negatively correlated with SOC, which could be expected since this study focused on autotrophic nitrification (Liu et al. 2017). Once again, CT presented values that can be interpreted as in between the AFS-R and AFS-I areas, showing that with time tillage practices can degrade soil, and highlighting the importance of adequate soil management strategies for achieving soil physical and biological quality in agroecosystems.