<p>Soil regulates plant productivity in terrestrial ecosystems and maintains the balance of biogeochemical cycles through biotransformations mediated by living organisms, which are responsible for 80 to 90% of these functions. Therefore, it is necessary to evaluate whether restoration/natural regeneration processes in land degraded areas may allow the soil to partially or fully recover its microbial functions reflecting thus, in the fertility of these soils and consequently in the regeneration of forests. The use of microbiological attributes combined with infrared spectroscopy (FTIR) offers many opportunities to understand temporal dynamics and spatial variability in the recovery of important ecosystems during forest regeneration stages.The present work aims to evaluate the evolution of microbial quality in soils under three Atlantic Forest areas at different stages of regeneration (R40 - advanced, R12 - intermediate and RP - early regeneration pasture area) located in S&#227;o Paulo state, Brazil. We used as indicators of the soil microbial quality the number of colony-forming units (CFU) of total bacteria and fungi, spore density and root colonization by arbuscular mycorrhizal fungi (AMF). We also analyzed these soils by Fourier Transform Infrared Spectroscopy (FTIR-UATR). For each area, seven soil samples and plant roots were randomly collected at a depth of 0-20 cm at the end of the dry season (October 2019). In terms of dry soil, the CFU bacteria for each area was, respectively, 7.7, 4.6 and 3.2 x 10<sup>5</sup> CFU g<sup>-1</sup>; fungi, 1.2, 1.0 and 0.6 x 10<sup>3</sup> g<sup>-1</sup>,&#160;and AMF spore density, 39, 33 and 27 spores 50 ml<sup>-1</sup>. On average, AMF root colonization was 26 (R40), 25 (R12) and 21% (PR). For the FTIR spectrum, the major bands and their assignments were identified as a 3.370 cm<sup>-1</sup> wide band assigned to the O-H groupings; a peak at 1.635 cm<sup>-1</sup> attributed to aromatic C=C vibration, with contribution of C=O of the COO<sup>-</sup> and a peak at 1.072 cm<sup>-1</sup> attributed to the carbohydrate C-O bond. No difference was attributed to the composition of the main functional groups (O-H, C=O, COO- and C-O) between the soils from R40 and R12, but this difference was more evident when compared to the RP area.&#160; The microbiological results show good similarity between the tree areas in terms of spores, fungi and root colonization. However, in terms of bacteria, there is a more pronounced difference between the recent (RP) and the older regeneration areas (R12 and RP). Similar pattern was pointed by the FTIR&#160;results. Considering pasture as a strongly degrading area, these results are interesting since they show the differences in the soil quality between the three areas is not highly pronounced. They also show that in twelve years of regeneration, in many aspects&#8217; soils become similar to the area with forty years regeneration. Given these results, a further investigation on soil physics of these areas is being developed to relate soil regeneration processes and soil physical properties such as porosity, density and water retention capacity, all of them important to the maintenance of vegetation and ecosystem services of water and climate regulation.</p>
