The effects of fertilization with organic composts on the development of six tree species in a restoring area after kaolin mining the eastern Amazonia did not overcome the control. Hence, the initial hypothesis that organic fertilization would increase plant growth was refuted. Overall, chemical and organic fertilization provide higher nutrient concentration to the rhizosphere and, together with the trenches opening, improve soil conditions and root development (Sloan and Jacobs 2013). Regarding the soil physical and chemical properties in the same study area, Ribeiro et al. (2021) found no improvements with trenches opening. Trenches provided less compacted substrate with the possibility of greater water infiltration, which was directly correlated with plant growth (Orozco-Aceves et al. 2017; Duncan et al. 2020). Thus, probably 18 months, is not time enough for conclusive responses about different substrates, because decomposition and nutrients release are directly influenced by soil features (physical, chemical, and microbiological), type of wood of the sawdust, and cow manure composition, which determines the nutrient availability.
Planted individuals commonly show higher growth than those from the natural regeneration in the Brazilian Amazonia (Salomão et al. 2014; Martins et al. 2020a), with mean increment higher than 1.40 m yr− 1 and 2.39 cm yr− 1 in height and diameter at soil height, respectively. The reestablishment of proper soil conditions comes through the insertion of pioneer species that accelerate forest succession and the structural characteristics of the forest over the years. Therefore, our results corroborate the choice of an appropriate restoring strategy with species that tolerate water stress and nutrient deficit conditions, helping N fixation and, consequently, favoring biogeochemical cycles of N and C, in order to return resilience conditions of the restoring area (Justino et al. 2017; Batterman 2018).
Legume species, for example, have N fixation characteristics, as is the case of Inga edulis, one of the most important multi-purpose tree legumes for cultivation in acidic and low fertility soils of the Amazonia (Jaquetti et al. 2014). Additionally, when combined with green and chemical fertilization, the establishment and growth of I. edulis can increase due to its photosynthetic performance (Jaquetti et al. 2014). Of the species used in the initial planting, C. fairchildiana, T. vulgaris and C. matourensis showed the best increment results for both height and diameter (Fig. 3; Fig. 4). These species belong to the ecological group of pioneers and/or initial secondary forest, with rapid growth, and C. matourensis was identified as one of the most representative in the natural regeneration of bauxite mining areas close to this study site (Martins et al. 2020a). The high mean annual increment in crown area of C. fairchildiana, is a favorable characteristic for the restoration of intensely degraded ecosystems since the soil is quickly covered by the treetops. This feature helps to promote better microsite conditions and control of invasive trees and grasses as a natural mechanism to reduce light intensity.
Probably the strong presence of grasses had effects in plant mortality, because the mortality rate was low and represented only by B. spicata. Andrade and Sanchez (2014) recommended that up to 10% of mortality dispersed in the planted area is acceptable. So, in this study, the mortality rate of 2.4% was low, likely to the ecology of the planted species, high rainfall, which is typical of the region, and the soil conditions promoted by the experiment. In natural regeneration, the low density of tree and shrub species under the sawdust coverage may have been influenced by the sawdust decomposition time and the high layer thickness (10-cm), which may have hindered light incidence over seeds, thus hindering germination and seedling emergence. Although sawdust possibly hinders erosion processes by minimizing the impact of rainwater directly on the soil, we do not recommend its use in excess due to prevent conditions for the establishment of spontaneous species.
The presence of S. crinitum, S. fulvidum, C. palmata, and V. guianensis in natural regeneration is a clear indication of the initial succession phase, especially in intensely degraded ecosystems (Massoca et al. 2012). It is expected that these species will be replaced during the succession, increasing plant species richness as well as the availability and diversity of resources for the fauna (Cruz et al. 2020). The biodiversity of forest fragments is often referred to as islands, which is related to size and distance of the forest matrix. Larger and less isolated fragments are more favorable for biodiversity maintenance (Almeida et al. 2011). Besides these species, L. pisonis and A. mangium were also sampled. L. pisonis is native to the Amazonia and one of the most frequent species in the predominant phytophysiognomy in the study site. It has commercial potential for both wood and non-wood products (Siviero et al. 2020) and is also indicated for planting in degraded areas of the Brazilian Atlantic Forest (Morais Junior et al. 2019). L. pisonis has long life and usually occupies the forest canopy. Its presence in the experiment is possibly due to forest remnants, where the closest remnant is less than 100 m South of the planting area. On the other hand, A. mangium is native to Australia, Papua New Guinea, and Indonesia, and is considered an invasive species of difficult control that can damage local biodiversity (Heringer et al. 2019; Koutika and Richardson 2019). This species was planted more than 15 years before the experiment (2003) in adjacent areas for restoration programs.
Increase in litter stock means the gradual return of biogeochemical cycles and nutritional self-sufficiency of vegetation (Londe et al. 2016), with greater capacity to infiltrate water into the soil and improve its fertility (Durigan et al. 2016). There was no significant difference between the organic fertilization treatments in planting pits and in the sawdust coverage (Fig. 6), but the dry mass values were similar to those found in studies carried out in the Amazonia (Martins et al. 2018b) and other biomes (Zhang et al. 2014).
Organic fertilization in planting pits did not differ among control and the composts sawdust and cow manure. However, monitoring with more indicators to assess the restoring process efficiency should remain, as the fertilization effects can emerge over the years. A financial viability study of the trenches to restore degraded areas would be an important step to consolidate the method. Trenches are initially more expensive than other methods, which can be balanced over the years with no need of further interventions, such as enrichment with seedlings.