Understanding the mechanisms that influence the regeneration capacity of tropical forests is highly relevant in the current context of great loss and degradation of ecosystems. Species of Fabaceae family present resilience after disturbances through the symbiotic mechanism of Biological Nitrogen Fixation (BNF), important for the maintenance of biogeochemical cycles of the ecosystems. The aim of this work was to investigate the role of biological nitrogen fixation in the natural regeneration of secondary forests and burned primary forests in the eastern-central Amazon region. For this, 15 transects (0,25ha each one) were studied, covering three forest classes: secondary forests (> 17.5 years), recently burned primary forests (17 months) and undisturbed primary forests. All individuals of the Fabaceae family (DAP> 10cm) with nodulation potential indicated in the literature occurring in the transects were selected and evaluated. Soil samples were collected to check the presence of active nodules, and nodulation was estimated at individual, species, forest class and ecosystem levels. The total carbon and nitrogen in the soil were evaluated. A GLM model was used to test the main predictor variables that explain the nodular mass increment, including carbon and nitrogen content in soils and dendometric variables (diameter at breast height, canopy area and root mass). Of the 133 individuals studied, 43 (32%) were detected with active nodules. This represented 18 species and 9 genera of Fabaceae family. The genus Inga presented the highest nodulation frequency, with the species Inga pilosula showing the largest nodule mass (6,14 g.m-2). A large intraspecific variation of nodular mass was found in the study. Secondary forests showed a probability of nodulation three to fivefold higher than the other forest classes. Moreover, the total nodule mass was higher in secondary forests (9.37 g.m2) in comparison to burned forests (6.54 g.m2) and undisturbed forests (3.05 g.m2). No strong evidences were found for an important contribution of the biological fixation of nitrogen in burned forests. The main explanatory variable for nodule mass was soil total carbon. Along with carbon, other variables were also selected as predictors of nodule mass: soil total nitrogen, diameter at breast height (DBH) and canopy area. The results suggested an important contribution of biological nitrogen fixation in the regeneration of forests in the Santarém region, mainly in secondary forests and the potential of nitrogen inputs into the ecosystem through biological fixation. The results of the present study are important to increase the understanding of the mechanisms that drive the Amazon forest recovery to disturbances.
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