Native species seedlings in forest restoration in the Southern Amazon rapidly increase soil carbon stocks

Forest degradation in the Amazon has led to severe environmental impacts, with active restoration, through mixed plantings of native and/or exotic tree seedlings, emerging as a key for ecosystem recovery. The study aimed to evaluate the soil organic carbon (SOC) and total nitrogen (TN) stocks of legal reserve (LR) restoration treatments in the southern Amazon after 10 years of implementation. The experiment was conducted from 2012 to 2022, followed a randomized block design with five treatments and four replicates. Treatments included three active restoration techniques using seedling planting, one passive regeneration treatment (area isolation), and a secondary forest as reference. Soil samples were collected from 60 × 80 m plots down to a depth of 1 m to determine C, N, bulk density, and SOC and TN stocks. Photosynthetically active radiation (PAR) was monitored from 2015 to 2019 using sensors equipped with automatic data loggers. Active restoration significantly increased SOC stocks compared with passive regeneration (p < 0.05), reaching up to 120 Mg ha - ¹ in seedling-based treatments versus ∼100 Mg ha - ¹ under passive regeneration (∼100 Mg ha - ¹). Seedling-based treatments also exhibited higher annual SOC accumulation rates (1.4–2.5 Mg ha - ¹ year - ¹). Total N (TN) was not significantly affected by the treatments, except in the surface layer, where the secondary forest showed the higher values. Principal component analysis (PCA) revealed a negative correlation between SOC and understory PAR, indicating that increased canopy development enhanced organic matter input and SOC accumulation. PCA also indicated that the inclusion of exotic species, or a higher proportion of them relative to native species, may delay increases in soil C and N. Planting native species was the most effective restoration strategy for rapidly increasing SOC stocks, highlighting its potential as a viable and sustainable approach for forest restoration. • C and N stocks were assessed 10 years after Legal Reserve restoration. • Seedling planting raised C stocks without changing N stocks. • Active restoration accumulated more C than passive restoration. • Carbon gains increased with canopy cover, as shown by reduced radiation transmissivity. • Planting native species was the most effective restoration strategy.