Conversion of forests to agricultural and pasture systems in the Amazon: impacts on soil carbon and nitrogen stocks under a tropical humid climate (Af)

The conversion of forests into agricultural and pasture systems in the Amazon affects the stability of soil carbon (C) and nitrogen (N) stocks, which can be influenced by edaphoclimatic conditions and management practices. This study aimed to evaluate soil C and N stocks under different land-use systems in the Eastern Amazon. The research was conducted on a rural property in the municipality of São Miguel do Guamá, Pará, Brazil, characterized by a humid tropical climate (Af), with well-distributed rainfall throughout the year. Soils in the area were classified as Yellow Latosols, with sandy to sandy loam texture in the surface layers. Four land-use systems were evaluated: secondary forest under fallow for 30 years, oil palm cultivation (Elaeis guineensis), intensive pasture with Urochloa brizantha cv. Marandu, and nominal pasture without fertilization or genetic improvement. In each system, four soil pits (1 m³) were excavated and randomly distributed within the respective land-use area. Soil samples were collected from two opposite walls of each profile, at eight depths (0–5, 5–10, 10–20, 20–30, 30–40, 40–60, 60–80, and 80–100 cm), including undisturbed samples (for bulk density) and disturbed samples (for C and N concentrations and physicochemical attributes). Soil carbon and nitrogen stocks were estimated considering bulk density variation with depth and calculated for the 0–30 cm and 0–100 cm layers. Statistical comparisons were performed using Welch’s ANOVA and the Games-Howell multiple comparison test, considering four soil pits randomly distributed in each land-use system. Carbon stocks ranged from 27.6 to 42.4 Mg ha⁻¹ (0–30 cm) and from 59.4 to 75.5 Mg ha⁻¹ (0–100 cm), with no statistically significant differences among land-use systems (p > 0.05). In contrast, nitrogen stocks were significantly higher (p < 0.05) under intensive pasture (3.2 Mg ha⁻¹ at 30 cm; 5.3 Mg ha⁻¹ at 100 cm), indicating a management effect on this element. The lack of variation in C stocks suggests that carbon retention may be more closely related to the physical properties of the sandy soil, which has lower capacity to stabilize organic matter. In this context, continuous monitoring of soil C and N stocks is essential to guide sustainable agricultural practices and promote soil conservation in the humid tropical regions of the Amazon.