Mechanized land preparation in eastern Amazon in fire-free forest-based fallow systems as alternatives to slash-and-burn practices: Hydraulic and mechanical soil properties

Soils of the Brazilian Amazon are particularly fragile due to the inherent soil properties, climate conditions, and the management systems adopted in the region. The aim of this study was to evaluate changes in the mechanical properties and water retention of soils under different tillage systems of secondary vegetation in an area kept fallow for 10 years in Eastern Amazon, Brazil. The treatments consisted of the slash-and-burn of secondary vegetation (SB), slash-and-burn, stump removal and plowing (SBSP), chop-and-mulch of secondary vegetation (CM), and original secondary vegetation (OSV). Undisturbed soil samples were collected immediately, and 15 and 30 days after site preparation (0, 15, and 30 DAP, respectively) to determine bulk density (BD), total porosity (TP) and marcoporosity (Mac), and water retention. Preconsolidation pressure (σp) and compression index (Cc) were assessed at 0 and 15 DAP, whereas soil penetration resistance (PR) was measured directly in the soil profile. The least BD and greatest Mac and TP and the highest water retention at −30 kPa matric potential were obtained immediately after preparation using the SBSP system. The σp at 0 DAP was lower than the pressure exerted on the soil surface by the machines used for the CM and SBSP systems, but increased numerically to 15 DAP. The PR differed among the systems until reaching 0.25 cm soil depth. Leaving plant residues from the mulching of secondary vegetation on the soil is not, by itself, an effective practice for maintaining structural soil quality in the short term. Soil tillage in the SBSP system decompacts the upper soil layer, which improves all physical indicators, but only in the short term (less than one month). Soil reconsolidates quickly with the lack or reduced effect of organic matter on soil structure and system equilibrium.