Aim of the study:
The rapid conversion of tropical secondary forests into intensive agriculture threatens the hydrological functions of the soil. Empirical data on this issue for watersheds in Wallacea remain scarce. This study investigates the impact of changes in land use on soil hydraulic architecture and infiltration capacity in the Wimbi sub-watershed in Central Sulawesi, Indonesia. The results provide a scientific basis for regional watershed preservation.

Material and methods:
Soil infiltration was measured in situ by taking stratified purposive samples (n = 15 per land use, total of 45 points) using a doublering infiltrometer across secondary forest, groforestry, and cultivated land (maize monoculture). Core physical properties, hydrodynamics, and soil organic carbon (SOC) were quantified using standardized laboratory protocols. High-resolution infiltration data were calibrated using the Horton model, which was solved via an iterative non-linear least-squares regression framework.

Results and conclusions:
Converting forests to intensive agriculture significantly degrades soil hydraulic properties. Secondary forests exhibited the highest steady-state infiltration capacity (7.74 ± 0.45 cm ⋅ h−1), followed closely by agroforestry (6.94 ± 0.38 cm ⋅ h−1), with both being categorized as moderately rapid. In contrast, cultivated land showed a 50% decline to 3.87 ± 0.21 cm ⋅ h−1 (moderate). Non-linear fitting demonstrated that the Horton model effectively captures infiltration dynamics (R2 ≥ 0.89). This agricultural restriction was directly associated with mechanical compaction, which increased bulk density (1.61 ± 0.07 g ⋅ cm−3) and depleted total porosity (33.18 ± 1.45%). The critical deficit in SOC (0.98 ± 0.08%) in cultivated land is highly indicative of accelerated surface sealing, which potentially restricts subsurface internal pore networks. Meanwhile, agroforestry sustained superior soil structural buffering through elevated SOC (4.20 ± 0.31%), mitigating matrix compaction and enhancing pore connectivity. Transitioning to tree-based farming is essential to reduce runoff and restore tropical watershed resilience.