Edaphic and climatic effects on soil water dynamics and infiltration patterns in tropical rainforests

The preservation of forests’ hydrological functions is pivotal for alleviating land degradation and enhancing ecosystem services. However, understanding of the hydrological processes in tropical rainforests remains inadequate, particularly in the context of a changing climate and complex edaphic conditions. This study aims to quantify the spatiotemporal variations of soil water content and infiltration patterns within tropical rainforests located on distinct slope gradients (flat 0°, moderate 10° and steep 30°), and elucidate the effects of climatic and edaphic factors on these hydrological parameters. Based on the one year observation, it was revealed that the moderate-slope site maintained higher soil water content levels and temporal stability (54.32) compared to the flat (41.85) and steep-slope (42.71) sites across the 0 – 110 cm soil depths. This phenomenon is likely attributed to the superior soil condition at the moderate-slope site, characterized by optimal soil physical properties, a rich content of soil organic matter and abundant root biomass. The regression analysis demonstrated that soil physical properties (represented by capillary holding capacity and bulk density) had significant positive effects on soil water throughout the entire year (R2 > 0.6, p < 0.01). The effect of sand content on soil water content was found to be significant only during the dry period (R2 = 0.48, p < 0.05), while the effects of soil organic matter and fine roots were significant during both transition and wet periods (R2 > 0.5, p < 0.05). In response to the increasing precipitation, the moderate-slope site displayed more equilibrium state of infiltration patterns than the flat and steep-slope sites, evident in the weaker correlations between the dye coverage and the lateral, preferential, and uniform flows. This result suggested that moderate-slope sites may hold greater capacity for retaining soil water. Furthermore, the linear mixed-effects models revealed significant slope and precipitation interaction influencing the infiltrating water flows (p < 0.01). Therefore, this study provides valuable insights into the ecohydrological processes in rainforests, which are crucial for the effective planning of future forest management in tropical areas.