Spatial dynamics of preferential flow (rapid movement via macropores like root channels) and matrix flow (slower movement through soil micropores) are critical to ecohydrological processes, soil and water conservation, and groundwater pollution. However, it remains poorly understood how these two flow types interact to regulate water and solute transport in soil profiles, especially under the distinct wet and dry seasons of tropical climates.

In a study published in Soil and Tillage Research, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and their collaborators investigated dynamic interactions between preferential flow and matrix flow in rubber-associated forest systems within tropical regions, and explored their impacts on soil water storage and cycling.

Researchers conducted dye-tracing experiments and soil moisture monitoring across four systems of increasing root biomass and complexity: rubber monoculture (RM) < rubber-Alpinia oxyphylla (RAO) < rubber-orange-tea (ROT) < artificial rubber rainforest (ARR). Through in-situ measurements, they analyzed the differences in soil water regulation mechanisms.

The results showed a substantial increase in soil water storage capacity with system complexity. Compared to RM, RAO, ROT, and ARR had an increase in gains of 18%, 23%, and 31%, respectively. Besides, they identified that continuous rainfall, rather than short, high-intensity storms, was the primary driver activating beneficial preferential flow paths.

Moreover, researchers found that preferential and matrix flows exhibited an opposite, wave-like relationship on an annual scale. Preferential flow dominated downward movement during the rainy season, while matrix flow facilitated upward water supply from deeper layers during the dry season. This bistable, bidirectional water movement throughout the soil profile enhanced the overall water transport, storage, and circulation, contributing to greater ecosystem resilience.

The study reveals the bistability hydrodynamic characteristics of dual-flow systems in rubber-based agroforestry complexes. It provides theoretical foundations for optimizing rainwater utilization efficiency and intercropping design in rubber agroforestry systems.