Effects of Soil Physical Structure on Solute Transport in a Weathered Tropical Soil

Effects of soil structure on solute transport were studied in a clayey, well-aggregated Oxic Dystropept under grass and secondary forest at La Selva, Costa Rica. Both fine pores (<0.1 µm) and coarse pores (>30 µm) are abundant, as indicated by a water content of 0.37 m³ m⁻³ at −1500 kPa matric potential, the shape of the moisture release curve, and an initial infiltration rate (at field capacity) averaging 3900 mm hr⁻¹. Field application of Rhodamine B dye without ponding (simulating heavy rainfall) showed preferential flow along decayed-root channels, animal burrows, cracks, and other macropores. Dye application to intact cores under conditions of ponded steady-state flow gave a good correlation between flow rate and total stained area (p <0.01 under forest; p < 0.05 under grass). Solute (CaCl₂) breakthrough occurred very rapidly, often after < 0.1 pore volumes had percolated; however, relative concentration of the effluent did not exceed 0.95 even after five pore volumes had percolated. These results indicate that most water flows between aggregates or through macropores (as preferential or channelized flow), even when the fine pores are not fully saturated, and in effect bypasses the fine pore space. When solution inflow was interrupted after ∼1.8 pore volumes, then resumed after 10 min pause, relative concentration of the effluent dropped by 10 to 40%, then rose again. This indicates that solute diffused slowly into the aggregates. Taken together the results suggest that this soil strongly resists leaching. Preferential water flow may serve to prevent nutrient loss from the matrix of all highly aggregated soils and of all noncultivated soils in which animal activity and turnover of woody roots create abundant macropores. Research supported by NSF Grants BSR 83-17198 and BSR 86-05047 to the Organization for Tropical Studies, Duke Univ., Durham, NC 27706.