Preferential Flow Patterns in Paddy Fields Using a Dye Tracer

Reports of chemical leaching from intensively cropped paddy rice ( Oryza sativa L.) field sites indicate that soil structure dynamics may possibly affect preferential flow and transport despite repeated “puddling” (i.e., mechanical homogenization of water‐saturated soil) and plow pan formation. Our objective was to identify preferential pathways of soils in regularly managed paddy fields, which were drained during temporal fallow. Dye tracer studies were conducted on two experimental 20‐yr‐old rice fields of about 300 m 2 located in the Sunjian watershed in southeast China (Jiangxi province). Brilliant Blue solution was applied by ponding a 50‐mm pulse onto four 1‐m 2 plots with intact and unleveled soil surfaces. Staining patterns were recorded for 11 to 13 vertical and 10 to 15 horizontal soil profiles. Dye tracer penetrated vertically via preferential pathways to depths ranging from 94 (vertical profiles) to 120 cm (horizontal profiles), while most of the soil matrix remained unstained. Stained biopores and cracks were found in the plow pan. Horizontal profiles with stained pathways indicate large spatial heterogeneity of hydraulic conductivity. Dye coverage vs. depth calculated from vertical and horizontal profiles suggests that horizontal spreading of the solution above the plow pan supports access to plow pan macropores. Penetration through the plow pan proceeded with little horizontal spreading. The results obtained here for initially drained paddy soils indicate that preferential flow is a phenomenon that may be relevant for water and nutrient cycling (and losses to groundwater) in paddy fields. If the observed macropores and crack networks persist, preferential flow might even occur under flooding conditions.