Field-scale bromide leaching as affected by land use and rain characteristics

Natural heterogeneity in soil properties limits the understanding on water and solute transport at a field scale. The objective of the current study was to adopt a new experimental design with scale-dependent treatment distribution and to assess the impact of land use and rain characteristics on Br^- leaching under field conditions. On a transect with two land use systems, that is, cropland and grassland, rainfall intensity and the time delay between solute application and subsequent rain were arranged in a repetitive pattern at different scales. Soil samples in 10-cm increments down to 1-m depth were collected along the transect for Br^- analysis after rainfall simulation. Owing to continuous macropores, supporting the development of preferential flow, soil Br^- was more evenly distributed with soil depth and reached greater depth in grassland. Increasing rainfall intensity enforced the deep leaching of Br^-. Frequency-domain analysis revealed that the dominant factor controlling Br^- leaching varied with depth. At 0 to 10 cm, rainfall intensity was strongly correlated with Br^- concentration; while in the soil layer below, application time delay was the main driver for the spatial distribution of Br^-. With increasing soil depth, the spatial behavior of Br^- was mainly caused by soil properties such as soil texture and topography, rather than rainfall characteristics. Nevertheless, rainfall intensity was found to be positively correlated with Br^- concentration in deep soil, indicating a great risk of deep leaching and groundwater contamination under heavy rainfall. These results have direct implications for the surface application of chemicals.