Assessment of Nitrogen and Phosphorus Pathways at the Profile of Over-fertilised Alluvial Soils. Implications for Best Management Practices

Contrasting soil profiles (coarse-textured and fine-textured) treated with brilliant blue (BB) dye tracer, inorganic nitrogen (N) and phosphorus (P) concentrations along and between stained preferential flow pathways were examined for an irrigated and overfertilised maize monoculture system at the Mediterranean central Chile. The PO4-P concentrations were 2- to 10-fold higher in areas with BB than in areas without BB below 0.5-m soil depth in both soils. This elevated concentration was attributed to transport through cracks in fine-textured soil and finger flow in coarse-textured soil. The highest PO4-P value (13 mg kg(-1)) was found in areas with BB at the fine-textured soil. There were no significant differences in inorganic N concentration between areas with and without BB for both soils, which suggest that the effects of preferential flow are less important for inorganic N forms. There was a strong significant (p < 0.01) positive correlation between PO4-P and NH4-N concentrations in the fine-textured soil, and the amounts retained were clearly proportional to the clay content. Strategies for reducing N and P losses must be placed on good agronomic management of irrigated maize cropping system including accurate calculation of N and P fertiliser rates and establishment of suitable mitigation measures such as cover cropping.