Nitrate transport and transformation processes in unsaturated porous media

• Published in: Journal of hydrology (Amsterdam) Vol. 169; no. 1; pp. 51 - 94
• Main Authors: Tindall, James A., Petrusak, Robin L., McMahon, Peter B.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.07.1995; Elsevier Science
• Subjects: agricultural soils; calcium nitrate; clay soils; denitrification; Earth sciences; Earth, ocean, space; Exact sciences and technology; groundwater contamination; leaching; losses from soil; nitrate nitrogen; nitrous oxide; preferential flow; sandy soils; soil depth; soil water content; Soils; Surficial geology; unsaturated flow; Midwest; Florida; United States; North America; Eastern U.S; Missouri; oxides; textures; fertilizers; fine-grained materials; concentration; clastic sediments; moisture; leaching; transport; nitrates; denitrification; mass transfer; unsaturated zone; clay; agriculture; sedimentary rocks; sand; transformations; water content; porous materials; soils
• ISSN: 0022-1694; 1879-2707
• DOI: 10.1016/0022-1694(94)02666-Y

A series of experiments was conducted on two contrasting agricultural soils to observe the influence of soil texture, preferential flow, and plants on nitrate transport and denitrification under unsaturated conditions. Calcium nitrate fertilizer was applied to the surface of four large undisturbed soil cores (30 cm diameter by 40 cm height). Two of the cores were a structured clay obtained from central Missouri and two were an unstructured fine sand obtained from central Florida. The cores were irrigated daily and maintained at a matric potential of -20 kPa, representative of soil tension in the rooting zone of irrigated agricultural fields. Volumetric water content (θ), concentration of nitrate-N in the soil solution, and nitrous oxide flux at the surface, 10, 20, and 30 cm were monitored daily. Leaching loss of surface-applied N0 3 − -N was significant in both the sand and the clay. In unplanted sand cores, almost all of the applied nitrate was leached below 30 cm within 10 days. Gaseous N loss owing to denitrification was no greater than 2% of the nitrate-N applied to the unplanted sand cores and, in general, was less than 1 %. Although leaching was somewhat retarded in the clay cores, about 60% of the applied nitrate-N was leached from the unplanted clay soil in 5–6 weeks. Under unsaturated conditions, the clay had little to no tendency to denitrify despite the greater moisture content of the clay and retarded leaching of nitrate in the clay. The planted sand cores had surprisingly large gaseous N loss owing to denitrification, as much as 17% of the nitrate-N. Results from both the clay and sand experiments show that the dynamics of nitrate transport and transformation in unsaturated soils are affected by small, localized variations in the soil moisture content profile, the gaseous diffusion coefficient of the soil, the rate at which the nitrate pulse passes through the soil, the solubility of N 2O and N 2 and the diffusion of the gasses through the soil solution, and development of a water content profile in the soil. Limited dentrification in the clay soil was due to a limited volume of soil available for infiltration after internal catchment and the development of denitrifying conditions resulting from the presence of an extensive macropore system.