NITRATE TRANSPORT AND TRANSFORMATION PROCESSES IN UNSATURATED POROUS-MEDIA

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 con ditions. 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 t wo were an unstructured fine sand obtained from central Florida. The c ores were irrigated daily and maintained at a matric potential of -20 kPa, representative of soil tension in the rooting zone of irrigated a gricultural fields. Volumetric water content (theta), 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-app lied NO3--N was significant in both the sand and the clay. In unplante d sand cores, almost all of the applied nitrate was leached below 30 c m within 10 days. Gaseous N loss owing to denitrification was no great er than 2% of the nitrate-N applied to the unplanted sand cores and, i n general, was less than 1%. Although leaching was somewhat retarded i n the clay cores, about 60% of the applied nitrate-N was leached from the unplanted clay soil in 5-6 weeks. Under unsaturated conditions, th e clay had little to no tendency to denitrify despite the greater mois ture 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 t he clay and sand experiments show that the dynamics of nitrate transpo rt and transformation in unsaturated soils are affected by small, loca lized variations in the soil moisture content profile, the gaseous dif fusion coefficient of the soil, the rate at which the nitrate pulse pa sses through the soil, the solubility of N2O and N-2 and the diffusion of the gasses through the soil solution, and development of a water c ontent profile in the soil. Limited denitrification in the clay soil w as due to a limited volume of soil available for infiltration after in ternal catchment and the development of denitrifying conditions result ing from the presence of an extensive macropore system.