Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/24/2003
Publication Date: 11/1/2003
Citation: SZOGI, A.A., HUNT, P.G., HUMENIK, F.J. NITROGEN DISTRIBUTION IN SOILS OF CONSTRUCTED WETLANDS TREATING LAGOON WASTEWATER. SOIL SCIENCE SOCIETY OF AMERICA JOURNAL. 2003. v. 67. p. 1943-1951. Interpretive Summary: Constructed wetlands are perceived as an on-farm alternative to the traditional land intensive spray field treatment of nitrogen-rich swine lagoon effluents. However, the technology is relatively new, and it is important to understand in detail the treatment function of wetland components such as the soil and plant interactions. To advance this understanding, we studied the nitrogen accumulation and distribution in soils of two wetland systems: one planted to bulrushes and another planted to cattail. Over a five-year period, the nitrogen load was increased to about 10 times of that of a traditional spray field (25 lb per acre per day). The soil N accumulation by depth was similar in both wetland systems, but represented a small fraction (<6%) of all the nitrogen applied to the wetland in five years. Annual samples of water stored in the soil pores revealed that nitrogen moved from the water to the soil, at the low loading grates. In contrast at the highest loading rate, nitrogen moved from the soil to the water. This movement indicated that the major treatment mechanisms were microbial processes that transform wastewater nitrogen into harmless atmospheric nitrogen gas. Nitrogen transfer was higher in the bulrush than the cattail wetlands. This was likely related to higher oxygen content and more favorable conditions for efficient microbial processes in the bulrush wetlands.
Technical Abstract: As constructed wetlands are being used for animal wastewater treatment, it is important to more fully understand the component functions of the treatment process. Our objective was to investigate the interaction of the plant, soil, and pore water in wetland accumulation and distribution of N. In a five-year study in North Carolina, we investigated two constructed wetland systems that treated swine lagoon wastewater. Wetland system 1 wa planted to bulrush (Scirpus sp.) and wetland system 2 was planted to cattail/bur-reed (Typha sp./Sparganium americanum). Nitrogen loading rates were increased from 0.6 g/sq.m/day in the first year to 2.7 g/sq.m/day in the fifth year. The pattern of soil N accumulation by depth did not differ significantly (P> 0.10) between the two wetland systems. Similarly, the two wetland systems did not differ significantly for the five-year net N accumulation which was <6% of total N applied over the five years. These results indicate that internal recycling satisfied most of the plant N requirements. Ammonium distribution profiles in the water-soil pore water column indicated that ammonium was transported from the surface water into the soil pore water when application rates were below 1.8 g/sq.m/day. At higher loading rates, 2.5 g/sq.m/day, ammonium was transported from the soil pore water into the surface water. This result indicated that the major mechanism of N removal was microbial nitrification-denitrification. Ammonium fluxes were higher in the bulrush than the cattail wetlands. This was likely related to higher oxygen content and more favorable conditions for efficient microbial processes in the bulrush wetlands.