Denitrification potentials in soils underlying a riparian forest and an agricultural field in the coastal plain of Virginia
While research has shown that riparian forests are effective in reducing shallow groundwater nitrogen levels, the relative importance of the mechanisms responsible for this reduction have not been adequately addressed. This project focused on the microbial mediated process denitrification, which has...
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| Format: | Others |
| Language: | en |
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Virginia Tech
2014
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| Online Access: | http://hdl.handle.net/10919/40729 http://scholar.lib.vt.edu/theses/available/etd-01242009-063055/ |
| Summary: | While research has shown that riparian forests are effective in reducing shallow groundwater nitrogen levels, the relative importance of the mechanisms responsible for this reduction have not been adequately addressed. This project focused on the microbial mediated process denitrification, which has been hypothesized to be a major factor responsible for decreased groundwater nitrate levels observed in forested regions.
The study site was located on Virginia’s Eastern Shore and incorporated a transect extending from a field under agricultural use through a mesic forest to a distance of 91.4 meters. Groundwater flowed from a well drained agricultural field of Bojac sandy loam (coarse-loamy, mixed, thermic Typic Hapludults) and Munden sandy loam (coarse-loamy, mixed, thermic Aquic Hapludults) to a poorly drained forest soil, Nimmo sandy loam (coarse-loamy, mixed, Typic Ochraquults). Previous work along this transect reported mean nitrate (NO₃-N) levels of 1,161 ± 393 μmol·liter⁻¹ for shallow groundwater underlying the agricultural field, whereas shallow groundwater 91.4 meters into the forest had a mean concentration of 2.2 ± 2.6 μmol·liter⁻¹. Groundwater nitrate (NO₃-N) levels below ~3 meters of the water table 91.4 meters into the forest 559.5 ± 101.9 increased to approximately 250 μmol·liter⁻¹. In addition to nitrate levels, other water quality parameters and soil characteristics suggested that vertical variations of soil environments existed and therefore, must be incorporated into experimental design.
Denitrification activiy was measured at various depth increments in the agricultural field and forest using an acetylene blockage technique. In addition, denitrification activity was measured after subjecting the soils to carbon and nitrate amendments. Denitrification activity from the forest was limited by nitrate at the water table and were carbon limited as vertical depth increased. Denitrification activity measured with nitrate amendments at the water table in the forest were two orders of magnitude higher than those in the field (7.37 nmol·g⁻¹·hr⁻¹ vs 0.074 nmol·g⁻¹·hr⁻¹). Dentrification activity measured with nitrate ± glucose amendments were higher at the water table in the forest, 6.88 nmol·g⁻¹·hr⁻¹, as compared to the field, 0.15 nmol·g⁻¹·hr⁻¹. Denitrifier microbial densities were measured at various vertical depths in the forest and agricultural field. Results demonstrated that denitrifiers densities at the water table in the forest were greater than those at the water table in the field. The number of denitrifying organisms per cubic centimeter of soil at the water table in the field averaged 2850 ± 1553(SD) as compared to 14,350 ± 13,369(SD) at the water table in the forest. At 0.91 meters below the water table in the field and in the forest the number of denitrifying organisms per cubic centimeter of soil were 1343 ± 1086(SD) and 3922 ± 3919(SD), respectively. The differences in denitrification measurements were due to location of the water table. The water table in the forest was located in the A horizon as compared to the water table in the field which was located in the C horizon. Results demonstrated that denitrification was an active mechanism that affected nitrate reduction in shallow groundwater in this system. Thus, riparian vegetation can be quite beneficial in reducing shallow groundwater nitrogen levels through microbially mediated processes such as denitrification. As a result nonpoint source nitrogen loadings from groundwater discharge can be reduced. === Master of Science |
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