Browsing by Author "J. Wendell Gilliam, Committee Member"

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Modeling Nitrogen Transport and Transformations in High Water Table Soils
(2004-02-03) Youssef, Mohamed A.; John E. Parsons, Committee Member; George M. Chescheir, Committee Member; R. Wayne Skaggs, Committee Chair; J. Wendell Gilliam, Committee Member
Development of management practices that reduce nitrogen (N) losses from agricultural lands has been the focus of research over many years. Development and testing of such practices is a complex task since it requires understanding of N dynamics in the soil-water-plant system, which is regulated by a large number of interacting physical, chemical, and biological processes. Nitrogen models are useful tools for developing and evaluating management practices for sustainable agriculture. The model, DRAINMOD-N was originally developed to simulate N dynamics in artificially drained soils. However, the model was based on a simplified N cycle, which restricted its applicability. A new version of DRAINMOD-N, referred to as DRAINMOD-N II, was developed and field-tested in this study. DRAINMOD-N II simulates N dynamics and turnover in the soil-water-plant system under different management practices and soil conditions. It considers a detailed N cycle, adds a simplified carbon cycle, and operates at different levels of complexity according to the conditions of the system being simulated. Processes considered in the model are atmospheric deposition, application of mineral N fertilizers, soil amendment with organic N sources, plant uptake, mineralization, immobilization, nitrification, denitrification, ammonia volatilization, and N losses due to leaching and surface runoff. DRAINMOD-N II driving hydrologic variables are predicted by the water management model DRAINMOD 5.1. DRAINMOD-N II was tested with a six-year data set from the North Carolina Lower Coastal Plain. The experimental site consists of eight 1.7-hectare instrumented, subsurface drained plots. The site was planted to a corn-wheat-soybean rotation. Water table depth (WTD) midway between the drains, subsurface drainage flow rates, and meteorological data were automatically measured and recorded. Flow proportional drainage water quality samples were collected and analyzed to determine N concentrations and loads. Results of the simulations showed good agreement between predicted and observed WTD. On average, predicted WTD was within 11.8 to 13.9 cm of observed values. The average coefficient of determination (R2) for WTD was in the range of 0.71 to 0.77. There was also good agreement between predicted and observed subsurface drainage rates. On average, predicting annual subsurface drainage was within 5.7-12.1% of observed values. The average R2 values for predicted versus observed subsurface drainage ranged from 0.65 to 0.73. The DRAINMOD-N II simulations showed good agreement between predicted annual nitrate-nitrogen (NO3-N) drainage losses and observed values. On average, predicted annual NO3-N leaching losses were in the range of 19.9-46.0%. The high errors in predicting annual NO3-N leaching losses were mostly induced by errors in predicting WTD and drainage rates. The model did an excellent job in predicting cumulative drainage and NO3-N leaching losses over the whole simulated period. Cumulative drainage and NO3-N leaching losses over the six-year period was within 1.3-13.2% and 2.2-10.1% of observed values, respectively. In spite of relatively large discrepancies on an annual basis, in some years, errors in predicting cumulative NO3-N losses over the six-year period were remarkably small. Results of this study indicate that DRAINMOD-N II can be reliably used to simulate N dynamics and turnover in agroecosystems. However, this evaluation of the model should be regarded as an incomplete test because it relied primarily on the literature rather than field and/or lab measurements to parameterize the model. Further research is needed to test DRAINMOD-N II based on independent measurement of the model input parameters.
Practices to Reduce Nitrate-Nitrogen Losses from Drained Agricultural Lands
(2003-04-30) Burchell, Michael Reed II; G.M. Chescheir, Committee Member; Stephen Broome, Committee Member; R.Wayne Skaggs, Committee Chair; J. Wendell Gilliam, Committee Member
Two practices were studied to reduce nitrate-nitrogen (NO3--N) losses from drained agricultural lands - shallow subsurface drainage systems and in-stream constructed wetlands. Data was collected between January 2001-September 2002 from two drainage systems near Plymouth, NC. Drains in Plot 1 were 1.5 m deep and 25 m apart, and drains in Plot 2 were 0.75 m deep and 12.5 m apart. Overall, decreased drain depth reduced drainage outflows by 42%. On average, NO3--N export from the shallow subsurface drains was 8 kg/ha in 2001 and 27 kg/ha in 2002. Nitrate export from the deeper drains was 6 kg/ha in 2001 and 37 kg/ha in 2002. Overall, an average of 8 kg/ha less NO3--N was exported from the shallow subsurface drainage system. Decreased export observed in 2002 from the shallow subsurface drainage system was significant at the 10% level, but not for the entire 21-month period. The model DRAINMOD was calibrated with these field observations. Long-term simulations indicated that shallow drains would reduce drainage outflows by 23% at this site, and based on observed drainage water NO3--N concentrations in 2002, NO3--N export could be reduced by as much as 16%. A wetland mesocosm experiment was conducted to determine if organic matter (OM) addition to soils used for constructed wetlands would increase NO3--N treatment. Eight batch studies, with initial NO3--N concentrations ranging from 10-120 mg/L, were conducted in 2001 and 2002 in 21 surface-flow wetland mesocosms. The results indicated that increasing the organic matter content of a Cape Fear Loam soil from 5% to 11% enhanced NO3--N wetland treatment efficiency in 7 of the 8 batch studies. Wetlands constructed with dredged material from Wilmington, N.C., with initial OM of 12%, showed improvement in NO3--N treatment efficiency when increased to 22 %.
Riparian Buffer Effectiveness in Removing Groundwater Nitrate as Influenced by Vegetative Type
(2005-12-22) King, Scott Edwin; Deanna L. Osmond, Committee Chair; Robert Evans, Committee Member; J. Wendell Gilliam, Committee Member
Nonpoint source contributions of nitrogen, particularly from agriculture, have become a serious concern for many watersheds in North Carolina. Recent regulatory action has increased the implementation of various best management practices (BMPs), particularly riparian buffer zones, for the purpose of reducing groundwater NO3-N pollution. However, the best design for such buffers has been the subject of great debate. The objectives of this project were to evaluate the relative effects of buffer vegetation and width on groundwater NO3-N removal and to determine if denitrification was the process most responsible. The main project consisted of four identically-designed buffer replications located on a farm in the Coastal Plain of North Carolina. The influence of vegetative type on buffer NO3-N concentration decreases were as follows; trees had an average decrease of 57% (from 8.79 to 3.78 mg NO3-N L-1), fescue had a decrease of 40% (from 6.33 to 3.77 mg NO3-N L-1), switchgrass had a decrease of 44% (from 5.52 to 3.09 mg NO3-N L-1), native vegetation had a decrease of 37% (from 6.47 to 4.07 mg NO3-N L-1), and the no-buffer control had a decrease of 27% (from 4.93 to 3.62 mg NO3-N L-1). These calculations are averages for each vegetation type from all of the wells from both widths and depths from all four buffer replications. For the 8 m buffer width, a total average NO3-N concentration decrease of 12% (from 9.97 to 8.75 mg NO3-N L-1) was observed for the intermediate well depth, while a 54% (from 5.26 to 2.41 mg NO3-N L-1) was observed for the deep well depth. For the 15 m buffer width, a total average NO3-N concentration decrease of 59% (from 6.42 to 2.61 mg NO3-N L-1) was observed for the intermediate well depth, while a 75% (from 4.31 to 1.06 mg NO3-N L-1) was observed for the deep well depth. Despite these apparent observed differences in the NO3-N concentration decreases, there were no overall statistically significant differences (p>0.05) between any of the vegetation types or between the two buffer widths or depths. The lack of significance is due to the variability of the results observed between the four buffer replications. An evaluation for buffer dilution using NO3-N:Cl ratio comparisons revealed that dilution appears to be a slight, if not inconsequential, factor in observed NO3-N concentration decreases. Redox monitoring probe results revealed low redox potential (Eh) values, indicating that substantial denitrification potential was present in all three of the buffer replications evaluated for redox. Dissolved organic carbon (DOC) concentrations indicate that the site has relatively low carbon present (overall average 3.1 mg L-1) and is considered to be an important limiting factor in the overall nitrate removal ability of the buffers. Additionally, a second riparian buffer study was conducted on a farm in the North Carolina Mountains to compare vegetative effect with the Coastal Plain. This buffer, with four vegetative treatment types of shrubs, fescue, native vegetation, and a no-buffer control was installed in April 2004 after one year of pre-buffer groundwater monitoring. Preliminary results are mixed and may be the result of significant preferential groundwater flow paths caused by the very rocky nature of the soil on site. However, the vegetation is not yet fully established in the buffer and the monitoring will continue in an effort to determine if any NO3-N removal trends develop between the vegetation types.