Sustaining the global population depends upon modern agricultural practices reliant on large inputs of nitrogen (N) fertilizer, but export of excess N from agroecosystems has negative environmental consequences, such as accelerated eutrophication and associated water quality degradation. The challenges posed by diffuse and widespread nutrient pollution in agricultural drainage waters necessitate cost-effective, adaptable, and reliable solutions. In this context, enhanced denitrification approaches developed over the last several decades have produced denitrifying bioreactors that harness the ability of ubiquitous soil microorganisms to convert bioavailable N into inert N gas, thereby removing bioavailable N from an ecosystem. Denitrifying bioreactors are edge-of-field structures that consist of organic carbon substrate and support the activity of denitrifying soil bacteria that remove N from intercepted nutrient-enriched drainage waters. The potential to improve bioreactor performance and expand their application beyond the Midwest to the agriculturally significant Mid-Atlantic region was investigated with a three-pronged approach: 1) a pilot study investigating controls on N removal, 2) a laboratory study investigating controls on emission of greenhouse gases nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2), and 3) a field study of one of the first denitrifying bioreactors implemented in the Atlantic Coastal Plain. The pilot and laboratory studies tested the effect of amending woodchip bioreactors with biochar, an organic carbon pyrolysis product demonstrated to enhance microbial activity. The pilot-scale study provides evidence that either hardwood- of softwood-feedstock biochar may increase N removal in woodchip bioreactors, particularly under higher N loading. The results from the laboratory experiment suggest the particular pine-feedstock biochar tested may induce greater greenhouse gas emissions, particularly of the intermediate product of denitrification and potent GHG nitrous oxide. The field study evaluated performance of a biochar-amended woodchip bioreactor installed on a working farm. Two years of monitoring data demonstrated that the bioreactor successfully removed N from drainage waters, but at relatively low rates constrained by low N loading that occurred in the absence of fertilizer application during continuous soy cropping at the site (10.0 kg NO3--N ha-1 yr-1 or 4.86 g NO3- -N m-3 d-1 on the basis of bed volume reached the bioreactor.) Removal rates averaged 0.41 g m-3 d-1 (8.6% removal efficiency), significantly lower than average rates in systems receiving greater N loading in the Midwest, and more similar to installations in the Maryland Coastal Plain. Greenhouse gas fluxes were within the range reported for other bioreactors, and of the N removed an average of only 0.16% was emitted from the bed surface as N2O. This case study provides useful measurements of bioreactor operation under low N loading that informs the boundaries of bioreactor utility, and may have particular regional relevance. The pilot and field studies suggest that wood-based biochars may enhance N removal and may not produce problematic quantities of greenhouse gases, respectively. However, the laboratory study raises the need for caution when considering the costs and benefits amending woodchip bioreactors with biochar and accounting for the effect on greenhouse gas emissions in this calculation, because the tested pine biochar significantly increased these emissions. / PHD / Modern agricultural relies on nitrogen (N) fertilizer to produce enough food for the global population, but losses of excess N from farmland has negative environmental consequences. Even with advances in best practices to reduce the environmental impact of agriculture, such as nutrient management planning where the right fertilizer is applied at the right rate at the right time, crops cannot use fertilizer with perfect efficiency and a portion will be lost to the environment. A relatively new agricultural best management practice removes this excess N before it enters surface water bodies by intercepting drainage water with high N levels at the edge of the field, slowing it down, to give the tiny creatures living in the soil the chance to use this N as energy. These naturally occurring soil bacteria remove the N fertilizer from the water by transforming it into harmless N gas that makes up nearly 80% of the atmosphere. These denitrifying bioreactors, named after the microbial N removal mechanism, are becoming established management practices in the Midwest, but they have not yet been widely adopted in other agriculturally significant regions, such as the Mid-Atlantic. In an effort to design more effective and flexible bioreactors, the effect of amending woodchip bioreactors with a charcoal-like material previously shown to increase the activity N-removing bacteria was tested and found to modestly increase N removal with sufficiently high drainage water N concentrations. However, a laboratory test of the effect of biochar on production of a harmful intermediate product of denitrification, the potent greenhouse gas nitrous oxide, found higher emissions from the biochar treatments than the woodchips alone, suggesting the N removal benefits may v not outweigh the costs. To evaluate performance under field conditions, a biochar-amendment woodchip bioreactor was installed in the Virginia Coastal Plain, and monitored for two years. N removal was significantly lower than reported rates, but this was due to a relatively low amount of N in the drainage waters. However, measuring performance under sub-optimal conditions provides useful information for determining the limits to conditions for which bioreactors are useful.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/91437 |
| Date | 18 January 2018 |
| Creators | Bock, Emily |
| Contributors | Biological Systems Engineering, Easton, Zachary M., Hession, W. Cully, Stephenson, Stephen Kurt, Eick, Matthew J. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/vnd.openxmlformats-officedocument.wordprocessingml.document |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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