Impacts of Dredging and Brush Cutting of Paired Agricultural Drainage Ditches on GHG Emissions and Nutrient Filtration Capacity
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Agriculture provides many beneficial and essential ecosystem services. Along with these beneficial services, the conversion of natural ecosystems into heavily modified agricultural ecosystems is also a source of disservice, including being a major source of global greenhouse gas (GHG) emissions and pollution of downstream waterways due to increased nutrient runoff. Carbon (C), Nitrogen (N) and Phosphorus (P) applied to agricultural fields as fertilizer are a source of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions. Nutrient runoff can lead to excess P in surface water bodies causing algae blooms, and excess N can lead to excess nitrate (NO3) in rural groundwater (GW) wells. There is a need to establish beneficial management practices (BMPs) to take into account agriculture-derived pollution with all agricultural practices. This thesis informs the development of BMPs by examining the environmental pollution aspects of both: 1) GHG emissions; and 2) nutrient export; resulting from the common practices of brush cutting and dredging of ditches to enhance drainage. Riparian vegetation in agricultural drainage ditches has been shown to decrease insolation, which decreases soil and water temperatures. This vegetation also hinders drainage by restricting flow, thus raising water levels, which decreases CO2 emissions and increases CH4 and N2O emissions. However, no previous studies have examined in detail the effects of removing drainage ditch vegetation. This study examines the GHG emissions from four ditch microplots in the South Nation Watershed in southern Ontario, Canada following the removal of riparian vegetation from two microplots. The trials took place over three field seasons, and the intervention methods were selected to observe the effects of brush cutting and of dredging on GHG emissions between years. The Control Shrub and Control Tree microplot sites were left unaltered. The Brush Cut Shrub and Brush Cut Tree sites were brushed in Spring 2018 and Dredged in Fall of 2018, with observations at all sites taking place over 2018-2020 growing seasons. Brushing increased CO2 emissions at the treed site but had little effect on the shrub site. Dredging decreased CH4 emissions. Riparian vegetation has also been shown to obstruct the path for water flow, decreasing water velocities and raising water levels, which increases the ability of ditches to filter and retain nutrients. Simultaneously with the GHG research above, this study also examines the N, P, and C v export from two adjacent watersheds within the South Nation Watershed following the removal of riparian vegetation from one of them. The trials took place over two field seasons and the intervention methods were selected to observe the effects of brush cutting and dredging on N and P export over two years. The southern watershed (Brush Cut) was brushed + dredged in 2018 and the northern watershed (Control) was left intact before flow monitoring took place in 2019 and the Fall of 2020. Tile drain discharge containing DOC, N and P, occurred during the Spring and Fall when the water table is higher, but was not observed during the summer. Brush cutting and dredging increased hydraulic outflow and reduced or eliminated NO3 retention capacity of agricultural drainage ditches by 320% in 2019 and 68% in Fall 2020. This increase in NO3 export may negatively affect rural water supplies. Lack of O2 and increased retention of DOC and SO4 in the Control watershed suggests that significant NO3 reduction occurred. Differences in P export between Brush Cut and Control in 2019 were small. There are more signs of P transformation in the Control watershed, but brush cutting and dredging may not significantly affect eutrophication. This thesis will help inform stakeholders about the environmental geochemical costs and benefits of brushing and dredging so that they can develop BMPs that minimize GHG production and maximize nutrient filtration. Future research is needed to determine how many years the effects of these intervention methods remain, and also determine other environmental impacts such as their effects on biodiversity.
Cite this version of the work
Andrew Schietzsch (2023). Impacts of Dredging and Brush Cutting of Paired Agricultural Drainage Ditches on GHG Emissions and Nutrient Filtration Capacity. UWSpace. http://hdl.handle.net/10012/19022