IMPACTS OF AGRICULTURE ON GREENHOUSE GASES AND NITROGEN CYCLING

Agricultural practices directly augment the emissions of greenhouse gases (GHGs) to our atmosphere and alter nutrient cycling in fields. Fertilizers (manure) applied to agricultural fields can increase GHG flux rates and the amount of nitrogen (N) and phosphorus (P) in soils. Fertilization can lead to leaching, volatilization, denitrification and water runoff of nutrients. Excess N and P can cause eutrophication in water bodies, which is a major issue in the Lake Champlain watershed in Vermont. While P has been widely studied throughout the Lake Champlain Basin, less is known about N. Yet, understanding N cycling is important for controlling and managing eutrophication. Best Management practices (BMPs), such as the incorporation of manure via aeration tillage, have been adopted in an attempt to minimize nutrient runoff, but little is known about the effects of such BMPs on GHG emissions. This study focused on comparing soil NO₃^- and NH₄⁺concentrations, and GHG fluxes of carbon dioxide (CO₂) and nitrous oxide (N₂O). While CO₂ emissions are the result of plant and microbial respiration, N₂O emissions result primarily from dentrification, a process that relies on low oxygen soils and high nitrate availability. Two pasture fields, adjacent to Lake Champlain, were selected for this four-year study in Shelburne, Vermont. Field 2 is a conventional field where manure is spread directly across the field (broadcast). Field 1 is a BMP where soil is aerated before manure is spread. After the second year of sampling, these practices yielded a difference in NO₃^- and NH₄⁺ concentrations and GHG emissions. Using static flux chambers and an Infrared Photoacoustic Spectroscopy gas analyzer, CO₂ and N₂O fluxes were measured on site. Soil temperature, air temperature, and soil moisture were recorded and soil cores were taken at times of gas sampling and analyzed in the lab for NO₃^- and NH₄⁺. The BMP decreased N₂O fluxes (P < 0.1) but had no impact on CO₂ fluxes relative to broadcast manure application alone. However, the broadcast field had 2x the amount of NH₄⁺ after manure was spread, compared to the BMP field. Differences in NO₃^- followed a similar trend. Our results suggest that aeration tillage may be used as a management technique to reduce post-manure N₂O fluxes, likely by increasing oxygen availability and reducing rates of N₂O production.