Enhancing edge-of-field woodchip bioreactors to reduce nitrogen leaching using bioaugmentation and biostimulation

View the project final report

Commodities

• Agronomic: corn

Practices

• Crop Production: fertilizers, nutrient management
• Education and Training: on-farm/ranch research, participatory research
• Natural Resources/Environment: drift/runoff buffers
• Soil Management: soil microbiology

In recent years, Minnesota has begun to tackle its significant nitrate pollution, brought to attention in particular by the Dead Zone in the Gulf of Mexico, to which Minnesota is the sixth largest contributor. The greatest input to nitrate pollution is agriculture, which accounts for an estimated 89-95% of the nitrate loads found in the Minnesota, Missouri, and Cedar Rivers, and Lower Mississippi River basins (MPCA 2013). Woodchip bioreactors, which provide the means for the biological reduction of nitrate to inert dinitrogen gas (i.e. denitrification), are a feasible means for reducing nitrate leaching. However, there is a need to enhance bioreactor efficiency in the spring when runoff and nutrient leaching are at their highest in Minnesota. Cold temperatures in early spring most likely limit the activity of denitrifying bacteria in woodchip bioreactors.

This study will employ two methods for enhancing an established woodchip bioreactor located in Willmar, Minnesota: (1) bioaugmentation, in which cold-adapted denitrifying bacteria are introduced to the bioreactor, and

(2) biostimulation, in which an additional carbon source is added to the bioreactor to stimulate microbial activity and denitrification. Bacteria will be isolated from the bioreactor itself and tested for denitrification abilities. The bacteria with the greatest denitrification rates at cold temperatures will be grown in the lab and subsequently reintroduced to the bioreactor. By measuring nitrate reduction over time at this bioreactor, we will be able to determine the optimal microbial community and additional carbon that gives the largest nitrate removal.

This study can be applied to other bioreactors in Minnesota and the Midwest to create the most efficient bioreactors. Enhancement of these bioreactors will contribute significantly to nitrate load reductions from agriculture currently polluting bodies of water. Going forward, this information will be used to establish best management practices to enhance bioreactor in a cost effective way. The overall goal of this research will particularly benefit small-scale farmers as it provides a cost-effective, edge of field option for reducing nitrate pollution.

Having an understanding of the microbial communities naturally present in woodchip bioreactors will allow researchers to optimize the woodchip bioreactors using specific strains of known denitrifying bacteria. Denitrifying bacteria from woodchip bioreactors have not been well characterized, so this study will pave the way for this research and provide impetus for further study of the microbial communities in woodchip bioreactors. Learning about the denitrifying bacterial communities will allow researchers to target those bacteria that are most active at lower temperatures and inoculate the woodchip bioreactors with these strains. Inoculating the woodchip bioreactor with the most efficient strains will contribute significantly to nitrate load reductions from agriculture currently polluting bodies of water, which can be quantified by measuring nitrate load reductions in the wastewater before and after entering the woodchip bioreactor, and it will be invaluable in the execution of the Minnesota Nutrient Reduction Strategy.

Developing a procedure for optimal denitrification will promote the establishment of woodchip bioreactors across the region. Throughout the course of this study, researchers will strive to engage with the Willmar community by informing farmers and community members of the mechanisms behind biological nitrate reduction and sharing progress on the enhancement of this woodchip bioreactor. This will occur through increased presence at community events, such as having a table and poster at a local farmer’s market. Learning about the economic feasibility and effectiveness of these systems will incentivize farmers to establish woodchip bioreactors on their own fields.