MN Impact: Model drainage system with multiple frameworks shows promise for reducing nutrient runoff
Agricultural production is intensifying to meet the increased demands on food security, availability and accessibility caused by global population growth. Climate and land use changes place stress on land and water resources. Opportunities for sustainable intensification of agricultural production, including water management strategies, have never been greater.
Researchers at the Southwest Research and Outreach Center in Lamberton, Minn., wanted to understand the individual and cumulative impacts of multiple, integrated best practices on water quantity and water quality in order to meet nutrient load reduction goals. In particular, researchers were interested in evaluating the response of in-field, edge-of-field and beyond-the-field/in-stream management practices on water quantity and water quality for a small watershed and upscale these results to watershed scale.
The team used unique research infrastructure situated in the Cottonwood River Watershed (CRWS) at the Southwest Research and Outreach Center to achieve this goal. The site included a series of components consisting of modular bioreactors, constructed wetlands, managed ditches and cover cropping strategies to assess in-field tile drainage and surface runoff. In addition to the field research, data from the experimental site was coupled with actual on-farm management data from farms in the CRWS that were enrolled in the Minnesota Agricultural Water Quality Certification Program (MAWQCP). Data collection and monitoring took place over a three-year period, 2017-2019.
Data for the experimental site supported that structural practices like bioreactors, constructed wetlands and managed ditches can have a positive effect on water quality through reduced nutrient loss (nitrogen and phosphorus) and can increase temporary water storage on the landscape. During the three-year experiment, ditch management using a minimally invasive low-grade weir resulted in, on average, 57 percent less flow than the unmanaged ditch. Similarly, on an annual basis, the managed ditch exhibited an average 67 percent reduction in nitrogen (N) load and a 27 percent reduction in dissolved reactive phosphorus (DRP) load. During the experiment, nutrient reduction varied by type of constructed wetland but all types reduced nutrient losses. Constructed wetland management resulted in a range of 54 to 60 percent reduction in N load and a range of 44 to 50 percent reduction in DRP load. Performance of corncob/woodchip modular bioreactors (MB) varied by treatment type but all types reduced nutrient losses. Nitrate load was reduced by 14 perfect when the MB consisted of corncobs and wood chips only. When a supplemental carbon source was added to MBs, the nitrate load reduction increased to 31 percent. Finally, when supplemental carbon and heat were added to MBs, the nitrate load reduction increased to 42 percent.
In much of the Midwest, including Minnesota, land use is dominated by row crop agriculture with the extensive use of artificial subsurface drainage systems and maintained ditches to manage soil water conditions. Excess water in the system can affect soil properties and plant growth and development. In order to make meaningful strides at mitigating and improving the impact of agricultural runoff on the environment, a multi-scale approach (in field, edge-of-field, beyond-field/in-stream) is needed and projects like this represent a significant step forward for agriculture.