The Nature Conservancy in Illinois has been engaged in long-term water quality monitoring in the Mackinaw River Watershed to understand how implementation of conservation practices affects water quality at multiple scales and to gain insights related to outreach, practice delivery, and partnerships that can inform TNC and partner work on other programs and in priority watersheds throughout Illinois and the larger Mississippi River Basin. Key partners for the TNC’s research in the Mackinaw River watershed include the McLean County Soil and Water Conservation District, McLean County Natural Resources Conservation Service, University of Illinois Urbana-Champaign, and Illinois State University. Our narrative describes ongoing projects at the Franklin Research and Demonstration Farm in Lexington, Ill., and the Paired Watershed Study in the headwaters of the Mackinaw River. For more information, visit www.nature.org/en-us/get-involved/how-to-help/places-we-protect/the-mackinaw-river-watershed.
The Mackinaw River is a major tributary to the Illinois River that drains into the Mississippi River. This 3,000-square-kilometer watershed contains some of the most productive agricultural lands in the nation and plays a key role in the livelihood of farmers and the Illinois economy. The Mackinaw River was selected as a priority site by The Nature Conservancy more than 25 years ago because of its status as a high-quality river that sustains some of the highest quality stream segments in Illinois and provides habitat for 60-70 native fish and 25-30 mussel species. Yet, urban development and intensive tile-drained row crop agriculture have stressed freshwater resources, leading to habitat loss and reduced water quality.
Subsurface agricultural tile drainage systems within the Midwestern region of the U.S. have been identified as major contributors of nutrients to the Mississippi River. Illinois is recognized as one of the highest contributors in the nation of total nitrogen and phosphorus flux to the Gulf, much of which is exported through one of the most extensive subsurface agricultural tile drainage systems that exist within the Upper Mississippi River Basin. The Nature Conservancy’s work in the Mackinaw River watershed includes research and monitoring at multiple scales to address and manage these nutrient losses.
At the field and farm scales, TNC studies the effectiveness of conservation practices in the context of a working farm. Established in 2003, the Franklin Research and Demonstration Farm in Lexington, Ill., is a collaborative effort between the TNC, University of Illinois Champaign-Urbana, McLean County Soil and Water Conservation District, McLean County Natural Resources Conservation Service, and the Franklin family. It serves as a model for sustainable agriculture, innovative research, and successful partnerships.
To better understand the scale of conservation practice adoption and implementation needed to provide measurable improvements in water quality, scientists from the TNC and Illinois State University designed a paired watershed study in the headwaters of the Mackinaw River. Since 2000, TNC researchers have led water quality and flow monitoring efforts in two small (approximately 10,000 acre) watersheds and have worked with local partners to facilitate outreach and conservation practice implementation in the project’s treatment watershed.
Multiple phases of research have occurred at the Franklin Farm and in the paired watershed study.
- Phase 1: Research revealed that conservation practices that reduced erosion and surface water runoff were not enough to solve water quality problems as nitrate and dissolved phosphorus runoff generally bypass these practices and enter river systems through tile-drainage systems.
- Phase 2: Research has focused on the effectiveness of constructed wetlands to treat excess nutrient runoff from tile drainage. The TNC’s research in the paired watershed study is designed to measure if wetlands can improve water quality at various watershed scales. Parallel research at the Franklin Farm with the University of Illinois has shown these wetlands are very effective at reducing both nitrate-nitrogen and dissolved phosphorus from agricultural drainage tiles at the field scale.
- Phase 3: Research focuses on stacked practices at the field and watershed scales. Cover crops have been established at the Franklin Farm annually since 2010. In 2022, the tenant farmer agreed to transition from fall to spring nitrogen application for one of the experimental fields. In the paired watershed project, cover crops have been established annually in priority areas of the treatment watershed since 2019.
Key Research Findings – Franklin Research and Demonstration Farm
In 2022, analyses of data collected over 12 years was published in the peer-reviewed Journal of Environmental Quality, available at doi.org/10.1002/jeq2.20316. The long-term data show that these treatment wetlands are very effective at removing both nitrate-nitrogen and dissolved phosphorus from subsurface tile drainage systems. This research showed that even the smallest wetlands representing 3% of the tile-drained farmland are very effective at reducing nitrate losses by 15%-38% and capturing 53%-81% of the dissolved phosphorus.
The Franklin Farm wetlands have remained very effective over many years, emphasizing how incorporating constructed wetlands into state and watershed-level conservation planning can significantly contribute towards reducing excess nitrogen and phosphorus export to river systems, and ultimately to the Gulf.
Results Summary
- Wetlands representing 3% of tile-drained farmland (Wetland 1) reduced 15%-38% of NO3-N export, with cumulative reductions of 39%-49% and 49%-57% as tile water flowed from the first wetland to wetlands representing 6% (Wetland 2) and 9% (Wetland 3), respectively. (Figure 1, below)
- Mass NO3-N retentions ranged from 28T-52% across the 12 years.
- 12-year total dissolved phosphorus load reductions for Wetland 1 ranged from 53%-81%, with cumulative reductions of 35%-91% and 32%-95% for Wetlands 2 and 3, respectively. (Figure 1, below)
- Mass dissolved phosphorus retentions ranged from 71%-85%.
- Nitrate export was significantly higher for corn than soybean production years, during which 80%-84% of annual NO3-N export occurred between March and June compared to 39%-56% during soybean years.
Cover crops: Cover crop success at the Franklin Farm has been highly variable over time. One important factor that has influenced cover crop success during this study is the timing between seeding and subsequent rainfall. In addition, researchers have observed that cover crop coverage is generally reduced in corn production years as a portion the cover crop established in the fall is disturbed with the application of fall anhydrous ammonia fertilizer. Additional data from several more years of successful cover crops during corn production years at the Franklin Farm are needed to quantify how much additional water quality benefit they are providing, in addition to their agronomic benefits.
Key Research Findings – Paired Watershed Study
The paired watershed study includes two approximately 10,000-acre watersheds. Bray Creek is the treatment watershed where the TNC and other project partners have provided extensive outreach and financial assistance to facilitate high levels of adoption of constructed wetlands and other conservation practices. In Frog Alley, the adjacent reference watershed, outreach related to conservation practices has been limited.
Results
- Water samples collected since 2010 from the first constructed wetland show that this wetland is reducing nitrate concentrations from tile drainage by 24% to 38% annually, thus, reducing the amount of nitrate-nitrogen that reaches Bray Creek from agricultural runoff. The results indicate that these wetlands are functioning as designed to remove excess nitrate export from tiles.
Preliminary analyses of spring (March to June) data from 2006-21 showed that under normal weather conditions, constructed wetlands that were established in 2008, 2014, and 2015 (Figure 2, left) are providing significant nitrate-nitrogen reductions in the treatment watershed compared to the reference watershed, which has no constructed wetlands (p=0.03, Figure 2, right). This analysis did omit data from 2014 due to exceptionally high rainfall, and monitoring is continuing to determine if the observed changes in water quality are consistent over time.
Cover Crops: Monitoring data from the paired watersheds show that approximately 70% of the dates in which nitrate concentrations were similar or higher in the treatment than reference watershed occurred when winter cover crops should function to retain excess nitrogen in the field (between October and early May). TNC researchers hypothesized that stacking winter cover crops with wetlands in the headwaters of the treatment watershed could further reduce nitrate export during this critical time. In 2019, the TNC partnered with the McLean County Soil and Water Conservation District to pilot a cover crop project that enrolled 500 acres of cropland into winter cover crops in the treatment watershed. An extremely rainy fall hindered harvest and cover crop seed application – even so, a total of 311 acres were successfully seeded with fall cover crops. Cover crop acres planted in Bray Creek increased to 855 acres by 2021, but adoption in 2022 was again hindered by poor field conditions. Results from the paired watershed stacked practices study are forthcoming.
