Conservation Drainage Network

Nitrogen

by

Design approach tradeoffs and annual performance prediction for denitrifying bioreactors treating subsurface drainage

Summary prepared by: Laura Christianson

Christianson, L.E., C.H. Hay, R.D. Christianson, B.M. Maxwell, and R. Cooke. 2026. Design approach tradeoffs and annual performance prediction for denitrifying bioreactors treating subsurface drainage. Journal of Environmental Management, 397, 128245. https://doi.org/10.1016/j.jenvman.2025.128245

Major Findings:

This paper describes monitoring at ten woodchip bioreactors that were built between 2016 and 2021 in Illinois. A total of 36 site-years of bioreactor performance were assessed. These results showed:

  • The ten bioreactors removed nitrate from the tile drainage water in all years, including both wet and dry years, which reiterates this practice’s dependability.
  • Across the dataset, the bioreactors removed 107 pounds of nitrate-nitrogen (N) per year which equated to reducing the annual N loss by 3.7 lb N/ac (median values; Figure 1a and 1b).
  • The bioreactors generally treated the majority of the annual drainage flow from the field and removed approximately half of the nitrate that entered (medians: 60% and 51% respectively; Figure 1c). This resulted in a median edge-of-field N removal efficiency of 17%.

Figure 1. Summary performance of 36 woodchip bioreactor site-years from Illinois, USA monitored at ten individual bioreactors. Boxes represent the 25th-75th percentiles, stems encompass the 10th-90th percentiles, and dots show the outliers. Inside each box, the solid line represents the median and dashed line represents the mean.

This study showed two notable tradeoffs in how bioreactors work and how we think about their design:

  • A given bioreactor was able to treat more water less well, or less water better. In scientific terms, annual bioreactor hydraulic retention time (or, the average length of time a parcel of water stayed inside the bioreactor for treatment) was negatively correlated with the annual percentage of flow captured but positively correlated with annual nitrate removal efficiency for the treated water. As you hold water longer inside a bioreactor, you achieve a greater nitrate removal efficiency for that water but then you treat a relatively lower fraction of water from the field because more flow will bypass the bioreactor.
  • Site-years with the highest edge-of-field N removal efficiencies (37–72 %) often removed less than 100 lb N annually. Site-years that removed the most pounds of N (for example, >220 lb N annually) exhibited a lower range of edge-of field removal efficiencies (7–36 %). This was because efficiency-based performance metrics such as % removal are limited in that they are, by definition, relative to inflow values. Across the dataset, as annual N loading from the drainage areas increased, for example in a wetter year or at a larger drainage area, the edge-of-field N removal efficiency decreased whereas annual N load removed by the bioreactors increased.

Contributions to the conservation drainage community

This study supported previous studies by showing that bioreactors effectively remove nitrate from tile drainage water.  These bioreactors worked in wet and dry years and at relatively small and large drainage areas (17 to 72 acres).

This study also highlighted a dichotomy in how we often think about bioreactor performance. Achieving a high nitrate removal efficiency (that is, a high % removal) was inconsistent with removing a high mass load of nitrate (that is, removing lots of pounds of nitrate). Design approaches need to better balance removing high nitrate loads for greater watershed benefit with appropriate site-level effectiveness targets.

This study points to the need for additional bioreactor design research to better tune design aims and performance objectives. In summary, (1) bioreactors work and (2) more research is needed to expand their application for greater water quality benefit at scale.

by

Effectiveness of saturated buffers on water pollutant reduction from agricultural drainage

Summary prepared by: Gabriel Johnson, Department of Natural Resources Ecology and Management, Iowa State University

Johnson, G., Christianson, L., Christianson, R., Davis, M., Díaz-García, C., Groh, T., … & Rogovska, N. (2023). Effectiveness of saturated buffers on water pollutant reduction from agricultural drainage. Journal of Natural Resources and Agricultural Ecosystems, 1(1), 49-62. https://doi.org/10.13031/jnrae.15516

Major Findings:

This paper reviewed published studies (2014-2022) on the performance and cost-effectiveness of saturated buffers in tile-drained landscapes. Results of this review documented:

  • Saturated buffers reduced annual nitrate losses by an average of 46% ± 24% at the edge of the field, corresponding to a rate of 8.4 ± 5.3 lb N/ac/y. Recent research shows that their performance may be even greater (Ghane 2025).
  • Within drainage diverted through the saturated buffer, 82% ± 22% of nitrate was removed on average.
  • Nitrogen reductions occur primarily through the process of denitrification, but plant uptake and microbial immobilization also play a role in the nitrogen dynamics within saturated buffers. In limited study, nitrous oxide losses were found to be nearly equal to traditional (unsaturated) buffers (8.4 lb N) and much less than adjacent cropland (32.4 lb N).
  • Cost-effectiveness for nitrate removal averaged $2.13 ± $1.27/lb N (ranging from $0.54 to $4.17/lb N), corresponding to $10 to $27/ac treated per year.

Figure 1. Saturated buffer operation diagram. Tile flows are routed through the control structure, with the diverted flow saturating buffer soils. Interactions between nitrate (red diamonds) and microbes contribute to denitrification and conversion to nitrogen gas (purple circles).

Contributions to the conservation drainage community

This review clearly demonstrates the value of saturated buffers as an effective practice in reducing nitrate-nitrogen loads from subsurface drainage discharge. A summary analysis of nitrogen removal performance as well as cost-effectiveness is provided.

An outline of future opportunities to advance saturated buffer research, performance and adoption is also provided. Key research areas include further exploring buffer nutrient and carbon cycling across varying contexts, sensitivity of performance to site suitability factors, and the influence of vegetation management in terms of nutrient budgets. Key areas for practice implementation and adoption include potential strategies to optimize designs and management for enhanced performance (e.g., alternative distribution pipe arrangements, optimized buffer widths, automated control structure management), as well as improved and streamlined processes to identify suitable sites for implementation. Future results from siting and design efforts should help inform existing conservation program standards (NRCS Conservation Practice Standards), while also identifying new, alternative saturated buffer applications (e.g., grass waterways, contour buffer strips). Also, innovative implementation models, such as the Batch and Build concept, can capitalize on efficiencies of scale, reducing implementation costs and improving cost-effectiveness.

Finally, to further support saturated buffer research, monitoring recommendations are outlined. These recommendations included recommending the use of 4-chamber control structures when monitoring flow rates across weir stoplogs, more intensive water sampling frequencies (particularly for phosphorus) to better understand concentration and load reductions, and the development of more robust methods to determine nutrient concentrations as discharge moves through the buffer to the adjacent stream.

by

Saturated buffer paired-field study reveals new insights into its water quality performance

Summary prepared by: Ehsan Ghane, Department of Biosystems and Agricultural Engineering, Michigan State University

Ghane, E., AbdalAal, Y., Tehrani, A. (2025). Paired-field evaluation of a saturated buffer reveals significant water-quality benefit through upstream weir management. Agricultural Water Management. 318, 109664. https://doi.org/10.1016/j.agwat.2025.109664

Ghane, E. (2025). Unveiling the true potential of saturated buffers (E3535). Michigan State University Extension Bulletin. www.canr.msu.edu/drainage

Major Findings:

We conducted a paired-field study at an on-farm site in Michigan, USA, comparing a saturated buffer (SB) system to a free drainage control field. Results showed:

  • The saturated buffer system significantly reduced nitrate-N load by 54.5% (11.1 kg/ha) annually compared to free drainage.
  • The upstream weir of the system, functioning as controlled drainage, was the primary driver behind the annual load reduction, accounting for most of the flow and nitrate-N load reduction.
  • High-level weir management (<35 cm weir depth below ground surface at structure) was more than twice as effective as low-level weir management (>35 cm weir depth below ground surface at structure), removing 0.076 kg/ha nitrate-N daily.

Contributions to the conservation drainage community

This study showed that in saturated buffer applications where a three-chamber structure is used, the upstream weir acts as controlled drainage reducing both flow and nitrate-N load in conjunction with the saturated buffer. Actual performance of saturated buffers can easily be underestimated when not acknowledging the functionality of the upstream weir. Using similarly structured paired-field studies in the future may help others explicitly quantify the influence of the entire saturated buffer system, which includes the upstream weir management as well as the flow through the buffer.

It was also clear during this study that even though the water-managed zone upstream of the control structure was only 6.6% of the total drainage area due upstream field slopes, the upstream weir management functioning as controlled drainage reduced nitrate-N load by more than 50% compared to the free drainage field. This suggests that prior assumptions around site suitability for controlled drainage requiring flat fields with slopes generally less than 1% may be overly restrictive, particularly when paired with complementary practices like saturated buffers.