Tile Water Monitoring 2018, Part 4: Bioreactors05/28/2019 | Soil Health, Water Quality
This is the fourth installment of a series on the Iowa Soybean Association’s tile water monitoring program and aggregated results from 2018.
By Anthony Seeman, ISA environmental research coordinator
Bioreactors are used for nitrogen reduction, not tied directly to production, by treating water at the edge of the field before it enters the local stream. Results from the 2018 Iowa Soybean Association (ISA) tile water monitoring program illustrate that bioreactors are successful at reducing nitrate-N but they pose a challenge when measuring amounts of water flowing through the structure.
Woodchip bioreactors use a control structure to divert some tile water into an underground trench of woodchips. Another control structure at the other end of the trench allows for water to exit slowly so naturally occurring bacteria convert the nitrate to nitrogen gas.
There are two keys to optimum bioreactor performance. The first is to divert a significant portion of the water through the woodchips. The challenge is that the control structures are designed for a set level of water to be diverted, and any additional water will bypass the trench. This allows for maximum drainage from the tile line and avoids excess water in the field. Before spring field operations begin, the water level should be lowered to ensure field operations can proceed normally. After planting, the level in the bioreactor should be raised to force more water through the woodchip-filled trench. However, if conditions are very wet, some water will likely still bypass the trench.
The second key for high performance is to slow the water down in the trench so the bacteria can do their job. If the water moves through the bioreactor too fast it doesn’t achieve the anerobic conditions the bacteria thrive on, and there isn’t enough time to consume a large amount of nitrates. This also requires careful management of the water level leaving the bioreactor.
As part of ISA’s tile water monitoring program, nitrogen concentrations are measured before entering and after exiting the bioreactor. This helps the producer and technical service provider know how well the bioreactor is working and whether management changes need to be made.
It is difficult to monitor how much water goes through a bioreactor. Expensive equipment is required to record the water level in the control structure every hour and therefore not practical to equip all bioreactors with this instrumentation. But ISA has collected data to explore the bioreactors’ general performance.
Bioreactors have been installed across the state using various systems, capturing a range of incoming concentrations. Figure 1 shows the 2018 average incoming and outgoing nitrogen concentrations from 19 bioreactors with at least five paired measurements. Individual nitrogen measurements ranged from 0.1 to 34.2 milligrams per liter (mg/L) incoming and 0.1 to 26.5 mg/L outgoing. Annual average reductions ranged from 25 to 91 percent.
Figure 2 shows percent reductions grouped by month. These values are similar to percentages recorded in the past. In the spring, high flows and cool temperatures limit the amount of denitrification. The same situation in the fall was unusual but expected with all the precipitation that kept water flow at high levels.
While not perfect, these results are generally in line with the values listed in the Iowa Nutrient Reduction Strategy (NRS). To achieve the maximum performance, bioreactors need to be designed and managed properly. Monitoring data from a bioreactor’s flow enables farmers to manage the settings to keep denitrification rates high.
Bioreactors work, but they are not a silver bullet solution for nitrogen reduction. Other edge-of-field practices used for nitrogen reduction include saturated buffers, wetlands and ponds. To meet the Iowa NRS goals, farmers and landowners will need to use a suite of practices, both in-field and edge-of-field, on many acres across the state.
Contact Anthony Seeman at email@example.com.
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