Robert Kröger, a South African Ph.D candidate who was completing his studies at the ARS laboratory in Oxford, decided to investigate whether drainage ditches could help mitigate nutrient loads in field runoff.
“You can manage nutrients that contribute to the development of oxygen-deficient ‘dead zones’ in downstream ecosystems in three ways,” says Kröger, who is now a research professor with Mississippi State University. “You can just not use them; you can use edge-of-field systems like buffer strips to capture them; or you can use drainage ditches—and every single agricultural landscape has a drainage system.”
For 2 years, Kröger and Moore collected runoff samples from two Mississippi drainage ditches adjacent to experimental no-till cotton fields. They collected monthly samples and also obtained samples of runoff generated by storms.
Sampling analysis indicated that the ditches alternated throughout the year between being a sink and source for dissolved inorganic phosphorus and particulate phosphorus. Around 5.5 percent of the fertilizer applied annually to the fields was transported into the ditches, where around 44 percent of inorganic phosphorus in the runoff was removed by attaching to ditch sediments or vegetation before the runoff was discharged.
The ditches reduced runoff concentrations of dissolved inorganic phosphorus during the growing season by 61 percent. When the fields were fallow, average loads were decreased 47 percent. But it wasn’t possible to determine whether some of that phosphorus load was from the accumulation of “legacy” nutrients—those that linger in the soil years after they are applied.
The team used the same experimental fields to determine whether the ditches also helped reduce inorganic nitrogen from field runoff. Runoff samples collected during the 2-year study contained 2.2 percent of the initial fertilizer application, but only 1.1 percent of the inorganic nitrogen remained in the runoff when it was discharged from the ditch. This means that the ditch was responsible for reducing runoff levels of inorganic nitrogen by 57 percent over 2 years.
The researchers wanted to see if they could make these good results even better. They were already familiar with the riser pipes producers placed at the edge of drainage ditches to create a dam that temporarily impounds runoff. This reduces runoff volume and velocity, which in turn reduces field erosion. It also helps raise the water table, which improves crop access to soil water.
The team installed low-grade weirs—small dams—at several points throughout three drainage ditches. They also placed riser pipes at the ends of three other ditches. Then they conducted two simulated nutrient runoff events and tracked nutrient loads in each impoundment pool for the next 7 days.
The scientists observed that levels of several nutrients in the trapped runoff dropped significantly 7 days after the runoff event began. Average median levels of dissolved inorganic phosphate dropped 93 percent, and average median levels of total organic phosphate dropped 87 percent. Average median nitrate levels declined 97 percent, and average median ammonium levels dropped 76 percent. This indicated that using either riser pipes or weirs to temporarily trap field runoff could increase the capacity of drainage ditches to lessen runoff nutrient loads.