Evaluation of Measurement Methods as Surrogates for Tile-Flow Nitrate-N Concentrations
Nitrogen management practice effects on nitrate loss to surface waters are best determined through measurement of nitrate-N concentrations in tile water flow at specially developed water quality sites. These sites, however, are expensive to develop and maintain. Plus, the number of treatments that can be compared is limited due to physical constraints on the number of plots. With a need to determine N reduction practice effects with multiyear rotations, such as corn following soybean, the number of drainage plots available for different practice evaluation becomes even more limited.
Surrogate methods need to be developed that allow evaluation of N management practices that reasonably estimate nitrate-N concentrations comparable to measurement in tile flow drainage. Such surrogate methods would allow many more practices to be evaluated, and support agronomic research evaluating effects on crop production and N use efficiency. If successful, surrogates could be utilized on land that is not suitable for tile drainage, but where ground water recharge supplies water, plus potential nitrate-N, to surface water systems.
This project will include two surrogate methods, measuring soil nitrate-N and soil solution nitrate-N concentrations. Sampling will be done by probing the soil profile and with suction lysimeters. To make the comparison with tile-flow nitrate-N concentrations, two existing tile drainage sites will be used, with contrasting N management practices. The project sites will be at the Iowa State University Northwest Research Farm, Sutherland, and the Ag Drainage Water Quality Site, Gilmore City. At Sutherland, N treatments studied will be fall-applied anhydrous ammonia with N-serve and spring-applied anhydrous ammonia. At Gilmore City, treatments will be springapplied N with and without a rye cover crop. Both sites will be in corn following soybean, and measurements will be taken in both crop phases.
Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.
This study was a unique opportunity to investigate different nitrate-N measurement methods (surrogate to tile drainage water nitrate-N – suction cup lysimeter water and soil profile nitrate-N) at two on-going water quality research sites utilizing tile drainage monitoring. And also the opportunity to study two different in-field nitrate loss reduction management practices, rye cover crop (with and without RCC) and fertilizer N application timing (fall and spring anhydrous ammonia). Following are highlights of the study findings.
- Lysimeter water nitrate-N concentrations were quite variable in time and space.
- Soil nitrate-N concentrations were variable and decreased with crop N uptake.
- Dry conditions limited lysimeter water sample collection, but not soil sample collection.
- Nitrogen-reduction practice effects on water quality (nitrate) would be best determined with monitoring of tile drainage flow, which integrates the entire soil volume being drained, across time scales and across small-scale spatial variation caused by some N management practices (example banded N). Suction cup lysimeters have potential use for determining nitrate-N reduction practice effects, especially for practices with large effects on nitrate-N. Predicting tile water nitrate-N concentrations has limitations, including larger variation and differences in relative concentrations. Use of lysimeters will require multiple and perhaps specific within plot locations, multiple depths, frequent and many sample water collection times, and measurement in multiple years, especially, when N is banded into concentrated zones.
- Sampling for soil profile nitrate-N has limited potential as a surrogate measure as related to tile water nitrate-N. Soil sampling has potential for determining differences between management practices, but sampling would likely need to be limited to the springtime before crop uptake becomes rapid and influences nitrate-N levels in soil or a management practice supplies nitrate back to the soil system (such as a cover crop).
- There was a quite different level of lysimeter surrogate measure success between the two sites studied, with the reasons not fully understood. Several aspects of surrogate measures need to be understood and carefully considered if implemented as a method to determine potential water quality effect of various in-field N reduction practices.
More details are available in a complete final report at: https://www.cals.iastate.edu/inrc/files/project/files/sawyer-_evaluation_of_measurement_methods_as_surrogates_for_tile-flow_nitrate-n_concentrations_finalreport_inrec_12_2019_0.pdf
Other activities and accomplishments
- 1 field day, 5 presentations