Evaluation of Measurement Methods as Surrogates for Tile-Flow Nitrate-N Concentrations

Aug 2016


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.

Project Updates

December 2019


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

August 2019

The project field component was completed as planned at both locations in the fall 2018: the Agricultural Drainage Water Quality site (Gilmore City) and the tile drainage study site at the Iowa State University Northwest Research Farm (Sutherland). Final soil and lysimeter water samples were collected in the fall 2018, with soil and lysimeter water nitrate-N concentration analysis completed, data checked and databases finalized. Data analyses were conducted, data summarized and a final project report was developed for the Iowa Nutrient Research and Education Council/INREC co-funding source for the project.

In the spring before nitrogen application and crop planting, all suction lysimeters were removed from both project sites.

Initial work began on writing a journal paper and writing activities continued for the INRC final project report. A presentation of preliminary project results was made to the INREC board.

December 2018

The project continued as proposed at both locations, the Agricultural Drainage Water Quality site (Gilmore City) and the tile drainage study site at the Iowa State University Northwest Research Farm (Sutherland).

As per the project protocol, lysimeter water samples were collected in approximately two week intervals and in each crop until soil freezing. Profile soil samples were collected approximately in two week intervals after crop harvest and until soil freezing. Collected water samples were submitted to the water quality lab for analysis, with analysis results received for all samples. Soil samples were dried, with submission to and nitrate-N analysis received from the soil test lab. Data entry was completed for 2018 sampling results. Exploratory results analysis continued.

Although not a direct component of the project, corn and soybean crops were harvested from all plots, with yields measured. Tillage following corn per the treatment protocol was completed at the Iowa State University Northwest Research Farm (Sutherland). The treatments sampled at the Agricultural Drainage Water Quality site (Gilmore City) are all no-tillage. Per the project protocol, the fall 2018 lysimeter and soil sampling ends the field research component of the project.

First year results were presented at the American Society of Agronomy and Crop Science Society annual meeting in November.

March 2018

A project database was created, with complete inclusion of all research data from 2017. Preliminary analysis of the first-year data was hampered by the dry conditions in 2017, which limited tile-flow drainage data from the two research sites and concurrent soil profile and lysimeter water collection. A review of methods for measuring nitrate in soil and soil water profile was completed with a report summarizing the methods. After review of data from an exploratory implementation of two resin-bead lysimeters in 2017, a decision was made to install six of these lysimeters at the Gilmore City Ag Drainage Water Quality Site for the 2018 crop year. 

December 2017

Water and soil sampling resumed in fall 2017 when the soil was rewetted by rainfall and water could be collected from the majority of lysimeters. However, tile flow had not resumed in most plots at either site. All water samples collected during the 2017 crop year were analyzed by the ISU water quality lab for nitrate-N concentration. Soil samples collected in the fall were prepared for submission to the lab for nitrate-N concentration. Data entry, management and exploratory results analysis continued. The review of methods for measuring nitrate in soil and soil water profile continued, and a summarization for that information developed. Lysimeters remain in place for the second year of study.

September 2017

Lysimeter water and soil samples were collected until mid-July when tile flow ceased. The collected water and soil samples were analyzed for nitrate-N concentration. It was impossible to continue to collect water from an adequate number of lysimeters due to the dry period beginning in July. The project plan was to stop soil water and soil sampling when tile flow ceased, which occurred at both locations. Sampling will resume after adequate precipitation to wet the soil profile. Data entry, management, and exploratory results analysis is in progress. 

June 2017

Corn and soybean were planted at two locations, the Agricultural Drainage Water Quality site (Gilmore City) and the tile drainage study site at the Iowa State University Northwest Research Farm (Sutherland). All suction lysimeters were installed in early May, soon after each crop planting and before emergence. Nitrogen management treatments being studied include rye cover crop or no rye cover crop, and fall versus spring anhydrous ammonia application. Profile soil and lysimeter water samples were collected in two-week intervals and in each crop. The collected water samples were submitted to the water quality lab for analysis. Soil samples were dried and prepared for submission to the soil test lab for soil nitrate analysis. Some additional field supplies were purchased, including a vacuum pump, to increase the efficiency of lysimeter water sampling. A few faulty lysimeters were identified, with adjustments that either fixed an issue or the lysimeter was identified for replacement. 

March 2017

A literature review of methods to measure soil nitrate and the potential for a relationship to nitrate-N concentrations in tile drainage was done. Investigation continued into the type and cost of suction lysimeters and associated supplies to use in the field. The suction lysimeters, associated supplies and soil coring equipment were purchased. Multiple project meetings were held to finalize lysimeter installation location within treatment plots, installation procedures and soil sampling procedures, and soil sampling location. Laboratories were selected to analyze the collected water and soil samples. Each water quality site was visited to become familiar with the plot layout, plot orientation and treatment locations.

December 2016

This project got underway in December with the hiring of a postdoctoral researcher. A literature review of methods to measure soil nitrate and potential for relation to nitrate-N concentrations in tile drainage began. There were initial investigations into the type of suction lysimeters and associated supplies to use in the field, including cost. A project meeting was held for personnel introductions and to start discussions about specific treatments to study, study locations, supplies needed, installation designs, and initial plans for purchase.