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. However, these sites are expensive to develop and maintain. In addition, the number of treatments that can be compared is limited due to physical constraints on the number of plots. With a common need to determine N reduction practice effects with multi-year 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. If successful, surrogates could be used on land that is not suitable for tile drainage, but where ground water recharge supplies water (and potential nitrate-N) to surface water systems.
The objective of this project is to relate two measurement methods as surrogates to tile-flow nitrate-N concentrations, then measure the effects of two N reduction management practices on nitrate-N via the surrogate measures and loss in tile drainage.
The research 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 water quality tile drainage sites in Iowa will be used, with two nitrogen management practices. Other on-going data being collected also will be available, such as crop production practices, crop yields, and grain N content.
Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.
Key Research Questions
This study was designed to investigate alternate nitrate measurement methods in lieu of nitrate in tile drainage flow. The objective was to determine if surrogate measurement methods would allow evaluation of in-field N management practices and provide reasonable estimate of nitrate-N concentrations comparable to tile flow drainage. This research included two measurement methods, nitrate-N in water collected with suction cup lysimeters and the soil profile.
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, 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 is a summary 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.
• There can be no estimate of discharge flow (nitrate-N load) with lysimeter or soil sampling.
• Cannot flow-weight lysimeter or soil nitrate-N concentrations across time.
• Lysimeter water nitrate-N concentrations, when restricted to the soil sampling dates, correlated to tile water nitrate-N concentrations, but only at one of the two water quality sites (NWRF only).
• Correlations improved when all lysimeter sample dates were included and separated by each crop phase (but again only for NWRF site in 2018).
• The often low correlation of lysimeter nitrate-N concentrations with tile water nitrate-N concentrations is partially explained by the relatively consistent tile water nitrate-N concentrations across time but the varying and sometimes much higher lysimeter water nitrate-N concentrations.
• Predictability of tile water nitrate-N concentrations is limited because of the trend for lysimeter nitrate-N concentrations to be higher than in the tile water, and more variable.
• Soil profile nitrate-N concentrations correlated (but poorly) to tile water nitrate-N concentrations, but only at one of two water quality sites and only one sample depth (only NWRF, r = 0.23 for the 12-24 inch depth).
• Soil nitrate-N concentrations correlated with lysimeter water nitrate-N concentrations at both water quality sites and both depths.
• Across measurement-yearsummary of lysimeter water nitrate-N concentrations indicated similar management practice trends as tile water nitrate-N concentrations in many but not all cases, but not the same concentration levels.
• Using only two N management practices for comparison per site limited ability to determine practice effects; more replications would be helpful and possibly more management practices studied.
• Banded N fertilizer (in this study anhydrous ammonia at both water quality sites) increases small-scale within-plot spatial variation and thus likely variation issues with use of lysimeters for sampling soil water for nitrate-N concentration and determination of N management practice effects.
• Nitrogen reduction practice effects on water quality (nitrate) would be best determined with monitoring of tile drainage flow.
Suction cup lysimeters have potential use for determining nitrate-N reduction practice effects, especially for practices with large effects on nitrate-N. Relating to (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. As discussed, 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.