The following methods were used to determine the minimum, mean, and maximum reduction in nitrate concentrations and the impacts on corn yield for each practice. These values were calculated using the same approach for most practices. However, for some practices the method was different, with those differences explained below. Nitrate-‐N concentrations were used rather than loads because tile,
subsurface, and overland flow can vary across the state, which would have an impact on calculated load reductions. See “Appendix A – Literature Reviewed” for more details on specific research studies used for each practice.
Although only nitrate-‐N reductions are used here, some of the practices may have other benefits such as phosphorus and sediment reduction (cover crops), or aesthetic and wildlife benefits (wetlands and buffers). Any additional benefits were not included in the economic analysis.
Nitrate-‐N Reduction Minimum and Maximum
Minimum and maximum values for the timing, source, nitrification inhibitor, energy crop, land retirement (CRP), cover crop, living mulch, extended rotation, bioreactors, and buffer practices were calculated based on individual site-‐years from each research study. For example, if there were 10 years of data for a
potential reduction practice and the highest resulting nitrate-‐N concentration for one of the years was 5% higher than the corresponding controlled comparison (control) practice, the nitrate-‐N removal of that practice in that year would be -‐5% (or a 5% nitrate-‐N concentration increase). If the lowest concentration for one of the years was a nitrate-‐N concentration of 25% lower than the corresponding comparison practice, the nitrate-‐N removal of the potential reduction practice would be 25% (or 25% decrease in nitrate-‐N concentration). The standard deviations for each practice were also determined based on the site-‐year data.
Nitrate-‐N Reduction Mean
The mean nitrate-‐N concentration reduction values were based on a corn-‐soybean rotation rather than individual crop years. In other words, the rotation concentrations resulting from the reduction practice were averaged, the result of which was divided by the average concentrations of the control practice and subtracted from 1. For example, assume there are 4 years of data for nitrogen application rate reduction in a corn-‐soybean rotation having a rotation average tile nitrate-‐N concentration of 2 for the first round of corn-‐soybean and 4 for the second round of corn-‐soybean. The comparison has 4 years of data at the “normal” nitrogen application rate with a nitrate-‐N concentration of 6 for the first round and 8 for the second round. The resulting mean tile flow nitrate-‐N reduction of the rotation due to reducing nitrogen application rate would be computed as in Equation 1.
Equation 1
Yield Calculations
Corn yields for the practices are calculated the same way for minimum and maximum values, however, the comparison is change in yield. Here a negative change is reduced yield, and a positive change is increased yield. Mean yield change for a potential reduction practice from the comparison practice is calculated by averaging all observed yields in the potential reduction practice, subtracting average observed yield of the comparison practice, then dividing by the average observed yield of the comparison practice.
Calculations Differing from Those Outlined Above
Reductions for other potential reduction practices required different approaches.
Nitrogen Application Rate
The nitrate-‐N concentration in tile flow water at a given fertilizer application rate was determined with an equation developed by Lawlor et al. (2008). Tile flow nitrate results from Lawlor et al. (2008) have been compared to other data from studies in Iowa and south-‐central Minnesota, and the data are in-‐line with the information from Lawlor et al. (2008) (Figure 1)
This data set was not adjusted for differences in rainfall, and, as mentioned earlier, long term increases or decreases in precipitation may influence this trend.
Figure 1. Nitrogen application rate effect from various studies on tile drainage nitrate-‐N concentration for a corn-‐soybean rotation compared to the tile-‐flow response curve developed by Lawlor et al. (2008).
Pastures
There was little pertinent data about nitrate-‐N concentrations coming from pastures in Iowa. The assumption was made that nitrate-‐N concentrations in water leaving the root zone are the same as for perennial energy crops.
Drainage Water Management
Drainage water management (controlled drainage) and shallow drainage have little, if any, impact on nitrate-‐N concentration. They do, however, reduce the amount of water leaving the system thus reducing the total nitrate-‐N load. In addition, there was little evidence that corn yield was significantly impacted by the practice. Minimum, maximum, and average load reductions are used instead of nitrate-‐N
concentrations. The values used are site averages, and do not include analysis across site-‐years.
Wetlands
Wetlands are dynamic systems and nitrate-‐N concentration reduction is dependent on design. A nitrate-‐N removal of 52% was assigned to this practice based on an annual project report by Helmers et al. (2008a) where the average wetland is 0.785% of the contributing watershed. Ultimately, practice performance will depend on the size of the wetland.
Bioreactors
Bioreactors also are heavily dependent on design, and could be sized to remove up to 50% or more of the nitrate load from a tile line. However, preliminary research in Iowa shows an average nitrate reduction of 43% from one study using the mean calculation procedure outlined above. These practices should have no impact on yield, as they are not installed in areas that would typically be farmed.