Improved Nitrogen use efficiency can be achieved through combinations of Source, Rate, Time, and Place. A recent roundtable meeting identified as a goal “broadening the focus of applied research beyond N rate to move toward more integrated agricultural systems,” and cited the 4R Nutrient Stewardship approach as a step to help represent the complexity of the farming systems (Reimer et al., 2017). Research continues to broaden the focus of nutrient research to address the 4Rs and not solely application rate.
Enhanced Efficiency Fertilizers (EEFs) as Part of 4R Strategy
The rate and source part of the equation was evaluated in a summary of research projects using EEFs in multiple management systems. The use of polymer coated urea (PCU) can reduce N loss as nitrous oxide (N2O) and ammonia (NH3) volatilization when managed correctly (Hopkins, 2016). In a recent summary of the use of PCU N fertilizer compared to untreated urea, Hopkins, 2016 at Brigham Young University reported N2O and NH3 losses across three management systems, laboratory, glasshouse, and field studies. The fertilizer treatment rates ranged from 80.3 lb/ac to 401.5 lb/ac across the potatoes, corn, and Kentucky bluegrass cropping systems (Hopkins, 2016). The results in the report focused on the different source on N applied across application rates and management systems.
When uncoated urea was applied, losses as a percentage of total N applied were 13 percent as NH3 and 2 percent as N2O (Hopkins, 2016). The use of a PCU versus uncoated urea resulted in lower NH3 and N2O emissions. On average NH3 emissions decreased by 300 percent, with a range of 64 to 574 percent (Hopkins, 2016). The percent reduction in N2O loss was lower, 120 percent on average, with a range of 38 to 201 percent and was significant for 11 of the 12 treatments (Hopkins, 2016). In addition to loss as NH3 and N2O, N applied as urea can convert to nitrate (NO3–) through nitrification and be taken up by plants, accumulated in the soil, or lost to leaching. In eight of the 12 treatments, PCU use resulted in less NO3– accumulated in the soils (Hopkins, 2016). Plant performance in these studies was not negatively impacted using PCU versus uncoated urea.
4R NH3 Mitigation Strategy Results
Understanding the relationships of mitigation strategies for NH3 volatilization is critical to improving global fertilizer N use efficiency, environmental quality, and climate change mitigation (Pan et al., 2016). Pan et al., 2016 identified 171 studies with 886 observations that were evaluated by N source, rate of application, timing or frequency of application, and surface versus deep placement of N fertilizer. The source of N fertilizer was evaluated two ways. First, as the type of fertilizer relative to urea. Second, as fertilizers treated with urease inhibitors and fertilizers treated with nitrification inhibitors (Pan et al., 2016).
Source – The source of N, when evaluated as the type relative to urea, affected NH3 volatilization. When a urea-mixed or non-urea fertilizer was used, there was a 75 percent and 31 percent decrease, respectively, in NH3 volatilization compared to when urea was used as the N fertilizer (Pan et al., 2016). Additionally, the use of a urease inhibitor or a controlled release fertilizer decreased the loss of NH3 by 54 percent and 68 percent, respectively (Pan et al., 2016).
Rate – The rate of N application was evaluated relative to the lowest N rate reported. Increased rates of N application increased NH3 volatilization (Pan et al., 2016). On average, increased N application rates increased NH3 volatilization by 186 percent (Pan et al., 2016).
Timing – Split applications of N fertilizer had a neutral effect on NH3 volatilization (Pan et al., 2016).
Place – NH3 volatilization decreased by 65 percent with deep placement of N fertilizer versus surface application (Pan et al., 2016).
4R Management Effects on N2O and NO3– losses
The fertilizer industry’s 4R Research Fund supported three meta-analysis, examining the literature for the effect of 4R management on the loss of N2O and NO3– in primarily corn and soybean systems in North America.
The effects of 4R management strategies on NO3– leaching losses to tile drainage discharge over 40 years of research in the Midwest was considered in the update of the Measured Annual Nutrient Loads for Agricultural Environments (MANAGE) database meta-analysis project (Christianson and Harmel, 2015).
Source – There were no dominate N sources used across the research projects included in the study nor was there equal representation of source across the projects. Further, the disparity in reporting the source of N fertilizer utilized did not allow for a comparison of NO3– loss by source (Christianson and Harmel, 2015).
Rate – Overall N application rate had the largest effect on the amount of dissolved N in the tile drainage discharge. As N application rate increased, regardless of source, NO3– loss to tile drainage increased (Christianson and Harmel, 2015).
Timing – Timing of N application did not affect the load of dissolved N in tile drainage discharge. However, in wetter conditions, dissolved N loss from tile drainage was higher than in drier conditions (Christianson and Harmel, 2015), indicating that timing should be considered as relative to rainfall or soil moisture.
Place – Placement or method of application of N fertilizer did not affect N loss as NO3– loss to tile drainage (Christianson and Harmel, 2015).
Two other meta-analysis have also been completed and are in the process of publication. Reports conducted by Alison Eagle of Duke University and Rachel Cook of North Carolina State University are available at http://www.nutrientstewardship.com/research. These two projects found results similar to Hopkins, 2016 and Christianson and Harmel, 2015.
Source – The use of an N EEF product reduced N2O losses.
Rate – N2O and NO3– losses increase with increased N application rates.
Timing – Timing had variable results depending on the type of N loss measured, N2O vs NO3–.
Place – N2O and NO3– losses were not influenced by where the fertilizer was placed.
The 4R Research Fund has field studies in Kansas, Iowa, and Illinois measuring the influence of different 4R practices on the losses on N2O and NO3– in corn and soybean cropping systems. Broad-scale evaluations like these meta-analysis are useful to identify the major trends in how 4R management impacts N cycling in the soil, plant, and water system, but site specific implementation based on good data collection is necessary to maximize N use efficiency at each location.
References
Christianson, L.E. and R.D. Harmel. 2015. 4R Water Quality Impacts: An Assessment and Synthesis of Fourty Years of Drainage Nitrogen Losses. J. Environ. Qual. 44:1852-1860.
Reimer, A. J. E. Doll, B. Basso, S.T. Marquart-Pyatt, G. P. Robertson, D. Stuart, and J. Zhao. 2017. Moving toward sustainable farming systems: Insights from private and public sector dialogues on nitrogen management. JSWC. 72:1, 5A-9A.