The goal of nitrogen (N) application is for crop uptake, resulting in improved yield and quality of the crop harvested. However, N fertilizer applications are also susceptible to emission losses, as ammonia (NH3) and nitrous oxide (N2O), and surface and groundwater losses as nitrate-N (NO3 –). In the past, research has focused on the rate of nutrient application on yield, quality, and N losses. As producers look to improve nutrient use efficiency, the need for research to address other factors in nitrogen application has grown.
Specifically, a growing number of researchers are looking at how the 4Rs of nutrient application—source, rate, time, and place—affect crop uptake and yields. The fertilizer industry established the 4R Research Fund to help establish sustainability indicators and environmental impact data for implementation of 4R nutrient stewardship across North America.
Improved N use efficiency can be achieved through combinations of rate, source, 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 as part of a 4R strategy
Enhanced-efficiency fertilizers (EEF) include slow- or controlled-release N fertilizers that are coated or encapsulated or fertilizers treated with nitrification and/or urease inhibitors. Selecting the use of an EEF in a crop management system encompasses the right source, right time, and right place components of the 4R nutrient stewardship concept (Snyder, 2016).
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 with untreated urea, Hopkins (2016) at Brigham Young University reported N2O and NH3 losses across three management systems in laboratory, glasshouse, and field studies. The fertilizer treatment rates ranged from 80.3 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% as NH3 and 2% 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%, with a range of 64 to 574% (Hopkins, 2016). The percent reduction in N2O loss was lower, 120% on average, with a range of 38 to 201%, 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.
By Sally Flis, Ph.D. and CCA Director of Agronomy, The Fertilizer Institute, Washington, DC