Can application of enhanced efficiency fertilizers at planting reduce N losses from grain corn production in Ontario?

Objective

1. To determine if applying a nitrogen fertilizer treated for a timed release at planting results in similar reduction in N₂O emissions and nitrate leaching compared to application at the 6th leaf-stage in corn;
2. To evaluate if nitrogen rate adjustments in these two practices can lead to further reductions in N₂O emissions and nitrate leaching;
3. To compare life-cycle economic and environmental impacts of these two 4R practices, considering management changes associated with each practice.

Justification
Nitrogen is a key input for sustaining high yields in crops, but the fertilizer N uptake efficiency (FNUE – percentage of fertilizer N recovered in aboveground plant biomass during the growing season) in crops is relatively low (<50%) with conventional production practices (Cassman et al. 2002). Part of the applied N is incorporated into soil organic matter and inorganic N pools, but N not taken up by crops may be vulnerable to losses during the growing season and after crop harvest. Consequences of low FNUE include reduced water quality due to NO₃^-, enhanced greenhouse effect and stratospheric ozone depletion due to N₂O (Galloway et al., 1995). In addition, low FNUE in cropping systems represents a significant economic loss to farmers.

Nitrous oxide (N₂O) is a long-lived greenhouse gas, an important driver of global climate change, and a dominant ozone-depleting substance that continues to have a key role in the delay of stratospheric ozone recovery (Chipperfield 2009; Ravishankara et al. 2009). Atmospheric concentrations of N₂O have risen by 15% since pre-industrial times and current levels are increasing linearly by 0.25% per year (Prinn et al. 2000; NOAA 2013,http://www.esrl.noaa.gov/gmd/hats/combined/N2O.html). Soils are the largest single source of N₂O worldwide, representing more than half of the total natural and anthropogenic N₂O sources (Denman et al. 2007). Agricultural soils alone contribute almost half of all the soil N₂O emissions, and therefore, the agricultural sector has a great potential to lower its climate change impact (Reay et al. 2012).

N₂O is produced by the microbiological processes of nitrification and denitrification (Firestone and Davidson 1989) which are promoted by ammonium and nitrate concentrations in soils, respectively. Studies consistently have shown that by providing additional N, fertilization can greatly increase N₂O emissions (Conrad et al. 1983; Bouwman 1990; Aulakh et al. 1992; Maggiotto et al. 2000). High-yielding crops such as corn, which require relatively large rates of nitrogen (N) fertilizer are major sources of N₂O and leaching. Indeed, a large proportion of the GHG emission (47%) associated with corn production in Ontario is due to N fertilizer input (34% from soil N₂O; 13% from fertilizer production and supply, Figure 1) (Jayasundara et al., 2014). Given that corn is a major economic crop in Canada with 11.7 Mt of grain corn, worth Cdn$1.5 billion produced from 1.2 million hectares, it is important to mitigate these emissions.

Figure 1: Carbon footprint of corn showing contributions of synthetic fertilizer nitrogen (SFN) and supply and manufacture of SFN in relation to other sources. Data from Jayasundara et al. (2014).


Matching amount and timing of application to crop uptake has been suggested as a mitigation measure to reduce N₂O emissions and leaching N losses, and are an integral part of the 4R Nutrient Stewardship program (Bruulsema et al., 2009). In a recent 2-year study funded by CFI, we found that during a typical wet spring in Ontario, N₂O emissions were reduced by 58% when N application occurred at the 6th leaf stage in corn instead of at planting (Roy et al., in revision). In the same study, the highest N rate (218 kg N ha-1) increased grain yield only by 6% but N₂O emissions by 64% revealing the importance of using appropriate N rate for N₂O emission reduction.

The use of N fertilizer products formulated for better matching of availability with crop uptake is also part of the 4R nutrient strategy due to its potential to reduce N₂O emissions and other N losses to the environment (Mikkelsen et al., 2009). Performing a meta-analysis of 113 datasets from 35 studies, Akyama et al. (2010) concluded that enhanced-efficiency fertilizer formulated with nitrification inhibitors reduced N₂O emissions on average by 38%, while those with urease inhibitors did not have a significant effect. In a recent study, Halvorson et al. (2012) concluded that N source can be an effective management option to reduce N₂O emissions in irrigated no-till corn in the semiarid western US. Of the controlled-release N fertilizers evaluated (polymer-coated urea, stabilized urea, SuperU, and urea-ammonium- nitrate + AgrotainPlus), UAN+AgrotainPlus consistently had the lowest level of N₂O emissions with no yield loss. However, as Venterea et al. (2012) pointed out, mitigation practices are very site specific, dependent on local soil and climate, making the extrapolation of results to other regions difficult.

As stated by Stewart et al. (2009) "As farm size has increased, the demand is greater than ever for growers to fine-tune logistics of planting and input timing". The application of UAN at the 6th leaf stage in corn is a feasible option for producers in Ontario but often application at planting is preferred due to reasons related to time management in large operations. In addition, particularly on soils of finer texture and slower drainage, excess soil moisture conditions at the 6^th leaf stage may sometimes preclude timely application. Extension specialists are often asked if applying UAN mixed with inhibitors of nitrification and/or urease at planting might be equivalent to applying regular UAN at the 6^th leaf stage. If effective, this strategy would improve N management while requiring only one field operation. To our knowledge, previous studies have not investigated this comparison under field conditions.

Measurement of N₂O fluxes under field conditions presents challenges due to the large temporal variability in emissions (Wagner-Riddle et al. 2007). Often, a large percentage (>50%) of the annual total emission occur over short-time periods such as spring thaw or after rainfall and fertilizer events. In addition, management during the growing season can affect emissions during the following non-growing season (Wagner-Riddle and Thurtell 1998). Hence it is important to monitor the effect of mitigation practices through year-round measurements of N₂O fluxes. In a review of challenges in mitigating N₂O emissions induced by inorganic fertilizers, Venterea et al. (2012) highlighted the need for studies on the effect of timing and source to also investigate the combined effect with reduced N rate. This is based on the rationale that alternative practices might achieve increased N use efficiency and therefore, allow for reduced N rate while achieving yield goals.

Few field studies have simultaneously measured N₂O emissions and N losses through leaching in year-round studies (Jayasundara et al. 2007). We have a long-standing experience with state-of-the-art methodology for continuously measuring N₂O flux at the field scale through the use of micrometeorological methods, while also quantifying nitrogen use efficiency, leaching losses and other important agronomic variables such crop biomass accumulation and yield (e.g. Jayasundara et al., 2007, 2010; Wagner-Riddle et al., 1997, 2007). For example, in Jayasundara et al. (2007) we demonstrated that cumulative NO₃-N leaching loss over a 4-year period was decreased by about 50%, from 133 kg N ha^-1 in a conventional (CONV) system to 68 kg N ha^-1 in a best management (BM) system. About 70% of the total NO₃ leaching loss occurred during corn years with fertilizer N directly contributing 11–16% of leaching in the CONV and less than 4% in the BM system. Higher soil derived N leaching loss in the CONV system, which occurred mostly (about 80%) in the non-growing season (November to April) was due to significantly higher (45–69%) soil derived mineral N left in the soil at crop harvest, and due to on-going N mineralization during the period from crop harvest to soil freeze-up. It is important to conduct similar comprehensive studies to provide solid scientific evidence for the 4R practices using year-round studies. This is true because mechanisms of N loss are interlinked in the N cycle and influenced differentially by soil-plant-water relations, thus measures to address one loss mechanism may lead to increasing losses from another. Our research group has the experience, expertise, and infrastructure to conduct the research needed to support the development of 4R practices that minimize N losses to the environment through both leaching and N₂O emissions.

In addition to environmental impacts of N use in cropping systems, it is important to consider economic and social aspects of the 4R Nutrient Stewardship program (Bruulsema et al., 2009). According to Wagner-Riddle and Weersink (2011) determining the socially optimal fertilizer management strategy requires understanding of the biophysical aspects related to best management practices (i.e. the 4Rs) and the profitability of those practices for the farmer. For example, Rajsic and Weersink (2008) and Rajsic et al. (2009) empirically examined the validity of the apparent reasons as to why producers may apply more than the recommended N rate. They examined the differences in ex-post optimal and ex-ante recommended application rates of N to corn on field trials over several years. Their results suggest farmers are not “wasting” fertilizer and that the over-application is a rational economic response due largely to risk. This highlights the importance of understanding economic and social drivers in producer decision making and potential adoption of the 4R Nutrient Stewardship program.

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Approach and Methodology
An experiment will be setup on four 2-ha plots to investigate objectives 1 and 2 using year-round continuous micrometeorological measurements over the 2015-2017 period. In addition, measured data on N₂O emissions and nitrate leaching will be combined to complete a full environmental impact assessment of the 4R practices including carbon dioxide use associated with the application method and product manufacture. An economic analysis of costs, and potential decision scenarios involved with the 4R practices studied will also be conducted.

Year-round Measurements:
We have pioneered an approach that takes advantage of the high temporal resolution and spatial integration of micrometeorological methods combined with paired plots for evaluation of management strategies (Wagner-Riddle et al. 1997, 2007). In this approach we can use up to four large plots (> 2ha) within a larger (~30 ha) homogeneous area (i.e. planted to the same crop) and set up individual monitoring towers in each plot for year-round measurements of N₂O flux at 30 min intervals. This high temporal resolution is tremendously valuable for monitoring the dynamic nature of N₂O emissions as induced by fertilizer, rainfall and spring thaw events. While the comparisons of treatments are limited in this approach, measurements provide a comprehensive assessment and data ideally suited for testing models of N₂O emissions.

The four 2-ha plots will be setup at the Elora Research Station managed by the University of Guelph, 15 km north of Guelph, Ontario. The soil at the site is classified as an imperfectly drained Guelph silt loam (29% sand, 52% silt, 19% clay), with average pH of 7.6 (water), organic carbon of 27 g kg^-1, total N of 2 g kg^-1, available P of 24 mg kg^-1 and available K of 146 mg kg^-1, in the 0-15 cm soil layer. Infrastructure such as an equipment trailer, high precision N₂O sensor, instrument towers and electricity are already in place at this site. The area will be planted with corn in each year of the study. Contrasting nitrogen fertilizer management will be applied to the plots as shown in Table 1. Two of the 2-ha plots will receive the recommended N rate (150 kg N ha^-1) applied at planting as UAN + Agrotain Plus, which contains urease and nitrification inhibitors, during the first two years of the experiment. The two other plots will receive UAN at the 6th leaf stage. The placement of the fluid fertilizer will be by surface broadcast using streamer nozzles similar to current farm practice for the UAN + Agrotain Plus (before planting), and using UAN injection at the 6^thleaf stage. Although this adds a confounding placement effect, we think it is important to use the typical method of application that is used by producers at these two different times. During the last 2 years of the project we will adjust the N rate of two of the plots according to a pre-plant nitrogen test. This will address the need to study practices that enhance NUE with reduced N application rate commensurate with the increased efficiency.

The N₂O flux will be obtained using the flux-gradient method which requires the measurement of differences in N₂O concentration between two heights above the crop or soil surface and turbulence parameters through the use of cup and sonic anemometers. The concentration of N₂O will be measured using a tunable diode laser trace gas analyzer (TGA100, Campbell Scientific, Logan, UT).


Table 1: Proposed nitrogen fertilizer management for the micrometeorological plots over the 2015 to 2017 period.

Years	Plot (2-ha each)	Fertilizer Treatment	Nitrogen Rate
2015, 2016	1, 2	UAN + Agrotain Plus at planting	150 kg N ha-1
	3, 4	UAN at 6th leaf stage	150 kg N ha-1
2017	1	UAN + Agrotain Plus at planting	150 kg N ha-1
	2	UAN + Agrotain Plus at planting	Adjusted according to requirements
	3	UAN at 6th leaf stage	150 kg N ha-1
	4	UAN at 6th leaf stage	Adjusted according to requirements

Soil solution samplers will be installed at 8 locations within each plot at a depth of 80 cm. Nitrogen loss by leaching will be estimated from the measured mineral N (NO₃ –N and NH₄-N) concentrations in soil solution and estimates of the drainage volume according to Jayasundara et al. (2007). In this approach drainage is estimated by the water balance method as described in McCoy et al. (2006). Briefly, hourly measurements of precipitation, soil water storage, and soil temperature are obtained. Runoff and interception are calculated as the difference between measured increases in soil water storage and total rainfall during each significant rain event when the soil is not frozen. Evapotranspiration is measured using the eddy covariance method. Instrumentation needed for all proposed measurements is available in-house.

Supporting data such as soil water content and temperature, soil nitrate and ammonium content, plant biomass and N content and LAI will be obtained throughout the year through detailed soil and plant sampling. Corn yield will also be measured.

Life-cycle Assessment of Greenhouse Gas Emissions:
For each 4R practice we will use measured N₂O emission and nitrate leaching data and coefficients of CO₂ emission associated with fossil fuel in field machinery use (e.g. UAN injection) to assess the environmental impact. These data will be compared to baseline data calculated at the county level in Ontario for 2006 (Jayasundara et al., in preparation) demonstrating the impact of 4R practices on the overall carbon footprint of corn production in Ontario. This analysis will also consider economic aspects and potential decision scenarios involved with the 4R practices studied. According to Bruulsema et al. (2009) there are a range of performance indicators (e.g. yield; N, energy and water use efficiency; net profit; farm income) that could be considered to assess the 4R Nutrient management practices. The relative importance of these needs to be determined with stakeholder input (Bruulsema et al., 2009) and how to combine the various facets of 4R nutrient stewardship remains a challenge. We will work with producer, government and industry representatives to conduct this LCA analysis, ensuring outcomes that address the current needs.