15Nitrogen Uptake and Use Efficiency in Corn in Response to Fertilizer Rate and Timing

J. A. Spackman1, F. G. Fernandez2
1University of Idaho, 2University of Minnesota
Background and Materials

Background

  • Midwest corn (Zea mays L.) demand for nitrogen (N) is often met by supplementing soil N with fertilizer additions.
  • Accurate estimates of the annual fertilizer-N rate are challenging because soil N supply varies with changes in soil temperature, moisture, crop management, etc. 
  •  Over-application of fertilizer-N represents economic loss, reduced fertilizer use efficiency, and increased risk of N loss to the environment. 
  • Urea is increasingly becoming the dominant N fertilizer source because of its high N content (46%), ease of transportation, and is relatively safer than anhydrous ammonia  

Methods

  • Corn was planted at six sites of contrasting soil textures during the 2014 to 2015 or 2015 to 2016 growing seasons
Treatments
  • Fertilizer treatments were 0, 45, 135, and 225 kg urea-N ha-1 broadcast applied at planting.
  • An additional treatment was split-applied as 45 kg aqueous urea-N ha-1 band-applied next to the seed immediately after planting and 90 kg urea-N at the four collared leaf stage (V4).
  • 5 atom% 15N enriched urea was applied to microplots within the treatment plots
Measurements
  • Grain yield, total aboveground plant N uptake, soil N (0-30, 30-60, 60-90 cm), and 15N content were measured at the eight collared leaf stage (V8), tasseling (R1), and physiological maturity (R6). 
Left: The microplot immediately after receiving 15N enriched urea fertilizer Left: The embedded microplot is visible as the darker colored soil. The 15N enriched urea fertilizer was broadcast applied as a urea solution. Right: The split application was done at the V4 corn development stage and broadcast applied while avoiding application to the corn tissue.
 
Left: Microplot residue was collected and removed from the field. Right: Following bulk plot harvest, the microplot residue was chipped and returned to the appropriate microplot area. The plots were maintained through the second growing season and received the same fertilizer treatments except no additional labeled 15N was applied.
 
Growing Season Precipitation, Soil Moisture Deficit, and Cumulative Water Loss
  • Growing season precipitation was generally greater-than-normal across all sites 
The charts below show daily precipitation and irrigation events, soil water deficit, and cumulative water losses (estimated using the checkbook method Steele et al., 2010 Appl. Eng. Agric. 6:983-996). On the bottom x-axis, F and f refer to fertilization events, PA is soil sampling eight days after 15N fertilizer application, V4 and V8 refer to the four and eight collared leaf stage, R1 is tasseling, R6 is physiological maturity, H is harvest and PH is post-harvest soil sampling.
  • Cumulative water losses were approximately 225 to 275 mm by the end of June following multiple large rain events that favored nitrate leaching. 
  • Irrigation maintained a low soil water deficit that favored additional water loss throughout the growing season on the 2014 loamy sand soil. A similar pattern was observed in 2015.  
  • On non-irrigated rainfed sites in 2014, the soil water deficit dropped below 50% (likely inducing water stress in the crop) mid-July. August rainfall replenished the soil profile only minimally increasing cumulative water losses. 
  •  Precipitation was more evenly distributed across the growing season in 2015 resulting in a generally low soil water deficit throughout the growing season and approximately 450 mm of cumulative water loss by post-harvest soil sampling.
  • Eight of the 12 field sites had linear grain yield responses to N rate indicative of significant N loss from the soil-corn system
  • It is expected that the Upper-Midwest will continue to experience greater-than-normal April through June precipitation that may favor significant N loss (USGCRP, 2017)
Fertilizer-Derived Nitrogen (FDN) Recovery in the Soil-Corn System
The value above each bar is the percentage of the total applied fertilizer-N recovered in the soil-corn system. PAY1 is soil sampling eight days post-15N application, V8Y1 is the eight collared leaf stage, R1Y1 is corn tasseling, PHY1 is post-harvest soil sampling and physiological maturity, PPY2 is pre-plant soil sampling in the second year, and PHY2 is post-harvest soil sampling and physiological maturity in the second year.
 
  • Fertilizer recovery in the soil-corn systems is greatest immediately following fertilization and is always less than 100%
  • Aboveground uptake of fertilizer-N and residual soil fertilizer-N increased with increasing N application rate. 
  • At V8Y1, the split application had at least two-fold more fertilizer-N recovered in corn biomass than any other treatment. Overall fertilizer-N recovery in corn biomass is similar between the 225 kg ha-1 and 45/90 kg ha-1 at PHY1.
    •   Split-applications apply part of the total N rate in-season avoiding early season precipitation events and coincides to actively assimilation of N by corn roots.  
  • Most fertilizer-N in corn biomass was exported in the grain at PHY1.
  • Averaged across sites, 32 to 39% of the fertilizer-N was recovered in soil-corn system indicating that 61 to 68% of the applied fertilizer-N was lost from the soil-corn system.
    • The mass of unaccounted fertilizer-N increased with increasing N rate and represents a large reactive N load potentially polluting ground- and surface-waters and greenhouse emissions.
    • The primary loss mechanisms were likely leaching on the loamy sand soils, volatilization on the silty clay loam soil (due to inadequate incorporation of surface-applied urea), and leaching and denitrification on the loam and clay loam soils.
  • Fertilizer-N in the soil at PPY2 is similar to soil + cob + stover at PHY1. This indicates that little fertilizer-N was lost from the system over the winter months when soils were frozen. 
  • By PHY2, only 13 to 16% of the applied fertilizer-N remained in the soil-corn system.
    • 1 to 4% of the first year fertilizer-N was exported in the grain. 
    • Fertilizer-N in the soil fraction was primarily organic N indicating stabilization of fertilizer-N in soil organic matter.
Corn Uptake of Soil- and Fertilizer-Derived Nitrogen at Post-harvest Soil Sampling in the Year of 15N Enriched Fertilization
Same letters between fertilizer treatments within a site-year are not significantly different at < 0.05. The red line denotes the change or lack of change of soil-derived N uptake with increasing N rate. 
 
No Added N Interaction
  • There was no difference in the amount of soil-N accumulated in aboveground biomass between the non-fertilized and single application fertilizer treatments at the sandy loam sites in 2014 and 2015, the silty clay loam site in 2014, and the clay loam site in 2014.
  • A lack of response between the non-fertilized and fertilized treatments indicates significant inorganic N loss forcing the crop to rely on soil mineralized N for uptake.
Negative Added N Interaction
  • The negative trend of soil-N uptake with increasing fertilizer rate was likely due to a high soil mineralization rate. Fertilizer application rates in excess of the amount needed to optimize corn grain yield resulted in pool substitution where labeled N substituted for unlabeled N taken up by the crop.
Positive Added N Interaction
  • The trend of increasing soil-N uptake with increasing fertilizer rate for the clay loam site in 2015 was likely due to pool substitution where labeled fertilizer-N was immobilized concurrent to soil-N being mineralized and available for crop uptake.
66 to 93% of the total corn N uptake across all six sites was derived from the soil.
  • While fertilizer N is an important supplement, this result re-emphasizes the importance of maintaining soil productivity and the soil's N supplying capacity.
Same letters between fertilizer treatments within a site-year are not significantly different at P <0.05.
  • Corn uptake of fertilizer-derived N increased with increasing N rate at all sites.
  • The split-application increased fertilizer derived N uptake 3.1-fold on average over the single pre-plant application on the loamy sand sites.
  • Fertilizer-N uptake was similar between the split application and the 225 kg N ha-1 treatment for the silty clay loam and clay loam sites.
  • In-season fertilizer applications improved plant fertilizer-N uptake relative to a single application at planting, but the benefit is greatest on soils prone to N loss or that have low mineralization potential.
Conclusion
  • Current fertilizer guidelines suggest that pre-plant urea is an acceptable N management strategy for most U.S. upper Midwest corn cropping systems. 
  • Precipitation and irrigation that maintains a low soil water deficit is likely to favor significant N loss from the soil-corn system.
  • In this study, <36% of the applied urea-N rate was recovered in the soil-corn system at harvest in the year of 15N enriched fertilizer application.
  • Split-applications significantly improved fertilizer-N uptake with the greatest benefit on loamy sand soils.  
  • If future spring conditions continue to saturate the soil profile favoring N loss, corn producers may need to shift away from pre-plant urea applications to other N sources or application timings to minimize environmental and economic losses.
Acknowledgments
We would like to thank the Minnesota Corn Growers for funding the project and the funders of the Hueg Harrison Fellowship, George Rehm Fellowship, and MnDRIVE Fellowship.
We also gratefully acknowledge the University of Minnesota Field Crew for their assistance in collecting and processing the soil and plant tissue samples.

Abstract

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