How does N rate affect nitrogen losses?

By Charles Wortmann

Nitrogen losses to the environment are related to fertilizer nitrogen rate. The University of Nebraska-Lincoln nitrogen rate recommendations are intended to maximize profit per acre from fertilizer N use or to apply at the economically optimal N rate.

However, the changes in yield because of N rate are small near EONR and applying a few pounds of N per acre more or less than EONR has little effect on grain yield. Small changes in N rate have much less effect on the marginal profit due to fertilizer N use than on grain yield.

This raises the question as to how N losses might be reduced with a reduction in N rate that results in 5% less marginal profit to fertilizer N use. The objective of this article is to address this particularly for greenhouse gas emission expressed in carbon dioxide equivalents (CO_2e). The practice would have effects on other N losses, especially due to leaching, but these are much more highly variable across Nebraska croplands than the nitrous oxide (N₂O) and carbon dioxide (CO₂) emissions associated with fertilizer N use.

The table gives a comparison, averaged across two fertilizer N costs, for fertilizer N applied at EONR and applied at a rate that results in 5% less marginal profit because of fertilizer N use. The mean reductions in N rates are 51 pounds N per acre and 31% for corn following corn, and 36 pounds N per acre and 31% for corn following soybean. This results in mean reductions in grain yield and marginal profits to fertilizer N use of 9.1 bushels per acre and $11.76 for corn following corn, and 6 bushels per acre and $8.40 for corn following soybean.

The mean corresponding effects on CO_2e due to N₂O emissions plus CO₂ emissions associated with fertilizer N use are estimated to be reductions of 189 plus 143 for a total of 332 pounds CO_2e per acre for corn following corn, and 133 plus 101 for a total of 234 pounds of CO_2e per acre for corn following soybean. The costs to achieve these reductions are the loss in marginal profit because of fertilizer N use.

Therefore, the costs per ton of CO_2e reduction are $71.11 for corn following corn and $71.95 for corn following soybean if no credit is given to the corresponding reductions in leached N and in other N losses. If the value of the reduced leaching and other N losses is $5 per acre, then the mean costs per ton of CO_2e reduction are $40.76 for corn following corn and $28.92 for corn following soybean.

These results demonstrate that the practice of reducing N rates has the potential to be a highly cost-effective means of reducing CO_2e emission to the atmosphere. For example, the total cost of CO₂ capture, transport and near perpetual storage, such as capture for a coal-burning power plant, are estimated to range from $100 to more than $200 per ton CO_2e.

These calculations for the effects of reduced N rate do not consider the eventual cost of reducing federal subsidies associated with yield. The calculations have not been extended to rain-fed corn because of inadequate data for representing the great diversity of rain-fed corn responses to N across Nebraska's cropland.

The reductions in N loss to other processes are not addressed because of these complexities. However, using Nitrogen Loss Assessment Tool (N-LAT) for Nebraska, NebGuide G2249, the mean leaching loss with the reduced N rate compared with the EONR was 40% less for corn following corn on occasionally flooded Invale loamy sand in Merrick County. This was with 43% of N spring-applied as anhydrous ammonia and 57% applied in-season as UAN. The reduced N rate results in an estimated 18% less N loss to denitrification for this loamy sand soil.

The benefits of such a reduced N rate practice to society and the cost in terms of reduced profit to the corn grower should be widely shared. The cost-effectiveness of the practice for reduced CO_2e and N loss can be used in a quest for reallocation of some federal and other subsidies to crop production to compensate for the reduced profit associated with this practice.

Wortmann is a Nebraska Extension soil and nutrient management specialist.