A new perspective when examining maize fertilizer nitrogen use efficiency, incrementally

Authors: Kitchen, Newell; Ransom, Curtis; SCHEPERS, JAMES - University Of Nebraska; HATFIELD, JERRY - Retired ARS Employee; MASSEY, RAYMOND - University Of Missouri; Drummond, Scott

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2022
Publication Date: 5/11/2022
Citation: Kitchen, N.R., Ransom, C.J., Schepers, J.S., Hatfield, J.L., Massey, R., Drummond, S.T. 2022. A new perspective when examining maize fertilizer nitrogen use efficiency, incrementally. PLoS ONE. 17(5). Article e0267215. https://doi.org/10.1371/journal.pone.0267215.
DOI: https://doi.org/10.1371/journal.pone.0267215

Interpretive Summary: For grain crop production, efficiently using plant-available nitrogen from any source (fertilizer, manure, irrigation water, soil mineralization) is a challenge because nitrate-nitrogen is soluble in water and thus subject to losses via agricultural field runoff and leaching through porous soils or losses to the atmosphere from waterlogged soils. This is especially important for corn production since of the six top grain crops globally, corn accounts for more than 40% of the total grain production and about 37% of the total nitrogen fertilizer used. Efforts to track and improve nitrogen fertilizer management have traditionally used equations based on fertilizer nitrogen use efficiency (NUE). These estimates are commonly based on the concentration of nitrogen in the grain associated with a single nitrogen fertilizer rate for a growing season. Expressed this way, it represents an average NUE. However, since corn yield response exhibits diminishing returns to increasing nitrogen fertilizer, so will NUE decrease. Thus, expressing NUE as an average over-simplifies the relationship between nitrogen rate and NUE. In this investigation we used empirical datasets of corn grain yield response to nitrogen fertilizer to create and interpret incremental NUE (iNUE), that is the change in NUE with each unit of fertilizer applied. We found, for those last few units of nitrogen fertilizer applied to reach an economic optimal nitrogen rate (EONR), iNUE is quite low, less than 10%. If low iNUE exists at EONR, then the problem is exacerbated when rates exceed EONR, as is the case when insurance nitrogen applications are made. Nitrogen fertilization creating low iNUE is environmentally problematic. Further analysis found that growing-season rainfall characteristics and secondarily soil properties had the most influence on iNUE. Since the forecast weather trends for the US Midwest are for greater annual precipitation, more of the annual total in the spring, and more variable summer rainfall, improved weather forecasting offers one of the best options for improving nitrogen management and subsequently improving NUE. Examining iNUE creates a unique perspective and ideas for how to improve nitrogen fertilizer management decision tools, educational programs, and public policies and regulations.

Technical Abstract: For maize, nitrogen fertilizer use is often summarized from field to global scales using average NUE (nitrogen use efficiency). But expressing NUE as averages is misleading because grain increase to added nitrogen diminishes near optimal yield. Thus, environmental risks increase as economic benefits decrease. Here, we use empirical datasets of maize (Zea mays L.) grain yield response to N fertilizer to create and interpret iNUE (incremental NUE). We found, for those last units of nitrogen applied to reach EONR (economic optimal nitrogen rate) iNUE for nitrogen removed with the grain is low, less than 10%. Nitrogen fertilization creating low iNUE is environmentally problematic. Examining iNUE creates unique perspective and ideas for how to improve nitrogen fertilizer management tools, educational programs, and public policies and regulations.