Bulletin #1086, Minimize Nitrogen Losses and Increase Nitrogen Use Efficiency

Developed by Sukhwinder Bali, Assistant Professor of Sustainable Agriculture, and Lakesh Sharma, Assistant Professor of Sustainable Agriculture, University of Maine Cooperative Extension

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Introduction

Potato field in blossom beside a river in Aroostook County

Nitrogen is an element that occurs in the atmosphere as dinitrogen gas (N₂). It represents 78% of all gas in the atmosphere. Ammonium (NH₄⁺) and nitrate (NO₃^–) are plant-available forms of nitrogen. Plants can absorb and use these forms of N through their roots. Nitrogen is an essential nutrient for plant growth and development. It is added to agricultural fields to increase crop yields, but it can be harmful to aquatic ecosystems.

The improper use or overuse of fertilizers — organic or inorganic — poses a threat to the environment. Excess use of N fertilizer throughout the world decreases the recovery and efficiency of N on arable lands, which ranges from 25% to 50% of applied N.^{1, 2} The rest of applied N is lost to the atmosphere, to soil leaching, and to runoff. Low nitrogen use efficiency (NUE) may lead to negative environmental and economic effects.

Apart from the increased use of N, the rapid increase of fertilizer prices is a significant concern for farmers. Savings on fertilizer costs equal increases in profits.

Here we discuss the various factors controlling NUE and potential methods for improving NUE to minimize environmental losses from agriculture.

How do we measure NUE?

NUE is the efficiency with which plants use and retain nitrogen in the soil. NUE measures how much nitrogen plants take in, and how much of this nitrogen intake is lost through gas exchanges of nitrous oxide from the plants. In the field, NUE is the amount of nitrogen a crop takes in and retains until harvest compared to the amount of nitrogen that was available for the crop to consume, based on how much fertilizer was applied to the soil.

NUE = N _Gain/N _Fertilizer

NUE = 1 when there is no loss to the environment

How can growers increase NUE and decrease costs?

So the question is how to minimize the losses and increase the efficiency of nitrogen use in the field. The first and most important principle should be to avoid overapplication of fertilizers. Growers should conduct soil testing every year to determine the soil status and nutrient needs for the next crop. The Analytical Lab and Maine Soil Testing Service provides standard soil testing for crop recommendations or specialized testing based on your needs.

Soil testing helps in making zone maps based on soil properties. Management zones are field areas with similar attributes in landscape and soil condition. Researchers suggest a variety of approaches for defining management zone boundaries. Geospatial data layers (e.g., cropping pattern, yield, and topography) combined using statistical analyses within a geographic information system (GIS) can specify zone boundaries. Geospatial data includes location information such as coordinates, address, and zip code. Satellite imagery is extracted using geospatial information. Land management, such as calculating fertilizer application rates, according to these management zones will help with nitrogen use efficiency.

Agronomic practices can reduce the need for chemical N fertilizers. One technique is planting a legume in rotation with the main crop. Legumes have symbiotic bacteria in their roots or root nodules that can fix atmospheric N and return it to the soil in plant-available form—ammonium. The more protein there is in the plant, the more nitrogen returns to the soil from nodules, roots, and plant material incorporated into the soil. It is estimated that two-thirds of N fixed by legumes is typically available for the next crop.⁴

However, natural and organic sources are not enough to supply the total crop need for N, so it is important to use the optimal amount of fertilizer at the right time for crop growth to reduce N losses to the environment. Typically, farmers apply N before planting. The most widely used fertilizer in Maine is ammonium sulfate. We know that fall N application is at risk for N loss in spring, which can lower crop yields. N loss occurs between application and absorption of N fertilizer through loss to the atmosphere, soil leaching, and surface runoff. In-season N application results in improved NUE as compared to preplanting or fall N application.

The most promising approach to improve NUE is to follow precision management, which is the management of inputs such as fertilizers to more precisely delineated needs. Outdated and uniform N recommendations that promote over-application of N to spatially and temporally variable landscapes produce low NUE and negative economic and environmental consequences.

Farmers need advanced functional tools to address spatial variation in fields. Sensors help in mapping soil sampling and properties such as soil pH, moisture, and nutrient levels, as well as crop yield. Sensors provide micro-scale information to farmers for accurate decision making about fertilizer application, irrigation, herbicide spraying, and harvesting.

Sensors and technologies that can identify crop nitrogen status throughout the growing season include remote sensing, ground-based active optical sensors (GBAO), satellite imagery, aerial imagery or photography, and leaf chlorophyll sensors. GBAO sensors and satellite imagery used to monitor plant health and adjust fertilizer application helps in developing yield prediction models. Site-specific recommendations differ for different soils and crops because available soil N and crop N uptake behavior vary with soil properties such as texture, structure, and development. Regional climate conditions and their interactions also cause variation in optimal N rates among growing seasons. GBAO sensors’ algorithms are successful for wheat, corn, cotton, sunflower, and sugar beet.⁵ GBAO sensors are important functional tools in agriculture for improving NUE.

Summary

Optimal use of N fertilizer significantly increases NUE and reduces the input cost for N fertilizer, ultimately benefitting farmers economically. Increasing NUE also protects surface water and groundwater quality from nitrate contamination and reduces ammonia emissions in the air. Rather than using one approach at a time to increase NUE, we suggest a combination of approaches, such as a sensor-based approach along with soil testing, to see faster improvement in NUE.

References

¹ Chien, S. H., L. A. Teixeira, H. Cantarella, G.W. Rehm, C. A. Grant, and M. M. Gearhart, 2016. “Agronomic effectiveness of granular nitrogen/phosphorus fertilizers containing elemental sulfur with and without ammonium sulfate: A review.” Agronomy Journal 108: 1203.

² Dobermann, A., J. L. Ping, V. I. Adamchuk, G. C. Simbahan, and R. B. Ferguson, 2003. “Classification of crop yield variability in irrigated production fields.” Agronomy Journal 95: 1105.

³ USDA-NRCS, 1998. Legumes and soil quality. Technical Note No. 6. Soil Quality Agronomy.

⁴ Scharf, P. C., and J. A. Lory, 2002. “Calibrating corn color from aerial photographs to predict side-dress nitrogen need.” Agronomy Journal 94: 397–404.

⁵ Blackmer, T., J. S. Schepers, G. E. Varvel, and E. A. Walter-Shea, 1996. “Nitrogen deficiency detection using reflected shortwave radiation from irrigated corn canopies.” Agronomy Journal 88: 1–5.

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