Nitrogen losses in agricultural systems pose significant challenges to sustainable farming practices and environmental conservation. As an essential nutrient for plant growth, nitrogen plays a vital role in agricultural productivity. However, various factors contribute to the loss of nitrogen from agricultural fields, such as leaching, volatilization, and denitrification. These nitrogen losses not only reduce crop yield and nutrient use efficiency but also have detrimental effects on water quality, air pollution, and ecosystem health
In 2050, the worldwide population is expected to reach 9 billion which requires a 70% increase in crop production to ensure the availability of food. Agriculture will continue to use more inorganic and organic amendments to meet this demand. Nitrogen (N) is essential for the growth of plants and the ongoing use of nitrogen fertilizers over the past five decades has significantly changed the global nitrogen cycle.
The nitrogen cycle consists of different processes, including the fixation of nitrogen by bacteria, ammonification, nitrification, assimilation into biomass from microorganisms, and denitrification. However, the over-application of organic manure and chemical fertilizers frequently exceeds crop needs, which causes nitrogen loss. Most nitrogen applied to agriculture is lost as NO3, NH3, or N2O, with NO3 being recycled into N2O and N2 through denitrification in the soil, water, and air system.
Only 45–50% of applied nitrogen is incorporated into agricultural products. Notably, N2O emissions are significant at the start of the crop season when N fertilizer is applied. These emissions have an impact on both human and environmental health.
Nitrogen Losses in the Environment:
Nitrogen losses in the environment are a pressing issue with far-reaching impacts. Various processes contribute to the loss of nitrogen, including leaching, volatilization, and denitrification. Excessive application of nitrogen fertilizers and poor management practices exacerbate these losses, leading to environmental pollution, eutrophication of water bodies, and greenhouse gas emissions.
Reactive nitrogen, also known as Nr, is a form of nitrogen that is present in many different ways and has a big impact on the environment and the world’s ecosystems. These reactive nitrogen species also include N2O, nitrate (NO3), nitrite (NO2), ammonia (NH3), and ammonium (NH4+). Many of these reactive nitrogen species are primarily anthropogenic in origin and result from practices like burning fossil fuels, growing legumes, and using commercial fertilizers in agriculture.
A significant source of nitrogen loss in agricultural fields all over the world is ammonia volatilization. Soil characteristics have an impact on the rate of NH3 volatilization. Even neutral or acidic soils are at risk of NH3 volatilization, particularly following the use of organic fertilizers like urine or inorganic fertilizers like urea. Soils with high pH are more prone to significant NH3 losses, but even neutral or acidic soils can.
Nitrous Oxide and Oxides of Nitrogen (NOx) Emissions:
The chemical reactions of nitrification (under aerobic conditions) and denitrification (under anaerobic conditions) in soil commonly generate nitrous oxide (N2O). Nitrification involves the microbial conversion of NH4-N to hydroxylamine (NH2OH), which occurs after the production of NOH and NO2. Both the NH2OH and NO steps of nitrification result in the production of N2O. Denitrification, on the other hand, changes NO3 or NO to N2 or N2O. N2O is considered a major contributor to climate change due to its long atmospheric lifetime (120 years) and higher heat-trapping potential than CO2.
By forming nitric acid from a reaction with stratospheric O2, it also contributes to the destruction of ozone. A known health risk is NO2, which is primarily released through the combustion of fossil fuels in vehicles and industrial facilities. A variety of respiratory and cardiovascular disorders, including damaged respiration in adults and children, have been linked to prolonged exposure to high levels of NO2.
Mitigation strategies for Nitrogen Losses:
In order to practise sustainable agriculture and safeguard ecosystems, agricultural practices must employ mitigation strategies to reduce nitrogen loss and environmental impact. Here are thorough explanations of the strategies mentioned above:
1. Split dose application:
To maximize nitrogen uptake and reduce nitrogen losses, it is effective to spread out the application of nitrogen fertilizers over the course of the growing season. Farmers can divide the dose into smaller portions and apply them at key growth stages when the plants are more in need of nutrients than they would if it were applied all at once. The risk of excessive nitrogen leaching or volatilization is diminished by timing the applications in accordance with the crop’s needs.
2. Application at critical growth stages:
Nitrogen fertilizers should be applied to crops at specific growth stages to maximise nutrient uptake and minimize nitrogen losses. During various growth stages, different crops have different nutrient needs. Farmers can make sure that the nutrients are available when the plants need them most by timing the fertilizer application to coincide with key growth stages. This strategy minimizes environmental contamination, lowers the likelihood of nutrient runoff, and improves nutrient use efficiency.
3. Eco-friendly approaches:
Environmental effects are lessened by using eco-friendly nitrogen management techniques. Applying coating materials to urea granules is one such method. To regulate the release of nitrogen into the soil, coating materials like polymers or oils can be applied to the surface of urea granules. These coatings serve as barriers that delay the breakdown of urea, gradually releasing nitrogen over time.
By using a controlled-release mechanism, nutrient losses due to leaching and volatilization are reduced and nitrogen is made available to plants for a longer period of time. Additionally, because the nutrients are released more gradually and in a manner that matches the crop’s capacity for uptake, coated urea granules lessen the chance of nitrogen runoff.
a. Polymer nutrient coatings:
Nitrogen fertilizers frequently use polymer nutrient coatings to increase their effectiveness. Based on the environment and plant demand, these coatings are made to release nutrients gradually. They can control the rate at which nitrogen is released, lowering the chance that too many nutrients will end up in water bodies or seep below the root zone. The urea granules are also given physical protection by polymer coatings, preventing them from degrading quickly when they come into contact with the soil.
b. Oil-based coatings:
The use of oil-based coatings is an additional strategy for reducing nitrogen losses. By limiting contact with moisture, these coatings create a thin film around the urea granules, slowing the release of nitrogen. The oil coating serves as a barrier, minimising the amount of the granules’ surface that is exposed to the soil and environment. Because of this, nitrogen is released gradually, meeting the needs of the crop and lowering the chance of nitrogen loss.
Minimizing nitrogen losses and managing nitrogen effectively in agriculture is crucial for sustainable and environmentally friendly farming practices. The continuous use of nitrogen fertilizers has significantly altered the global nitrogen cycle, resulting in negative environmental impacts. Implementing effective mitigation strategies, such as split dosing and application during critical growth stages, can enhance nutrient use efficiency and reduce nitrogen losses.
Coating materials on urea granules, through polymer and oil-based coatings, controls the release rate of nitrogen, preventing excessive leakage and promoting gradual release to meet crop needs. By adopting these strategies, farmers can improve nutrient utilization, decrease nitrogen losses, and mitigate the environmental impact of nitrogen fertilizers, ensuring long-term ecosystem productivity and health in agricultural systems.
Hafiz Abdul Wahab, Sher Afghan, Imran Khan, Muhammad Umer Chattha and Muhammad Saqlain
Department of Agronomy, University of Agriculture, Faisalabad