Nitrogen fertilization
Nitrogen fertilization refers to the practice of enhancing plant growth through the addition of reactive nitrogen compounds, such as ammonia and nitrate, to the soil or foliage. These compounds are essential for various biological processes, including the synthesis of proteins, enzymes, and chlorophyll, which plays a critical role in photosynthesis. The application of nitrogen fertilizers, commonly produced through chemical processes like the Haber-Bosch method, can significantly stimulate photosynthetic activity and overall plant growth.
However, while nitrogen is abundant in the atmosphere, it is predominantly in an inactive form that plants cannot utilize directly. The acceleration of reactive nitrogen availability, primarily through human activities such as fertilizer production and fossil fuel combustion, affects ecosystems significantly. Increased nitrogen deposition can enhance plant biomass and carbon storage, but its impact on soil carbon storage remains controversial, with varying effects across different ecosystems.
Nitrogen fertilization also has implications for climate change, potentially altering land-surface energy balances and contributing to greenhouse gas emissions, including nitrous oxide and ammonia. Additionally, excessive nitrogen can lead to environmental issues like eutrophication and hypoxia in aquatic systems. As such, while nitrogen fertilization is vital for agricultural productivity, it requires careful management to mitigate negative environmental impacts.
Subject Terms
Nitrogen fertilization
Definition
Nitrogen fertilization is a phenomenon in which plant growth is unusually stimulated by the addition of reactive nitrogen compounds, such as ammonia and nitrate. Nitrogen compounds are usually applied as fertilizers to soil for uptake by plant roots. They are also often applied through air deposition from the atmosphere to foliage for uptake through leaves and to surface soil for plant-root uptake. Nitrogen fertilizers are usually manufactured through chemical processes, such as the Haber-Bosch process, to produce ammonia. This ammonia is applied directly to the soil or used to produce other compounds, including ammonium nitrate and urea. Reactive nitrogen compounds in the atmosphere are side-products of fossil fuel combustion.
Nitrogen is an essential element of all proteins, enzymes, and metabolic processes for the synthesis and transfer of energy. Nitrogen is also an element in chlorophyll, the green pigment of plants that facilitates light harvest and photosynthesis. Thus, nitrogen is essential for plant survival, growth, and reproduction. Many experiments have shown that is linearly correlated with nitrogen concentration in leaves. Increased nitrogen availability via fertilization or deposition generally increases photosynthetic and stimulates plant growth.
Significance for Climate Change
Nitrogen is very abundant in the Earth’s atmosphere but mostly not active for plants to use. Most plants take up only reactive nitrogen compounds, mostly from soil, for photosynthesis and growth. In natural ecosystems, inactive nitrogen in the atmosphere is converted to biologically useful forms mainly via nitrogen fixation by lightning or by a limited number of plant and microbial species. activities, such as the manufacture of nitrogen fertilizer, cultivation-induced nitrogen fixation, and combustion of fossil fuels, have accelerated the addition of reactive nitrogen to ecosystems, to 5 times the natural level.
Nitrogen addition, particularly through fertilization and deposition, has profound implications for climate change and global warming. Nitrogen deposition has been suggested to be a major mechanism underlying terrestrial ecosystem carbon sequestration. Extensive experimental evidence supports the theory that plant growth is limited by nitrogen in almost all ecosystems, and nitrogen addition to ecosystems often stimulates plant biomass growth and increases carbon storage in plant pools. The nitrogen limitation is usually persistent in ecosystems, largely because of the transient nature of biologically available forms of nitrogen. Biologically available nitrogen enters natural ecosystems primarily by biological fixation, but it is highly susceptible to loss by leaching and volatilization.
Although nitrogen stimulates growth and carbon storage in plant pools, the effects of nitrogen fertilization on soil carbon storage are controversial. In several studies, nitrogen addition did not significantly affect soil carbon storage. It caused significant increases in soil carbon content in European and North American forests, and it stimulated substantial carbon loss from soil in other ecosystems. Nitrogen addition not only stimulates plant biomass growth and carbon input to the soil systems but also potentially stimulates microbial decomposition of litter and soil organic matter. Particularly in forests, most litter and soil have high carbon contents and low nitrogen contents.
Microbial activities are strongly limited by nitrogen availability. Nitrogen deposition can relieve nitrogen stress and stimulate microbial activities. As a consequence, soil carbon content can decrease with additional nitrogen input from deposition. In an ecosystem with low carbon content and high nitrogen content in litter and soil organic matter, nitrogen-induced increases in plant biomass growth and carbon input can result in increases in soil carbon sequestration. Since carbon in terrestrial ecosystems is mostly stored in soil, the inconsistent, often opposite responses of soil carbon storage to nitrogen addition suggest that stimulation of carbon sequestration in terrestrial ecosystems by nitrogen deposition may be minor.
Nitrogen fertilization and deposition may indirectly affect land-surface energy balance and then feedback to climate change. Nitrogen addition usually results in increases in leaf chlorophyll content and greenness of plant and land surface. Green land surface has low albedo, so it readily absorbs solar radiation, and its high levels influence local and regional water vapor dynamics and air circulation.
Nitrogen fertilization and deposition can cause emissions of nitrous oxide (N2O), ammonia gas (NH3), and nitrogen oxides to the atmosphere, leading to greenhouse effects. The production and application of nitrogen fertilizers also consume energy and emit carbon dioxide (CO2) into the atmosphere. Reactive nitrogen in the atmosphere can influence tropospheric aerosols and stratospheric ozone, resulting in cooling effects. Moreover, nitrogen-induced increases in tropospheric aerosols can reduce plant carbon uptake and ecosystem carbon storage.
Nitrogen fertilization and deposition can result in degradation of ecosystems. Runoff with nitrogen-rich compounds from high-deposition regions and fertilized fields causes eutrophication in rivers and lakes and hypoxia in coastal zones.
Bibliography
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"What's the Problem with Fossil Fuel-Based Fertilizer?" Union of Concerned Scientists, 5 Dec. 2023, www.ucsusa.org/resources/whats-wrong-fossil-fuel-based-fertilizer. Accessed 17 Dec. 2024.