Nitrogen is the most abundant element in our atmosphere. It’s crucial to life. Nitrogen is found in soils and plants, in the water we drink, and in the air we breathe. It is also a key building block of DNA, which determines our genetics. It’s essential to plant growth, therefore necessary for the food we grow.
But as with everything, balance is key. Too little nitrogen and plants cannot thrive, leading to low crop yields. But too much nitrogen can be toxic to plants, and can also harm our environment. Plants that do not have enough nitrogen become yellowish and do not grow well and can have smaller flowers and fruits.
Too much nitrogen can hurt plants and animals, and pollute our water systems.
To help us grow healthy crops and protect our environment we must understand the Nitrogen Cycle. The Nitrogen Cycle shows how nitrogen moves from the atmosphere to earth, through soils and back to the atmosphere in an endless cycle.
Nitrogen gas exists in large quantities in the atmosphere taking up about 78 percent of the volume of the atmosphere. Nitrogen is a part of many molecules essential to life like proteins, DNA, and some vitamins.
Plants and animals cannot absorb nitrogen from atmosphere
Plants and animals need nitrogen for the formation of proteins, vitamins, and DNA. Plants and animals cannot absorb nitrogen from the atmosphere.
The process of changing atmospheric nitrogen into a different biological form usable by plants and animals is called nitrogen fixation. Special organisms in the soil change gaseous nitrogen to ammonia that can be used by plants. Most nitrogen fixation is a natural process carried out by organisms. Some nitrogen is fixed inorganically by lightning or certain industrial processes, including the burning of fossil fuels.
The second step of the nitrogen cycle is nitrification. It is the process carried out by nitrifying bacteria, transforming ammonia absorbed by the soil during nitrogen fixation into nitrite then to nitrate. Special organisms convert ammonia in the soil to nitrite ions and nitrate ions. Nitrate ions are easily absorbed by plants into their own tissues.
The process through which plant roots absorb ammonium ions and nitrate ions is called assimilation. Plants and animals use these ions to make amino acids that make proteins and formulate DNA. The DNA makes up the genetic material and life’s design for all cells. Animals get their nitrogen by eating plants.
When an organism excretes waste or dies, the nitrogen in its tissues is in the form of organic nitrogen (for example, amino acids and DNA). Various bacteria then decompose the tissue and release inorganic nitrogen back into the ecosystem as ammonia in the process known as ammonification. The ammonia then becomes available for uptake by plants and other microorganisms for growth.
Nitrogen gas returns to atmosphere
Nitrogen leaves the soil in a process called denitrification. Denitrification is the process that converts nitrate to nitrogen gas, thus removing nitrogen and returning it to the atmosphere. Denitrification is important in that it removes nitrate from the ecosystem and returns it to the atmosphere. The released nitrogen gas to the atmosphere begins the cycle again.
Denitrification is very useful in sewage treatment, where unwanted nitrates are removed from the discharge. Removing nitrates reduces possibility of the discharged water being fertile ground for algae to grow.
Although, nitrogen is the ultimate product of denitrification, other intermediate gaseous forms of nitrogen exist. Some of these gases, such as nitrous oxide, are considered greenhouse gasses, reacting with ozone, and contributing to air pollution.
Nitrous oxide molecules stay in the atmosphere for an average of 100 years before being removed by a sink or destroyed through chemical reactions.
How human activities have changed the Nitrogen Cycle
Human activities have led to changes in the climate and atmosphere and hence affected the whole Nitrogen Cycle.
Agricultural activities. Commercial use of inorganic fertilisers account for the largest nitrous oxide gas emissions. Nitrogen and phosphorus are among the elements considered most limiting to plant growth and productivity. This is because nitrogen and phosphorus are often present in small quantities in the soil or are present in a form that cannot be used by the plant. Most soils lack a variety of required mix of micro and macro nutrients needed by plants.
Excessive irrigation and heavy rains cause soils to have inadequate levels of nitrogen due to over watering. Farmers apply manufactured inorganic nitrogen-based fertilisers or organic waste from plants and animals to the soil.
Inorganic and organic application is used to supplement naturally made ammonium ions and nitrate ions by the soil to increase food production. Excess nitrogen not absorbed by plants, reacts with the soil to produce nitrous oxide, a dangerous greenhouse gas.
Agricultural runoffs and discharges. Applying too much fertiliser and erosion of nutrient rich soil due to heavy rain or over irrigation has damaging effects to aquatic life. Large amounts of nitrogen may drain from the soil into drainage ditches, streams, rivers and eventually oceans. This leads to unnatural enrichment of water bodies with organic and inorganic nutrients, leading to the unbalancing of the local marine.
Other human activities changing Nitrogen Cycle
Land use and changes in land use. Humans are draining wetlands and swamps to use the land for settlement or agricultural purposes. This affects the balance of the Nitrogen Cycle. Because denitrification, which is an anaerobic process (occurs in absence of oxygen), takes place in such areas.
Burning of fossil fuels. Coal fired electricity plants account for the largest amounts of stationary emissions. When burning coal, oil and gas, nitrogen that is in the fuel and surrounding air gets oxidised creating nitrous oxide emissions. Nitric oxide in the atmosphere is converted to nitrogen dioxide and nitric acid which result in acid rain and smog.