Plants constantly seek to absorb nitrogen, but they are also active participants in the nutrient cycle, returning the element to the environment. This occurs through various mechanisms while they are alive and after they die, ensuring nitrogen remains in circulation. This continuous exchange highlights the dynamic nature of nitrogen cycling in ecosystems.
Nitrogen’s Essential Role in Plant Life
Nitrogen is a foundational component for plant structure and function, incorporated into molecules like chlorophyll, which powers photosynthesis. It is also an integral part of amino acids, the building blocks of proteins and enzymes necessary for growth.
Plants absorb nitrogen primarily as inorganic nitrate (\(\text{NO}_3^-\)) and ammonium (\(\text{NH}_4^+\)) ions from the soil solution using specialized transport proteins. These inorganic forms must then be converted into organic compounds through assimilation.
Assimilation involves reducing nitrate to ammonium, which is rapidly incorporated into amino acids. This conversion prevents the buildup of toxic free ammonium. The resulting organic nitrogen compounds are transported throughout the plant to support growth.
Active Release: Nitrogen Exudates and Volatiles
Living plants actively release nitrogen compounds into the soil for biochemical communication and nutrient management. This discharge occurs mainly through root exudates, a diverse mixture of organic compounds released into the rhizosphere, the narrow zone of soil influenced by the roots.
The release of these nitrogen compounds is a strategic tool for manipulating the surrounding microbial community. Plants may release amino acids to stimulate beneficial microbes that aid in nutrient acquisition. Certain root exudates act as biological nitrification inhibitors (BNIs), slowing the conversion of ammonium to nitrate.
By suppressing nitrification, plants conserve nitrogen in the less-leachable ammonium form, managing the local nitrogen supply. Plants also release volatile organic nitrogen compounds (VONCs) into the atmosphere, primarily from their leaves.
This gaseous release can shed excess nitrogen, especially under high nitrogen availability or environmental stress. Although the total amount lost this way is small compared to root exudation, it is a direct, active pathway for nitrogen return.
Passive Return: Nitrogen Release Post-Death
The most substantial pathway for nitrogen return is passive, occurring when the plant dies and its organic matter decomposes. Plant tissues are rich in organic nitrogen, locked within proteins and structural compounds. This nitrogen becomes available only after the plant’s structural integrity fails.
Decomposition begins with the breakdown of complex organic molecules by external agents, primarily fungi and bacteria. This initial stage is mineralization, where organic nitrogen is converted into simpler inorganic forms.
The specific step where amino groups from proteins are converted into ammonia (\(\text{NH}_3\)) and then into ammonium (\(\text{NH}_4^+\)) is called ammonification. This passive return is entirely reliant on the activity of soil decomposers.
The rate of nitrogen release depends heavily on the carbon-to-nitrogen ratio of the dead material. Residues with a low ratio, such as those from legumes, decompose and release nitrogen much faster. The resulting ammonium becomes a new nitrogen source for other soil organisms.
Microbial Partners: Converting Nitrogen Back to the Soil
The final stages of nitrogen cycling are governed by diverse microbial communities in the soil. The ammonium released during ammonification is a preferred nitrogen source for many plants, but it is also targeted by specialized bacteria. These bacteria drive nitrification, a two-step conversion requiring aerobic conditions.
First, ammonia-oxidizing bacteria convert ammonium (\(\text{NH}_4^+\)) into toxic nitrite (\(\text{NO}_2^-\)). Next, nitrite-oxidizing bacteria rapidly convert the nitrite into nitrate (\(\text{NO}_3^-\)), a form readily taken up by most plants. Nitrification makes the nitrogen released from plant decomposition fully accessible for new plant growth.
Not all nitrogen stays in the soil; some is lost back to the atmosphere through denitrification. Under waterlogged or oxygen-poor conditions, denitrifying bacteria use nitrate as a substitute for oxygen for their metabolism.
This reaction converts nitrate into gaseous forms, such as nitrous oxide (\(\text{N}_2\text{O}\)) and eventually inert dinitrogen gas (\(\text{N}_2\)). This effectively removes the nitrogen from the soil ecosystem and completes the atmospheric cycle.