The tundra is a vast and cold biome characterized by low-growing vegetation, short growing seasons, and a layer of permanently frozen soil known as permafrost. Nitrogen is a fundamental nutrient for all living organisms, required for the creation of proteins and nucleic acids. In the tundra, the availability of nitrogen is particularly constrained, making its cycle a process that governs the productivity and functioning of this ecosystem.
Nitrogen Inputs and Fixation
New nitrogen primarily enters the tundra ecosystem through two main pathways: atmospheric deposition and biological nitrogen fixation. Atmospheric deposition occurs when nitrogen compounds are transported through the atmosphere and deposited on the tundra landscape via rain and snow. While the Arctic has low rates of nitrogen deposition, it can still represent a significant portion of the total nitrogen input in this nutrient-poor environment.
A more substantial source of new nitrogen comes from biological nitrogen fixation, a process carried out by specialized microorganisms. In the tundra, these nitrogen-fixing organisms include free-living cyanobacteria, as well as cyanobacteria that form symbiotic relationships with mosses and lichens. These organisms convert atmospheric nitrogen gas, which is unusable by most life, into ammonia, a form that can be taken up by plants. Despite the cold temperatures that slow down this process, biological nitrogen fixation can contribute up to 90% of the new nitrogen in some tundra ecosystems. In areas with nesting seabirds, bird droppings can also provide a concentrated source of nitrogen to the local environment.
The Slow Process of Decomposition
The vast majority of nitrogen in the tundra is not newly fixed but is locked within the large reservoir of dead organic matter in the soil, and its release depends on decomposition. This internal cycling is driven by soil microbes that break down organic material, a process known as mineralization, which converts organic nitrogen into inorganic forms like ammonium. This process dictates the amount of nitrogen available to support plant growth.
Decomposition in the tundra is an exceedingly slow process for several reasons. The cold temperatures that define the biome significantly inhibit microbial activity, reducing the rate at which they can break down organic matter. Additionally, many tundra soils are waterlogged, creating low-oxygen conditions that further slow decomposition. The quality of the plant litter itself, often low in nutrients, also contributes to the slow pace of decay.
The Influence of Permafrost and Soil Temperature
Permafrost strongly controls the nitrogen cycle. It is a thick layer of soil that remains frozen year-round, and all biological activity is confined to the shallow layer of soil above it that thaws each summer, known as the active layer. This shallow active layer physically restricts the depth to which plant roots can grow and limits the volume of soil that microbes can access for decomposition. As a result, the immense stores of nitrogen locked in the frozen organic matter of the permafrost are largely unavailable.
The temperature of the soil within the active layer controls the rates of both nitrogen mineralization and uptake by plants. The constant freezing and thawing of the active layer also causes cryoturbation, which is the churning and mixing of the soil. This can bring deeper, nutrient-poor soil to the surface and bury organic matter, further influencing the distribution and availability of nitrogen. In tundra ecosystems with intact permafrost, the nitrogen cycle is often described as “closed” because there is very little loss of nitrogen from the system.
Nitrogen Cycling in a Warming Tundra
The tundra is warming at a faster rate than the rest of the planet. Rising temperatures are causing the active layer to deepen and the permafrost to thaw, unlocking previously frozen organic matter. This exposes the stored nitrogen to microbial decomposition, increasing the amount of available nitrogen in the soil. This process can act as a fertilizer, stimulating plant growth and leading to a “greening” of the tundra as vegetation becomes more productive.
This influx of nitrogen may not be entirely beneficial. The release of nitrogen from thawing permafrost may exceed the capacity of plants and microbes to absorb it, leading to an “opening” of the nitrogen cycle. Excess nitrogen can be lost through leaching into rivers and lakes, or it can be converted by microbes into nitrous oxide, a potent greenhouse gas. Changes in plant communities, such as the increasing dominance of shrubs, can also alter the dynamics of nitrogen cycling and carbon storage in a warming tundra.