THE NITROGEN CYCLE

By Laura Jamieson

From BC Wetlands Network March 1995

In order to understand how wetlands function as "kidneys," it helps to know a bit about how nitrogen is cycled through the ecosystem. All living things need nitrogen. It is used to make the basic units of all life: DNA, proteins and amino acids. However, most nitrogen exists as a gas in the atmosphere, which very few living things can use. There are a small number of bacteria that are able to transform nitrogen gas into ammonia, through a process called nitrogen fixation. These bacteria use ammonia for their own growth, or they excrete it into the surrounding soil or water, where other bacteria transform it into nitrates. Nitrates are absorbed by plants and used for growth. Animals obtain their nitrogen by eating plants, or other animals. Once a plant Or animal dies, the nitrogen in its proteins is broken down to ammonia again by decomposing bacteria, transformed into nitrate by nitrifying bacteria, and absorbed again by other plants. In this way, nitrogen in the soil and water is recycled over and over. At every turn of the cycle, nitrogen fixing bacteria add a little nitrogen, and some nitrogen is released back into the atmosphere by another group of bacteria in a process called denitrification.  These different processes occur in different places in the landscape. Nitrogen gas is transformed into ammonia at the surface of soils and water or just below. In the soil, ammonia and nitrates exist in an equilibrium transformed back and forth by different groups of bacteria.  While ammonia stays with soil, nitrates are water-soluble and leach out of the soil, flowing into groundwater and collecting in lakes and wetlands. There, in the waterlogged soils, denitrifying bacteria transform nitrates back into nitrogen gas and release it into the atmosphere. The nitrogen cycle of a landscape is self-adjusting.  The activity of all of these microorganisms in the soil and water maintains equilibrium between the amount of ammonia and the amount of nitrate in the system, and between the amount of nitrogen fixed in upland areas, and the amount released back into the atmosphere in wetlands. If the there is an excess of one form of nitrogen, there are microorganisms that will quickly, transform it into another form, thereby retaining a balance. Everything works fine, until we start overloading the system on a continual basis.