Nitrogen

One of the many great speakers we had at this year's High Desert Gardening & Landscaping Conference was Dr. Tom Thompson of the Department of Soil, Water, and Environmental Science at the University of Arizona. Dr. Thompson's lecture, Nitrogen in the Environment: Where It Comes From, Where It Goes, and Why, inspired me to do a little Web surfing on the topic. An Alta Vista search on the term "nitrogen AND cycle" turned up hits on over 21,000 documents. Of these, one of the most informative and interesting was fact sheet from the Ohio State University Extension titled Nitrogen and the Hydrologic Cycle. This document provided much of the content for this article and can be downloaded from: http://ohioline.ag.ohio-state.edu/aex-fact/463.html. Nitrogen is one of the 16 chemical elements required by all plants and even though it is very abundant in the environment - the atmosphere comprises 78 percent nitrogen gas - nitrogen is the element that most often limits plant growth. There are two reasons for this. First, nitrogen is a basic and essential constituent of proteins, amino acids, and chlorophyll in plant tissues. Second, nitrogen gas (N2) is not directly useable by most plants but must be changed into other chemical forms that can be taken up by plant roots.

Nitrogen in the environment goes through a complex set of chemical reactions is called the nitrogen cycle. Ammonium ions (NH4+) and nitrate ions (NO3-), the inorganic chemical forms of nitrogen that can be 4 directly used by plants, are produced from atmospheric nitrogen gas naturally by the action of bacteria and lightning and artificially by industry in processes called nitrogen fixation. Once nitrogen gas has been transformed into ammonium and nitrate and incorporated into plant tissues, it is unavailable to other plants until the plant containing it dies and its tissues are processed by other bacteria back into ammonium and nitrate in a process called mineralization. As you can see, bacteria play an important role in changing nitrogen from one chemical form to another. A third group of bacteria in the soil convert ammonium into nitrate in a process called nitrification, and a fourth group change nitrate back into the form we began with, nitrogen gas as well as another gas called nitrous oxide (N2O), in a process called denitrification. By now I suppose you are wondering where this is leading. All these loops and cycles are interesting, but what's the point?

One of the first things to appreciate is that the natural processes of nitrogen fixation and mineralization produce relatively small amounts of ammonium and nitrate. Only around two percent of the nitrogen in the soil is in the inorganic form that can be directly taken up by plants. The other 98 percent is taken up in plant (and animal) tissues and is unavailable for direct uptake. This is slowly released by the mineralizing bacteria. Since our desert soils are very low in organic material, our soils are chronically short of nitrogen. This is why so many desert plants are leguminous (i.e., they host nitrogen-fixing bacteria that provide them with their own private supply of ammonium and nitrate). It is also why we have to artificially add sources of nitrogen to our soils to get many plants to grow.

When adding sources of nitrogen to the soil, we have a choice of several different kinds of materials. In order to mimic nature and provide sources that slowly release nitrogen over long periods of time, we can add organic materials. When doing this we want to choose materials that have a fairly low carbon to nitrogen ratio such as compost and manures which promote mineralization and avoid materials with higher carbon to nitrogen ratios such as straw which do not. Alternatively, we can add chemical fertilizers.

Next month I will discuss the use of chemical fertilizers and some of the environmental dangers associated with their incorrect use. In the meantime, have fun surfing.