Nitrogen in snow?

Our recent heavy snowfall has been welcome to replenish some of our regions dry soils. The snow also brought comments and questions concerning the nitrogen (N) within the precipitation in that it will help green up our pastures and winter wheat.

We measured and analyzed the snowfall received in Brookings about 30 hours after it had fallen. We found about 9 inches of snow on the level that contained the equivalent of about 2 inches of water (this is a high water content probably resulting from compaction of the snow); the nitrate-N content of the snow was 0.4 ppm while the ammonium-N content was 0.3 ppm. This would be equivalent to only 0.3 lbs. per acre of available N for this snowfall. Not exactly a windfall of nitrogen for our crops, but also very typical nitrogen precipitation concentrations for this area.

The National Atmospheric Deposition Program (NADP) has measured N and other nutrients in precipitation for a number of stations around the country for over 30 years. The annual level of nitrogen deposits from precipitation will range from about 5 lbs./acre on the western edge of the Corn Belt to 12 lbs. per acre in the eastern Corn Belt.

Why the difference? Contrary to common perceptions, most of the N in precipitation does not come from lightning. There are two main forms of N in precipitation – nitrous oxides (nitrate-N) and ammonium-N. About 5-10% of the nitrous oxide forms originate naturally (i.e. lightning) and the remainder comes from human activity such as emissions from motor vehicles, electric power plants and industrial sources. Ammonium -N in precipitation can originate naturally from soil microbe activity (about 20%) while the remainder comes from manure or fertilizer (mostly urea forms) emissions of ammonia. The ammonium forms can make up from 25 to 75% of the total N in precipitation. Since most N in precipitation is from human activity, there tends to be higher levels occurring nearest large cities with industrial centers and near agricultural areas.

While the added N in precipitation is not a large contributor to the N needs of our major crops it can cause large changes in some environments. Some plants can be favored over others by the larger N additions. Acid rain, which is a result of more N and sulfur in rainfall, can cause changes in some freshwater ecosystems as well as harm some forest plant species. For more information on nitrogen deposition check out the website at: http:nadp.sws.uiuc.edu.

The bottom line? Most snowfalls contribute little to our overall crop N needs but can significantly influence some sensitive ecosystems.