by Jochum Wiersma, Small Grains Specialist, University of Minnesota
When it snowed in Brooking, SD in the early spring of 2013 they received 9 inches of snow . This contained the equivalent of about 2 inches of water. The nitrate-N content of the snow was 0.4 ppm while the ammonium-N content was 0.3 ppm. This was equivalent to only 0.3 pounds-per-acre of available nitrogen. Not exactly a windfall of nitrogen, but also very typical nitrogen precipitation concentrations for this area.
The National Atmospheric Deposition Program (NADP) has measured nitrogen 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 pounds-per-acre on the Western edge of the Corn Belt to 12 pounds-per-acre in the Eastern Corn Belt.
Why the difference? Contrary to common perceptions, most of the nitrogen 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 S 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 National Atmospheric Deposition Program website.
The bottom line? Most snowfalls contribute little to our overall crop N needs, but can significantly influence some sensitive ecosystems.