Soil nitrate-N leaching and denitrification

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Recent, heavy rain in some areas has produced some flooded soils and increased concerns about nitrogen (N) status in the soil in North Dakota. At this time (June 3, 2014), most of the N in the soil has probably been converted to nitrate-N (NO3-N) which has a negative charge and is mobile in the soil. Water has to enter the soil and move downward for leaching to occur. However, with the recent rainfall events the intensity (rate of precipitation) has been so great that considerable run-off has occurred. Therefore, it is hard to estimate how much water has moved through the soil profile. Prior to these recent rainfall events, precipitation received has been near normal. When nitrate leaching was measured in the past, it occurred over a long period of time when much above normal precipitation was received that entered the soil and moved downward through the profile. Soil sampling at the South East Research Farm near Beresford in 1995 before and after 15.2 inches of rain was received over a 6 month period showed that most nitrate-N moved downward 3 to 4 feet.

Another concern for soil nitrogen from the recent heavy rain is denitrification. Soil denitrification occurs when soil voids are completely filled with water (absent of oxygen) in the presence of carbon (soil organic matter) and rate controlled by temperature. Optimal soil denitrifying temperatures are above 50 degrees Fahrenheit and greatest near 75 degrees. Anaerobic soil micro-organisms reverse the nitrification process by converting nitrate-N (NO3-N) to nitrite-N (NO2-N), nitric oxide (NO), nitrous oxide (N2O), and finally N gas (N2) that is lost to the atmosphere. Just because water is ponding doesn’t mean the denitrification process is fully operational. Research work from the University of Nebraska (C. Shapiro) reported that when soil temperatures were 55-60 degrees Fahrenheit, 10% of applied N was lost at 5 days and 25% at 10 days of ponding. When soil temperatures were increased to 75-80 degrees F, the denitrification losses of applied N were 60, 75, 85, and 95 % at 3, 5, 7, and 9 days, respectively. Soil temperatures at Baltic SD during the last 4 days were 71, 72, 73 and 69 degrees Fahrenheit. Therefore, the potential for denitrification is possible if ponding occurs for a few days.

A soil sampling technique to determine if enough plant available nitrogen is remaining after leaching or denitrification is the PSNT (pre-sidedress nitrate-N test). After the ponded areas dry and can be soil sampled, one could obtain a composite soil sample from the 0-1 and 1-2 foot soil depths for nitrate-N analysis. Interpretation of the PSNT soil test results by Iowa State University and the University of Wisconsin and are summarized very well by Charles Shapiro with the University of Nebraska.

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