A combination of monitoring methods are used to measure nitrate-nitrogen in soil water and groundwater.
During growing seasons, soil water is collected weekly from 15 suction tube lysimeters, and samples are analyzed for nitrate-nitrogen. The lysimeters were installed at a 4-foot depth to collect soil water that has moved past crop rooting depth and could potentially move down to groundwater. The tips of the lysimeters are made of a porous ceramic material that water flows through when suction is placed on the tubes.
The graph shows the average soil pore water nitrate-nitrogen concentrations observed four feet beneath Byron #1 in 2014 under soybeans, 2015 under corn and 2016 under soybeans. Other fields in central MN managed with best management practices have shown nitrate-nitrogen concentrations that range from 30-70 ppm throughout a corn/russet/bean rotation. This preliminary data shows nitrate-nitrogen concentrations from Byron #1 measuring in a range comparable to or less than other locations.
Samples are analyzed for nitrate-nitrogen concentration using an ultraviolet spectrophotometer. Results are averaged to gain more representative information. For quality assurance, 10% of the samples are sent to a certified lab for analysis.
Installed below the crop rooting zone, at a 4-foot depth, drain gauges measure the volume of water that moves through the soil profile. This information combined with nitrate-nitrogen concentration data from lysimeters can provide an estimate of groundwater loading in terms of pounds of nitrate-nitrogen per acre.
As part of this field study, seven shallow monitoring wells were installed around the field perimeter in early November 2014. Over time, these wells will help provide a better understanding of the movement of shallow groundwater below this field and provide a way to measure the nitrate-nitrogen concentration of this water throughout the year.