Gorans Blog Series: Nitrogen movement.

This the sixth post in the Gorans Blog Series about the Gorans Discovery Farm Project. You can read the previous posts here.

In the last post, we explored how much sediment and phosphorus are carried in runoff and drainage water into the waterways. Now we are going to look more closely at nitrogen in the water, specifically the nitrate and ammonium forms.

Comparing the Fertilized Field and Unfertilized Field

We first want to discuss and compare the two fields, one you’ll remember was fertilized and the other was not. We know commercial fertilizer and manure (in this case, turkey manure) are important nitrogen sources for crop production. However, we also know there is a lot of concern around nutrients from those sources leaving the fields in water drainage and impacting water quality.

Nitrate Concentration (NO3-)

We found the median concentration of nitrate in the drainage water for the Fertilized Field (17.88 ppm) was 1.7 times higher than the Unfertilized Field (10.49 ppm). The higher nitrate concentration for the Fertilized Field than the Unfertilized Field was caused by the application of manure and commercial fertilizer. The impact of manure and fertilizer applications on the soil can be seen in higher fall residual soil nitrate-N tests, approximately 2 to 3 times higher in the Fertilized Field. Although the nitrate concentration from the Unfertilized Field was lower than the Fertilized Field, it is important to note that even without fertilization there can still be significant nitrate in drainage water. The source of nitrate from the Unfertilized Field could be attributed to legacy nitrogen from previous manure applications and mineralization of soil organic matter.

We found a significant general trend for the Fertilized and Unfertilized Fields in the drainage outflow—the nitrate concentrations significantly decreased over the seven years of our study. The reason for the decrease may be due to improved nutrient management practices implemented by the farmer.

Ammonium Concentration (NH4+)

Ammonium concentrations were below detection limit for about 85% of the 663 and 907 sampling days for the Unfertilized and Fertilized Fields, respectively. Thus, the median concentration for each site (0.03 ppm) was very low. The vast majority of the ammonium is bound to soil particles and therefore less prone to transport.

Loads of Nitrate and Ammonium

To calculate the loads, we used the concentrations of nitrate and ammonium, and multiplied that by the volume of water coming from the fields.

Nitrate load. Average daily nitrate load per unit area from the Fertilized Field was significantly higher than that from the Unfertilized Field. This is the result of the higher nitrate concentration for the Fertilized Field, since flow depths were not significantly different. Although the Unfertilized Field did not receive fertilizer nor manure since 2005, the cumulative load per area was still 58% of that of the Fertilized Field. Our results show that under management practices similar to those of this study, nitrate load in drainage water from an unfertilized field could be considerable relative to that of a fertilized field.

Ammonium load. The average daily ammonium load per unit area was not significantly different between the Unfertilized and Fertilized Fields, which can be explained by the field having similar flow depths and similar median ammonium concentrations. Results also showed that the ammonium loads from both fields were very low.

Nitrate and Ammonium Loads

Cumulative load from April to October of 2007 to 2013, and paired t-test comparisons of daily loads between Unfertilized and Fertilized Fields after adjusting for serial correlation (n=1071).

Description	Cumulative Load (lb/ac)	Average Daily Load ± SD (lb/ac)	One-sided p-value (for daily load)
Nitrate-N
Unfertilized Field	89.7	0.084 ± 0.164	<0.001
Fertilized Field	155.4	0.145 ± 0.252
Ammonium-N
Unfertilized Field	0.29	0.0003 ± 0.0006	0.120
Fertilized Field	0.37	0.0004 ± 0.0012

Comparing the City Stormwater to the Fertilized Field

Because we are most interested in the Fertilized Field results, we wanted to compare them to the City Stormwater measurements as well. Nitrate is one of the most common contaminants found in farm field drainage water, while ammonium is more prevalent in urban stormwater.

Nitrate Concentration (NO3-)

The median nitrate concentration in the water from the Fertilized Field (17.88 ppm) was 27.9 times higher than that for the City Stormwater (0.64 ppm), which can be explained by the greater inorganic and organic sources of N in the soil. Furthermore, the nitrate concentration in the City Stormwater was always below the nitrate concentration standard of 10 ppm for drinking water in the USA.

Ammonium Concentration (NH4+)

The median ammonium concentration for the City Stormwater (0.27 ppm) was 9 times higher than for the Fertilized Field (0.03 ppm). The City of Willmar’s ammonium concentration in the stormwater is similar to the median (0.44 ppm) from cities throughout the United States. Possible sources of ammonium in the city include deposition from automobiles, rainfall, and atmospheric deposition. In contrast to farm fields, the impervious surfaces in the city facilitate the transport of ammonium into the water.

Loads of Nitrate and Ammonium

Nitrate load. Average daily nitrate load per unit area from the Fertilized Field was 12.4 times higher than from the City Stormwater. The loads were higher because concentrations were so much higher for the Fertilized Field even though the flow depth was significantly lower than for the Stormwater site.

Ammonium load. The ammonium average daily loads per unit area from the City Stormwater were significantly higher than from the Fertilized Field. The higher loads were the result of higher concentrations and higher flow depth for the City Stormwater. These results show that urban areas are capable of transporting higher ammonium loads than farmland on a per area basis.

Nitrate and Ammonium Loads

Cumulative load from April to October of 2007 to 2012 (excluding 2008), and paired t-test comparisons of daily loads between City Stormwater and Fertilized Field after adjusting for serial correlation (n=792).

Description	Cumulative Load (lb/ac)	Average Daily Load ± SD (lb/ac)	One-sided p-value (for daily load)
Nitrate-N
City Stormwater	9.8	0.012 ± 0.029	<0.001
Fertilized Field	121.9	0.154 ± 0.269
Ammonium-N
City Stormwater	3.23	0.0041 ± 0.0099	<0.001
Fertilized Field	0.30	0.0004 ± 0.0020

In our next post in the series, we will continue to compare the nutrient runoff from the three sites and start to look at the bigger picture – scaling this information up to the watershed level.

Field Day November 10^th

Want to see the Gorans Discovery Farm for yourself and get a sneak peek at the experimental bioreactor? Join us for a field day on Friday, November 10^th at 10 a.m. Learn more on the Department of Soil, Water, and Climate’s website.