Atmospheric nutrient deposition research update: Q&A with Paulo Pagliari

Extension nutrient management specialist Paulo Pagliari is currently in the third year of a study measuring atmospheric nutrient deposition in Minnesota. Last September, we wrote about preliminary results from the first year of the study (2019). This year, we sat down with Pagliari to get an update on the study’s 2020 results. The project is funded by Minnesota’s fertilizer check-off dollars through the Agricultural Fertilizer Research and Education Council (AFREC).

You made some changes to the study after the first year. What were the differences between the 2019 study year and the last two years of the project?

Yes, we did some fine-tuning of what we're looking at. One thing that was a major change was one of the locations. We were originally doing this study at Becker, Morris, Lamberton, Waseca and Crookston, and those are all areas surrounded by agricultural fields. Then we decided to add an area where there's little influence from grain agriculture, so we replaced the Morris location with the Cloquet Forestry Center (CFC) which is a very wooded area in northwest Minnesota with very little, if any, influence from grain crops.

Paulo Pagliari

Another change we made is that we are not measuring sulfur anymore. It's been too troublesome. It's taken a lot of time and we weren’t really getting reliable data, so I think it's better to drop that one for now.

We are measuring other nutrients now, though. We’re trying to figure out what the source of the higher pH levels we’re seeing is. We're now tracking potassium, calcium, magnesium, aluminum, and a few other elements.

Lastly, in 2019, we only analyzed the inorganic forms of nitrogen in rainfall, which are ammonium and nitrate. Then we decided to also do a digestion on those rain samples and look at the total, which includes inorganic and organic forms of nitrogen.

How did your results differ from 2020 to 2019?

The nitrogen portion is the main one that we are measuring. We saw about a two-fold increase in the amount of total nitrogen that is being deposited. I believe that first year, when we were only measuring inorganic nitrogen, our range was between four to 13 pounds per acre. Now that we are measuring both inorganic and organic nitrogen, we are between eight and almost 26 pounds of nitrogen per acre, so it's roughly a two-fold increase. So that range of four to 13 pounds of N from 2019, depending on the location, was definitely an underestimate. I don't know what the real numbers would be. I'm guessing if you double those numbers, you'd be close to the real number, just from what we saw in 2020.

The pH in 2020 showed the same trend as 2019. The pH is still much more alkaline than what we expected it to be. We expected it to be around 5.5 and below but we are observing pH as high as 7.5, 7.7, I believe even 8.0 or higher at some locations in certain rain events. There's no way to explain why the pH is that high when everything we know about rainfall is that it should be acidic, including documentation that the EPA has. Now, being this high, it suggests that other nutrients, likely alkali metals, are probably controlling the pH of rain and snow. All those extra nutrients have a bigger impact on the pH than what the natural concentrations of gases in the atmosphere do.

Are there specific nutrients you think are contributing more to the higher pH levels?

If the pH is 8.0, that means there are earth metals in there that are making the pH high. That's why we are measuring aluminum, potassium, and calcium, because those will potentially have that impact. At some locations, we found quite a bit of calcium and potassium in 2020. Twenty pounds of potassium per acre fell at Waseca and 18 pounds at the CFC. But in the other locations, it was much lower. It was only around four pounds in Lamberton and Crookston. I don't know why. It's pretty interesting. At Lamberton, 20 pounds of calcium was deposited. It's very, very interesting data for sure. It varies a lot from location to location. It varied from 16 to 34 pounds of earth metal nutrients being deposited from those three sources (aluminum, potassium, and calcium). Nitrogen does have an impact on pH, but it will actually drag it down. When there's a lot of lightning, it creates nitrous oxide, and that converts into nitric acid. That can bring the pH of rain below 5.0.

Table 1. Nutrient load in rain and snow, 2020-21 season (pounds/acre): Nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg)

Location	Total N	K	Ca	Mg
Cloquet	N/A	18	3.39	0.7
Waseca	8.5	19	11.9	2.9
Lamberton	22.1	4	19.2	3.4
Crookston	25.4	4	6.8	4.8
Becker	8.3	N/A	N/A	N/A

2019 was a record wet year in Minnesota and then 2020 was pretty dry. Did that influence your results at all?

There was a significant drop in rainfall at most of the locations in 2020. Eight inches less at Becker, seven inches less at Crookston, five inches at Lamberton, nine inches in Waseca. But the amount of rainfall doesn’t have a big impact on the amount of nutrient deposition. Whenever it rains, whatever is up there will come down with the rain. If there's a light shower, it's probably going to be more concentrated. If there's heavy rainfall, it's going to be more diluted. But it appears that the amount of those nutrients in the atmosphere is somehow pretty consistent.

Have you seen any evidence for agriculture’s role in nutrient deposition?

For variability within the season, it looks like you get more ammonium in the spring while nitrate is more consistent year-round. This makes sense for ammonium because early spring is when a lot of ammonium fertilizer is being applied, or late fall, but ammonium usually stays in the soil in the fall and then is released in the spring when things warm up. And that’s assuming that ammonium fertilizer was applied according to U of M Extension best management practices. You can see that the amount of ammonium we measured at the CFC, where there is very little agriculture, is only one pound per acre. Then for Becker, which is the second lowest, it's five. There's five times more ammonium falling at Becker than at the CFC.

Another thing I find interesting is that half of the total nitrogen is organic. I don't know what form of the organic N it would be. I think urea would be a reasonable molecule to look for, because there have been some reports of urea in rainfall. I think urea would probably be the first target to see what the percent of urea in that organic fraction is.

Again, as far as nitrogen falling on fields, the highest we saw was 26 pounds of nitrogen per acre, which isn’t a lot when it comes to agriculture. And that won’t make a difference for the farmer anyway because that's already accounted for in the university’s corn nitrogen rate calculator. Whenever we do a trial to look at nitrogen rates, we don't know that that's being added. It's already there, so you don't subtract or increase anything on a farmer's fertilizer requirements, because that's accounted for.

What’s next for this project?

We are continuing the study this year, in 2021, and I'll apply for funding from AFREC for next year, 2022, as well. The methodology this year is the exact same as it was in 2020 and will be next year as well. It will be interesting to see how the dry growing season this year affects things. I think after we do this for four or five years, then we're going to have a pretty good average, and then we can start looking at those trends to see how consistent the measurement is.

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