By Alan Kraus and Dane McKittrick Clean River Partners, Northfield, MN
The graph above shows the average nitrate level of all 2021 sampling locations from March 25th through August 10th
Nitrate nitrogen, a plant food in fertilizers important for crop production, is one of only two pollutants that are increasing in the Cannon River. That’s why it’s so impressive that a field project to encourage farmers in Dundas, MN to plant more cover crops is showing a 34% reduction in nitrates from fields planted with cover crops compared to fields without cover crops.
Local farmers are also seeing the benefit of using cover crops on their fields. John Becker, one of the area farmers, said that he is excited about this project because cover crops protect the soil and prevent erosion. “Once the soil gets amended, the soil grows better crops. This is good for the soil, good for the crops and good for the stream,” said Becker.
2021 marks the third year that Clean River Partners, a non-profit organization in Northfield, MN, is measuring how cover crops impact water quality in Rice Creek, the only trout stream in Rice County and the most western trout stream in all of Minnesota. The 2021 mid-season results show nitrate concentration levels in 2021 have generally been the lowest levels recorded since the start of the study in 2018. Low rainfall in 2021 may have played a role in this result, but continued lower nitrate concentration in tile drainage from fields with cover crops compared to fields without cover crops supports that land use, in this case planting cover crops, also impacts water quality, even in dry years.
For the past three years, a dozen farmers in the Rice Creek watershed have been planting cover crops on about 1000 acres (30%) of farmland in the 4,100 acre watershed. Clean River Partners, Rice SWCD and St. Olaf College have been comparing nitrate concentration in tile drainage from fields planted with cover crops and fields without. They have also been testing nitrate concentration in Rice Creek as well as the watershed’s main drainage ditch. The stream sampling location, which is downstream from all other sampling locations and therefore considered to be a good representation of all the water entering from upstream, averaged 34% less nitrate concentration by mid-summer 2021 compared to 2020, 42% less than 2019 and 66% less than the 2013 level. Nitrate concentration levels from tiles draining fields with cover crops fields also had 34% less nitrate concentration compared to tiles draining fields without cover crops by mid-summer 2021.
Much of Minnesota has experienced extreme drought during most of the 2021 growing season. Precipitation in the Rice Creek watershed was about 4 inches less by mid-August 2021 compared to the same time period in 2019 and 2020. During that period, tiles that drained fields with cover crops averaged 28% less nitrate concentration compared to fields without cover crops. The mid-summer 2021 result is 34% less for fields with cover crops.
Although some of the nitrate reduction seen so far this year could be attributed to reduced rainfall, cover cropping has played a significant role. With just three exceptions on fields, the corn-soybean crop rotation within the overall study area was the same in 2021 as it was in 2019. This is important to consider because corn and soybeans uptake nutrients at different rates and will likely lead to different levels of nitrate concentration in the drainage water. Again, while low rainfall in 2021 likely reduced the total amount of nitrate discharging from tile drainage, the results in 2021 show that even with the same crops growing on the same land as in 2019, average nitrate concentration in the stream was 42.8% lower in 2021 compared to 2019. This finding further supports that cover crop history plays an important role in nutrient runoff reduction and suggests that the watershed health benefits of using cover crops increases with the number of years that the practice is in use.
The results of this study are evidence that planting cover crops reduces nitrate discharge in tile drainage, and when planted on a significant portion of the watershed, cover crops can improve water quality in streams. Thank you to the collaborating farmers and landowners in the Rice Creek watershed. They contributed about 40% of the cover crop cost and their leadership has provided real world, farm level results that are critical to understanding how agricultural practices can benefit water quality. Other important partners include Fishers and Farmers Partnership, Rice SWCD, Minnesota DNR, Minnesota Pollution Control and Compeer Financial. Bridgewater Township joined this list in 2021 to continue this project through 2024.