Nitrate on a slow decline: Watershed water quality response during two decades of tallgrass prairie ecosystem reconstruction in Iowa

Authors: Tomer, Mark; SHILLING, KEITH - University Of Iowa; Cole, Kevin

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/31/2019
Publication Date: 3/4/2019
Publication URL: https://handle.nal.usda.gov/10113/6563711
Citation: Tomer, M.D., Shilling, K.E., Cole, K.J. 2019. Nitrate on a slow decline: Watershed water quality response during two decades of tallgrass prairie ecosystem reconstruction in Iowa. Journal of Environmental Quality. 48:579-585. https://doi.org/10.2134/jeq2018.07.0258.
DOI: https://doi.org/10.2134/jeq2018.07.0258

Interpretive Summary: Opportunities to evaluate long-term water quality change in watersheds following substantial changes in land cover are rare, particularly in agricultural watersheds of the U.S. Midwest. At the Neal Smith National Wildlife Refuge in central Iowa, more than 3,000 acres of tallgrass prairie were seeded between 1993 and 2006, all in the Walnut Creek watershed. Changes in the losses of nitrates to Walnut Creek have now been tracked for 20 years, and this study updates a past report made after 10 years of observation, when many of the prairie seedings were still becoming fully established. Losses of nitrate continued to decline at a slow rate, only -0.15 ppm (parts per million) of nitrate-nitrogen per year, which was greater than, but not statistically distinguishable from, the rate reported after the first decade. There was also evidence that stream flow volumes declined slightly with the transition from annual cropping to prairie cover, further contributing to the trend of decreased nitrate loads. However, variability in climate, given two years with flooding and a major drought both occurred during the second decade of observation, combined with the slow rate of water quality change, challenged any notion that a watershed water quality record can clearly stabilize, more than 10 years after a third of this watershed was retired from crop production. This result is of interest to agricultural conservationists, producers, and policy makers who are trying to better understand the linkage between conservation efforts and long-term timing of water quality responses.

Technical Abstract: Opportunities to evaluate long-term water quality change in watersheds following substantial changes in land cover are rare, particularly in agricultural watersheds of the U.S. Midwest. The Neal Smith National Wildlife Refuge (NSNWR) was established as a tallgrass prairie ecosystem reconstruction in Walnut Creek watershed (5238 ha), Jasper County Iowa, with >1200 ha of prairie plantings initiated between 1993 and 2006. This study updates the documented decreases in watershed NO3-N losses that accompanied this change in land cover to a 20-year record. Annual flow-weighted NO3-N concentrations declined by 0.15 mg NO3-N L-1 yr-1, which was not significantly different than the rate of 0.07 mg NO3-N L-1 yr-1 reported after the first decade of monitoring. There was also evidence, at monthly and annual measurement scales, that watershed water yield declined with the transition from annual cropping to prairie cover, contributing to the trend of decreasing NO3-N loads. However, variability in climate, including two years with significant flooding events followed by a major drought during the second decade of monitoring, combined with the slow rate of water quality change, challenged any notion that a watershed water quality record will stabilize even >10 years after a substantial change in land cover. Efforts to document progress towards water quality goals will need to consider multi-decadal time lags, a changeable climate, and uncertainty in the effectiveness of management changes.