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Nitrate on a Slow Decline: Watershed Water Quality Response during Two Decades of Tallgrass Prairie Ecosystem Reconstruction in Iowa

Mark D. Tomer, Keith E. Schilling, Kevin J. Cole
Journal of environmental quality 2019 v.48 no.3 pp. 579-585
climate, conservation areas, drainage, drought, ecosystems, floods, glacial deposits, groundwater flow, habitat conservation, land cover, land management, losses from soil, monitoring, nitrate nitrogen, nitrates, streams, tallgrass prairies, water quality, water yield, watersheds, Iowa
The Neal Smith National Wildlife Refuge was established as a tallgrass prairie ecosystem reconstruction in the 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‐yr record. Annual flow‐weighted NO₃–N concentrations declined by 0.15 mg NO₃–N L⁻¹ yr⁻¹, which was not significantly different from the rate of 0.07 mg NO₃–N L⁻¹ yr⁻¹ reported after the first decade of monitoring. There was also evidence (p < 0.1) that prairie reconstruction led to a declining trend in annual watershed water yield, which would have contributed to the trend of decreasing NO₃–N loads. However, variability in climate, including 2 yr with significant flooding events followed by a major drought during the second decade of monitoring, challenged any notion that a watershed water quality record will stabilize even >10 yr after a substantial change in land cover, in this naturally drained watershed underlain by fine grained glacial deposits that exhibit multidecadal groundwater transport times. Efforts to document progress toward water quality goals will need to consider dominant flow paths and associated travel times, uncertainty in the effectiveness of management changes, and a changeable climate. CORE IDEAS: Nitrate in streamflow draining a prairie reconstruction was tracked for 20 yr. Nitrate loss decreased with time, at a slow rate of only 0.15 mg NO₃–N L⁻¹ yr⁻¹. Even with substantial land use change, water quality response may take decades.