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