Contribution of wetlands to nitrate removal at the watershed scale

Abstract

Intensively managed row crop agriculture has fundamentally changed Earth surface processes within the Mississippi River basin through large-scale alterations of land cover, hydrology and reactive nitrogen availability. These changes have created leaky landscapes where excess agriculturally derived nitrate degrades riverine water quality at local, regional and continental scales. Individually, wetlands are known to remove nitrate but the conditions under which multiple wetlands meaningfully reduce riverine nitrate concentration have not been established. Only one region of the Mississippi River basin—the 44,000 km2 Minnesota River basin—still contains enough wetland cover within its intensively agriculturally managed watersheds to empirically address this question. Here we combine high-resolution land cover data for the Minnesota River basin with spatially extensive repeat water sampling data. By clearly isolating the effect of wetlands from crop cover, we show that, under moderate–high streamflow, wetlands are five times more efficient per unit area at reducing riverine nitrate concentration than the most effective land-based nitrogen mitigation strategies, which include cover crops and land retirement. Our results suggest that wetland restorations that account for the effects of spatial position in stream networks could provide a much greater benefit to water quality then previously assumed.

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Fig. 1: Nitrate observatory in the MRB.
Fig. 2: Effect of wetland cover/crop cover on riverine nitrate.
Fig. 3: Effect of wetland connectivity and streamflow on riverine nitrate.
Fig. 4: Effect of wetland spatial patterning on riverine nitrate.

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Acknowledgements

This research was funded by the National Science Foundation (NSF) through a NSF Science, Engineering and Education for Sustainability (SEES) Fellows grant (EAR-1415206) to A.T.H. and through a Water Sustainability and Climate Program (WSC) Observatory grant (EAR-1209402): REACH (Resilience under Accelerated Change). C.L.D. was additionally funded by an environmental grant from the Mortenson Family Foundation. We thank the many members of the Finlay Lab who assisted with sample collection and processing, especially A. Keeler, E. Senyk, K. Kemmit and M. Rorer.

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A.T.H. and J.C.F. conceived of the project. A.T.H. and C.L.D. conducted field work and analysed laboratory data. E.F.-G., J.C.F. and A.T.H. interpreted results. A.T.H. wrote the original paper while C.L.D., J.C.F. and E.F.-G. contributed significantly to the final version.

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Correspondence to Amy T. Hansen.

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Hansen, A.T., Dolph, C.L., Foufoula-Georgiou, E. et al. Contribution of wetlands to nitrate removal at the watershed scale. Nature Geosci 11, 127–132 (2018). https://doi.org/10.1038/s41561-017-0056-6

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