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Denitrification potential in floodplain forests: Investigating water quality improvements to reduce negative impacts in downstream communities
YEAR: 2022
INVESTIGATORS: William Beck, Emily Zimmerman

The US Cornbelt is a leading global producer of intensively managed, row-crop corn and
soybeans – yet, the agricultural production of these commodities is often linked to challenges
associated with nonpoint source nutrient pollution that negatively impacts water quality and
highly altered hydrology that negatively impacts surface water discharge. Excess nitrogen and
phosphorus in surface water can have regional and local impacts on aquatic ecosystems,
recreational opportunities, livelihoods, and human health. Altered hydrology can lead to excess
discharge, resulting in extreme flood events. While these challenges affect many downstream
regional and local communities, underrepresented, underserved communities and communities of
color are often disproportionately impacted by these negative environmental externalities.
Floodplain management offers a unique opportunity to address challenges related to water
quality and water quantity to reduce downstream impacts. Floodplain forests represent a
potentially powerful water quality and quantity conservation practice, as rigid upright stems and
associated downed woody material and understory vegetation act to reduce flood flow velocities
and encourage water retention and infiltration. Deposition of sediment and associated
phosphorus are associated with reduced flood velocities, and deposition acts to transfer these
potential pollutants into long-term floodplain storage. Often overlooked is the ability of
floodplain forests to address riverine nitrogen loading. Through a combination of floodwater
retention, raised water tables, and sediment and organic matter deposition, floodplain forests
create zones of nitrate removal by encouraging processes such as denitrification. In addition to
larger water retention sites often associated with woody floodplain vegetation (e.g., oxbows),
individual trees and downed woody material provide abundant water retention microsites
through tree windthrow (e.g., pit and mound microtopography) and localized scour. Although
floodplain forests represent potentially significant nitrate sinks, research that quantifies nitrogen
removal within these areas is rare – especially in the agricultural Midwest. In addition, studies
that seek to link water quality and quantity impact with current forest species composition,
structure, and condition, and provide management recommendations to maximize water quality
and quantity return, are exceptionally rare.
The primary objectives of this project are to: 1.) estimate riverine nitrogen removal performance
of individual floodplain forest sites along major Iowa rivers, 2.) combine floodplain forest water
quality and quantity data with forest stand inventory to estimate watershed-scale impacts of
floodplain forests, and 4.) increase awareness and value of floodplain forest systems within
impacted, underserved downstream communities. To meet these objectives, we will employ a
combination of in-field monitoring and inventory, hydraulic floodplain modeling, a conservation
planning tool, and extension efforts. The results and outcomes from this project will be used to
create floodplain forest management recommendations, and maximize water quality (i.e.,
nitrogen) and flood mitigation returns for underserved downstream communities. Field data will
also be coupled with GIS-based modeling used by co-PI Zimmerman to quantify landscape-level
outcomes associated with floodplain forest management and key opportunities to maximize
restoration benefits. Associated extension programming seeks to increase awareness of
floodplain forest value to underserved audiences, and sustain management through promotion of
natural resource careers (e.g., forestry) within underserved downstream communities.