Rapid Nitrate Loss and Denitrification in a Temperate River Floodplain

Abstract

Nitrogen (N) pollution is a problem in many large temperate zone rivers, and N retention in river channels is often small in these systems. To determine the potential for floodplains to act as N sinks during overbank flooding, we combined monitoring, denitrification assays, and experimental nitrate (NO ⁻₃ -N) additions to determine how the amount and form of N changed during flooding and the processes responsible for these changes in the Wisconsin River floodplain (USA). Spring flooding increased N concentrations in the floodplain to levels equal to the river. As discharge declined and connectivity between the river and floodplain was disrupted, total dissolved N decreased over 75% from 1.41 mg l⁻¹, equivalent to source water in the Wisconsin River on 14 April 2001, to 0.34 mg l⁻¹ on 22 April 2001. Simultaneously NO ⁻₃ -N was attenuated almost 100% from 1.09 to <0.002 mg l⁻¹. Unamended sediment denitrification rates were moderate (0–483 μg m⁻² h⁻¹) and seasonally variable, and activity was limited by the availability of NO ⁻₃ -N on all dates. Two experimental NO ⁻₃ -N pulse additions to floodplain water bodies confirmed rapid NO ⁻₃ -N depletion. Over 80% of the observed NO ⁻₃ -N decline was caused by hydrologic export for addition #1 but only 22% in addition #2. During the second addition, a significant fraction (>60%) of NO ⁻₃ -N mass loss was not attributable to hydrologic losses or conversion to other forms of N, suggesting that denitrification was likely responsible for most of the NO ⁻₃ -N disappearance. Floodplain capacity to decrease the dominant fraction of river borne N within days of inundation demonstrates that the Wisconsin River floodplain was an active N sink, that denitrification often drives N losses, and that enhancing connections between rivers and their floodplains may enhance overall retention and reduce N exports from large basins.