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 experi- mental nitrate (NO3 � -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 dis- charge 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 NO3 � -N was attenuated almost 100% from 1.09 to 60%) of NO3 � -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 NO3 � -N disap- pearance. 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 denitri- fication often drives N losses, and that enhancing connections between rivers and their floodplains may enhance overall retention and reduce N exports from large basins.