Using denitrification to assess the functional significance of two-stage restoration for removal of agriculturally-derived nitrogen

Floodplains connect streams to riparian areas and often function as nitrogen sinks.  The addition of floodplains to incised agricultural channels is a potential management strategy promoting the removal of excess agriculturally-derived nitrogen.  We tested the two-stage restoration strategy in a northern Indiana agricultural stream using seasonal denitrification assays comparing stream sediments to soils collected from newly-constructed floodplains.  Denitrification rates were generally lower in floodplain soils compared to stream sediments, although denitrification occurred deeper in the floodplain.  In-stream denitrification rates were controlled by stream water nitrate concentrations (r² = 0.55, p < 0.007) and sediment organic matter content (r² = 0.34, p < 0.001).  Floodplain denitrification rates varied seasonally; there were no consistent relationships between denitrification and porewater nitrate, organic matter content, or soil saturation.   The addition of stream water to floodplain soils stimulated denitrification when floodplains were vegetated, suggesting that some aspect of plant growth augmented floodplain nitrogen removal.  Despite lower denitrification rates per unit organic matter in floodplain soils, the addition of bioreactive surface area resulting from floodplain construction increased reach-scale nitrogen removal by 50%.  Although results are encouraging, in high-nitrogen streams, floodplains alone may not be able to remove sufficient nitrogen to reduce export to downstream systems.