Influence of geomorphological variability in channel characteristics on sediment denitrification in agricultural streams

Within fluvial systems, the spatial variability of geomorphological characteristics of stream channels and associated streamhed properties can affect many biogeochemical processes. In agricultural streams of the midwestern USA, it is not known how geomorphological variability affects sediment denitrification rates, a potentially important loss mechanism for N. Sediment denitrification was measured at channelized and meandering headwater reaches in east-central Illinois, a region dominated by intensive agriculture and high NO₃-N stream export, between June 2003 and February 2005 using the chloramphenicol-amended acetylene inhibition procedure. Sediment denitrification rates were greatest in separation zones, ranging from 0.6 to 76.4 mg N m ^-2 h^-1, compared with riffles, point bars, pools, and a run ranging from 0 to 36.5 mg N m^- h^-1. Differences in benthic organic matter (r = 0.70) and the percentage of fine-grained sediments (r = 0.93) in the streambeds controlled much of the spatial variations in sediment denitrification among the geomorphological features. Although two meandering study reaches removed 390 and 99% more NO₃-N by sediment denitrification than adjacent channelized reaches, NO₃-N loss rates from all reaches were between 0.1 and 15.7% d^-1, except in late summer. Regardless of geomorphological characteristics, streams in east-central Illinois were not able to process the high NO₃-N loads, making sediment denitrification in this region a limited sink for N.