NITRATE UPTAKE BY FORESTED FLOOD PLAIN SOILS OF THE BLACK RIVER, LA 
CROSSE, WISCONSIN

Rebekka Steidler 1, 2, Emy Monroe 1, and William Richardson1.
1 Upper Midwest Environmental Sciences Center, U.S. Geological Survey, La Crosse, 
WI 54602; and 2 Department of Chemistry, Winona State University, Winona, MN 
55987.

While patterns of nitrogen loading to the Gulf of Mexico are well documented, little is 
known about nitrogen cycling in the Mississippi River.  This is particularly true for the 
Upper Mississippi River where some connectivity between main channel and 
backwaters still exists.  The preliminary work reported here is part of a larger study 
by scientists from the U.S. Geological Survey's Upper Midwest Environmental 
Sciences Center to more fully understand the nitrogen cycle in the Upper Mississippi 
River (UMR).  The purpose of this work was 1) to measure rates of nitrate (NO3) 
uptake by sediments from UMR; and 2) to test the hypothesis that dissolved organic 
carbon (DOC) is a limiting factor in the uptake of nitrate from overlying water by 
sediments.  Carbon limitation was tested by 1) evaluating nitrate uptake rates in 
sediments along a suspected sediment carbon gradient, and 2) by testing the effect of 
controlled carbon enrichment on the sediment uptake of NO3 from overlying water. 
During July and August 1999 sediment cores were collected from a forested flood 
plain adjacent to the Black River, a tributary of the UMR, at La Crosse, WI.  The 
transect ran perpendicular to the Black River along a suspected carbon gradient from 
dry flood plain forest, through a wet slough, a dry sand dike (natural), and into the 
Black River proper.  Nitrate uptake experiments were conducted and sediment carbon 
was measured on these cores; cores taken later from along this same transect were 
used to measure nitrate uptake with two DOC amendments, glucose as a labile 
carbon source, and leaf leachate as a ubiquitous source of carbon in the river.  Nitrate 
uptake in sediments along this transect ranged from 0.14-0.90 mg NO3/L/hr and 
sediment carbon (by loss on ignition) ranged from <1-29%.  Nitrate uptake was 
greater in sediments with higher carbon content.  For example, sediments in the wet 
sough had the highest carbon content (29%) and the highest nitrate uptake rates (0.9 
mg NO3/L/hr); sediments with the lowest carbon content (0.3%) had the lowest 
uptake rates (0.14 mg NO3/L/hr).  Sediments from the river channel, however, had 
low carbon content (0.98%) and high uptake rates of 0.71 mg NO3/L/hr.  Carbon 
amendments did increase uptake rates of some sediments.  For example, uptake rates 
in sediments from the dry sand dike and the dry flood plain forest showed an increase 
in rates when both forms of DOC were added.  Sediments from the sand bar and 
floodplain forest sites, without carbon amendment, had nitrate uptake rates of 0.08 
and 0.06 mg NO3/L/hr respectively, and rates increased to 0.44 and 0.74 mg NO3/L/hr 
in the carbon amended sediments.  Nitrate uptake rates in river sediments were 
unaffected by carbon amendment: rates were 0.06 mg NO3/L/hr without amendment, 
and 0.49 and 0.64 mg NO3/L/hr with glucose and leaf leachate amendments.  These 
results suggest that: 1) floodplain sediments may represent a significant sink for 
nitrate but uptake rates vary with sediment carbon content and location, and 2) rates 
of nitrate uptake from low carbon floodplain sediments are likely carbon limited.

Keywords:  nitrate uptake, floodplain forest, Black River, carbon, sediment
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