Nitrogen Cycling in Freshwaters IV
Presiding: M N Gooseff, Colorado School of Mines; S Findlay, Institute of Ecosystem Studies
NB24F-01 15:30h
A 15N Tracer Study of Nitrate Retention in a Third-Order Stream Flowing Through Different Land Use Conditions, Willamette Valley, Oregon
We conducted 15NO3- tracer studies in three reaches of Oak Creek, a third-order stream in the Willamette Valley, Oregon, in summer 2003 as part of the Lotic Intersite Nitrogen eXperiment II (LINX II). Nitrate-nitrogen uptake length was longest (960 m) in a forested reach (discharge = 7.5 L s-1), and declined to 670 m in an agricultural reach (5.5 L s-1) and 570 m in an urban reach (3.9 L s-1), corresponding to differences in discharge among reaches. When uptake rates were corrected for stream velocity and depth, nitrate-nitrogen uptake velocity declined from the forest (0.22 mm min-1) to the agricultural (0.19 mm min-1) and urban (0.13 mm min-1) reaches. Denitrification, determined through 15N2 and 15N2O gas production following tracer additions, was not detectable in the forest but accounted for 2 and 5% of nitrate-nitrogen uptake in agricultural and urban reaches, respectively. Other sinks for retained nitrate-nitrogen varied among the reaches. In the forest, allochthonous organic matter had the highest biomass-specific uptake. In the agricultural and urban reaches, autotrophic components played increasingly important roles in nitrate retention. Collectively, these results indicate significant shifts in retention pathways of nitrate-nitrogen in response to adjacent land use practices along this stream.
NB24F-02 INVITED 15:45h
Hydrologic and Biological Controls on the Fate of Watershed-derived Nitrogen in Estuaries: Insights from "Whole-estuary" Stable Isotope Enrichments
In situ nitrogen isotope tracer additions are becoming an increasingly widespread tool for understanding nitrogen fate and turnover in stream ecosystems. The approach however has only been attempted twice in estuaries. Isotope enrichment of these systems represents a several-fold increase in scale, and added challenges of bidirectional flow. Despite these potential limitations, the approach has yielded insights into nitrogen uptake, trophic transfer, and recycling on the scale of tens to hundreds of thousands of cubic meters of water and up to 4 kilometers of benthic area. The large-scale use of 15N tracer additions to estuaries has provided the unique ability to quantify multiple nitrogen (N) flow pathways under natural hydrologic and geochemical conditions. Estuarine 15N enrichment studies have been performed in two estuaries in the Plum Island LTER (Parker and Rowley Rivers) that differed in geomorphology, hydrology and biological structure. In each experiment 15N-nitrate tracer was added at the head of the estuaries in order to label watershed N entering the estuary and provided a "tag" for that N as it was processed in the estuary. 15N tracer released into the long water residence time Parker River was processed primarily (approx. 100 percent) through phytoplankton which subsequently supported high rates of secondary production in both the water column and benthos. In contrast, tracer released into the short water residence time Rowley River was primarily (75-80 percent of added 15N) advected, unaltered out of the estuary as nitrate. The 15N sinks that did exist within the Rowley were divided evenly between assimilation by benthic autotrophs (including uptake by marsh macrophytes) and denitrification. Recycling of the assimilated 15N back into the water column was a more important mechanism of N transfer than support of secondary production in the Rowley. Collectively, the use of large-scale stable 15N isotope additions in these contrasting estuaries has allowed us to examine how geomorphology, hydrology, and biotic structure interact to regulate estuarine N processing.
NB24F-03 16:00h
What controls nitrate removal in streams of varying land use?
In the 2nd year of a 3 year, multi-biome project, we quantified 15N-nitrate removal in streams of varying land use (native vegetation, agriculture, and urban) in southwestern Michigan and Jackson, Wyoming and we found distinct patterns in nitrate removal compared to the first year of the study. Water-column nitrate concentrations ranged from 1-165 ugN/L in Wyoming and 80-648 ugN/L in Michigan. Atypically, agricultural streams had the lowest nitrate concentrations in both biomes at the time of tracer additions. Whole-stream nitrate uptake was highest in the urban stream in Michigan (3.4 ugN/m2/s) and lowest in the stream draining pasture in Wyoming (0.15 ugN/m2/s) but was not simply related to areal denitrification rate or whole-stream metabolism (e.g. primary production or community respiration). However, relative nitrate demand (as uptake velocity) and whole-stream metabolism were related to stream width and discharge. Denitrification was highest in the native vegetation stream in Wyoming (0.66-1.1 ugN/m2/s), and lowest in the agricultural stream in Michigan (0.03-0.06 ugN/m2/s), but denitrification was not related to nitrate concentration. In Wyoming, denitrification represented a larger proportion of nitrate uptake than in Michigan streams. Understanding the relative importance of mechanisms controlling water-column nitrate removal will be critical in managing watershed nitrogen export.
NB24F-04 16:15h
Denitrification Rates in Streams Determined in an Intersite 15N Addition Study
Several recent studies have suggested that denitrification in streams may be a significant sink for fixed nitrogen in the landscape. Most previous studies of denitrification have relied on either acetylene block techniques using laboratory sediment core incubations not reflective of in situ conditions or indirect estimates involving changes in streamwater concentrations of nitrate. In this study, we used a new field 15N tracer addition approach that directly measures in situ rates of denitrification at the scale of entire stream reaches by determining tracer 15N-nitrate flux into nitrogen gas. We measured denitrification rates in headwater streams in 8 regions across the U.S. draining reference, agricultural, and urbanized catchments. Partial results from the first 2 years of this 3-year study showed denitrification rates ranging from undetectable to 1.1 ug N m-2 s-1 and, on average, accounting for about 20% of total nitrate uptake. In general, denitrification rates were higher in urbanized streams than in reference and agricultural streams. Denitrification rates also were positively related to streamwater nitrate concentrations. Results to date indicate that while denitrification is an important sink for nitrate in streams with high nitrate concentrations, it is generally low relative to total loss of nitrate from water.
NB24F-05 16:30h
Using Natural Hydrologic Variability to Separate the Effects of Water Availability on Hydrologic Transport From the Effects on the Biogeochemical Processing of Nitrogen and Carbon
The cycling of water, carbon, and nitrogen are intimately linked at a wide range of spatial and temporal scales complicating the interpretation of catchment solute export. Although carbon and nitrogen can affect the hydrologic cycle on longer time scales through changes in vegetation, water availability affects the cycling of carbon and nitrogen on time scales from minutes to centuries. The timing and amount of water variability affect carbon and nitrogen directly through hydrologic transport and indirectly through controlling the biogeochemical processes that effect the size, form, and mobility of carbon and nitrogen species available to be transported to surface water. A large number of terrestrial ecosystems are characterized by a pronounced seasonality in precipitation, water availability, and hydrologic transport of carbon and nitrogen in surface water. This seasonality results in an extended period of time when the importance hydrologic transport on carbon and nitrogen cycling are relatively low while the potential for biogeochemical processing to affect the size of the potentially mobile solute pool is high. In contrast, the importance of hydrologic transport on solute dynamics during the first increases in water availability is high while biogeochemical modification may be delayed or minimized. In these systems the effects of antecedent water availability on biogeochemical processes during the relatively dry season can be evaluated separately from the effects of transport at the initiation of the wet season aiding interpretation of catchment solute export. We will present examples of using this approach to help explain solute dynamics in snowmelt dominated cathments in Colorado, semi-arid systems in Arizona and New Mexico, and mediterranean catchments in California.