Labile Dissolved Organic Carbon Availability Controls Hyporheic Denitrification: a 15N Tracer Study 1 Dept. of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331, USA* email:
[email protected]
A.
Methods ● Instrumented HZ: well network (n=11), wells screened from 20‐40 cm deep to capture lateral HZ flowpaths across gravel bar. ● Tracer Experiment: A 48 h steady‐state injection of a conservative tracer, chloride, and 15NO3‐ was used to quantify ambient HZ denitrification Instrumented via 15N2 production. Following ambient plateau measurements of denitrification during the first 24 h, a second conservative tracer, bromide, Gravel Bar and labile DOC source, acetate, were co‐injected for an additional 24 h to measure HZ denitrification under increased labile DOC supply (Figure 4). ● Sampling: We collected samples b f before (n=5) and after (n=5) DOC ( ) d f ( ) OC amendment (Figure 5; sampling time interval ~2 h) for solutes relevant to denitrification (15NO3, 15N2(g), as well as NO3, DOC, DO, Cl‐, Br). Hydraulic transport parameters (head, flow rates, flowpaths, and residence times) were also measured along instrumented HZ. Figure 4. Basic experiment design.
Injection Setup HZ Sampling
Br (mg-Br L-1)
● Previous research shows: 1. This gravel bar is a stream denitrification hot spot. 2. Denitrification and DOC dynamics are related to flowpath length and residence time (Figure 3). 3. DOC declines most rapidly at small residence times as microbial respiration utilizes the labile C fraction; remaining less labile DOC becomes more conservatively transported at larger residence times indicating C quality limitation on anaerobic processes.
DOC (mg-C L-1)
Figure 3. Denitrification dynamics across HZ transport (Zarnetske et al., 2008).
Figure 2. Study site: A. Study stream, B. Study HZ and well network.
INJ WELL
NO3- (mg-N L-1)
● We investigated the HZ of a lateral gravel bar located in Drift Creek, Marion County, OR, USA (Figure 2) ● Drift Creek is a 3rd‐order, upland agricultural, pool‐ riffle stream with: a thin coarse bed (boulder to fine sand) slope of 0 007 mean width of 5 3 m and sand), slope of 0.007, mean width of 5.3 m, and regionally high background NO3‐ (0.3‐0.5 mg‐N L‐1). Discharge during experiment was 15.5 L s‐1.
INJ WELL
(o/oo)
Study Site & Background
2
B.
DOC Treatment
15N
Figure 1. Respiratory denitrification.
Cl- (mg-Cl- L-1)
PreDOC Condition
Figure 6. Spatial concentrations of solutes for the PreDOC and DOC treatment periods.
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15N (o/oo) 2
● Conservative tracers injected at well H1 were observed at 4 of the 9 down gradient wells (H2, J2, K2, K3). ● J2 and K3 were most connected to injection well (H1). ● Receiving wells represented HZ median residence times of 7.0 7 0 to 13.1 13 1 h, h nominal flowpath lengths of 0.7 to 3.7 m, and hypoxic conditions (7.5 to 9.3 mg-O2 L-1 deficit). ● Despite injecting acetate at 42 mg-C Figure 5. Representative sample results L-1, no acetate was detected in from experiment (well J2). receiving wells.
NO O3- (mg-N L-1)
Stream‐groundwater (hyporheic, HZ) interactions are critical to understanding the transport and fate of nutrients, such as nitrogen, through catchments. However, the connections between HZ biogeochemical and physical transport conditions that control nutrient dynamics are poorly understood. We used in situ, steady‐state injections of acetate,15N‐labeled nitrate (15NO3) and conservative tracers (Cl‐ and Br) to assess the influence of labile dissolved organic carbon (DOC) availability on microbial d denitrification (Figure 1) in the HZ of an upland agricultural stream. f ( ) h f l d l l
15NO - (o/oo) 3
Results & Discussion
Introduction
H41E‐0941
Utah State UNIVERSITY
DOC ((mg-C L-1)
Water Resources Program
Pacific Northwest Research Station, Olympia Forestry Sciences Lab, 3625 93rd Ave SW, Olympia, WA 98512, USA 3 Dept. of Biology, Utah State University, 5305 Old Main Hill, Logan UT 84322, USA
Cl- (mg-Cl- L-1)
2
Br (mg-B Br L-1)
OSU Graduate
‐‐‐‐‐ Jay P. Zarnetske1,*, Roy Haggerty 1, Steven M. Wondzell2, Michelle A. Baker3 ‐‐‐‐‐
Ecosystem Informatics NSF IGERT
Flowpath Length Figure 7. Changes in concentrations of solutes for the PreDOC and DOC treatment periods. Note: * denotes significant at p=0.05, df = 4. All analyses above include 2 levels of period (PreDOC,DOC). We treat repeated samples within a well as (independent) replicates. Per well, we run paired t-tests to determine how strongly means of PreDOC and means of DOC differ. We exclude wells H3, I1, I3, J1, and J3 because they did not receive a significant experimental treatment.
● All 4 receiving wells demonstrated 15N2 production during PreDOC conditions indicating that the HZ was an active denitrification environment (Figure 6, 7). ● Acetate addition stimulated significant increases in 15N2 production by factors of 2.7 to 26.1 in all receiving wells, and significant decreases of NO3- in the two most connected wells (J2 and K3) (Figure 7). ● Increases of bromide and 15N2 production occurred without detectable concurrent increases in acetate indicating that 100% of acetate was retained in the HZ (Figure 6, 7).
Conclusions ● The use of 15NO3- enabled the direct measurement of changes in denitrification conditions due to a labile DOC addition (acetate). ● Labile DOC supply is limiting denitrification (N2 production) in this this HZ system. ● Results support our hypothesis that microbial denitrification in anaerobic portions of the hyporheic zone is limited by labile DOC supply. Acknowledgments Support for this project was provided by NSF grant EAR‐041240 and DGE‐0333257. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. Special thanks to: Phoebe Zarnetske for tireless statistical/field/lab help, Vincent Adams & Jun Yin for field help, and Cam Jones/Kathryn Motter of CCAL and OSU IWW Collaboratory for help with analyzing general water chemistry. References: Zarnetske et al. 2008. Hyporheic Denitrification in an Upland Agricultural Stream: a 15N Tracer Study. Eos Trans. AGU.
