Nitrate Isotopic Composition, Hubbard Brook Experimental Forest, Watershed 3, 2011
TITLE
Nitrate Isotopic Composition, Hubbard Brook Experimental Forest, Watershed 3, 2011
PRINCIPAL INVESTIGATOR(s)
Christine Goodale
Cornell University
Dept. of Ecology and Evolutionary Biology
E215 Corson Hall
Ithaca, NY 14853
USA
Sarah Wexler
University of East Anglia
School of Environmental Sciences
Norwich NR4 7TJ
UK
Peter M. Groffman
Cary Institute of Ecosystem Studies
Box AB
Millbrook, NY 12545
USA
OTHERS INVOLVED
Kevin McGuire
Virginia Tech
Blacksburg, VA 24061
USA
Scott Bailey
Hubbard Brook Experimental Forest
234 Mirror Lake Road
North Woodstock, NH 03262
USA
ABSTRACT:
Despite decades of measurements, the nitrogen balance of temperate forest catchments remains poorly understood. Atmospheric nitrogen deposition often greatly exceeds streamwater nitrogen losses; the fate of the remaining nitrogen is highly uncertain. Gaseous losses of nitrogen to denitrification are especially poorly documented and are often ignored. Here, we provide isotopic evidence (δ15NNO3 and δ18ONO3) from shallow groundwater at the Hubbard Brook Experimental Forest indicating extensive denitrification during midsummer, when transient, perched patches of saturation developed in hillslopes, with poor hydrological connectivity to the stream, while streamwater showed no isotopic evidence of denitrification. During small rain events, precipitation directly contributed up to 34% of streamwater nitrate, which was otherwise produced by nitrification. Together, these measurements reveal the importance of denitrification in hydrologically disconnected patches of shallow groundwater during midsummer as largely overlooked control points for nitrogen loss from temperate forest catchments.
KEYWORD SET: Hubbard Brook Ecosystem Study LTER
15N, 18O, catchments, denitrification, forests, groundwater, HBEF Watershed 3, HBR, Hubbard Brook LTER, hydrologic, isotopes, nitrification, nitrogen, precipitation, streamwater.
KEYWORD SET: LTER Core Research Areas
inorganic nutrients.
BEGIN DATE
2011-07-03
END DATE
2011-07-14
LOCATION
HBEF watershed 3 (WS3)
West bounding coordinate: -71.725029
East bounding coordinate: -71.716533
North bounding coordinate: 43.962196
South bounding coordinate: 43.954650
Research Summary
During July, 2011, solutions were collected throughout Hubbard Brook watershed 3 (WS3) to provide a baseline characterization of nitrate isotopic composition across a range of landscape positions throughout the watershed.
Site Description
This study focused on watershed 3 (WS3), a 41.2-ha hydrologic reference watershed at the HBEF. Temperatures average −9 degreesC in January and 18 degreeC in July, and annual precipitation averages 1,400 mm, with approximately 70% falling as rain. WS3 is steep and south facing, with an elevation range of 537–732 m. The HBEF is covered by second-growth northern hardwoods naturally regenerated after harvesting between 1910 and 1917. WS3 is underlain by mica schist bedrock of the Silurian Rangeley Formation, and covered by Wisconsinan glacial tills. Spodosols of sandy loam to loamy sand texture comprise approximately 80% of catchment soils, and Inceptisols and Histosols make up the rest. The C horizon occurs at approximately 70 cm depth and has variable although generally lower hydraulic conductivity compared with the overlying B horizon.
A shallow groundwater system with a transient saturated zone develops within the solum throughout the catchment. A more consistent saturated zone is present in the near-stream region that is typically hydrologically connected to surface water in perennial stream reaches and is at or near the surface beneath ephemeral and intermittent stream reaches. WS3 is drained by Paradise Brook, a second-order perennial stream fed by several ephemeral and intermittent tributaries, which together comprise 79% of the stream length within the catchment. Solutions from weirs of other watersheds at HBEF were collected for comparison.
Sampling Design
Sampling encompassed 11 locations along Paradise Brook and 8 locations on tributaries, along with 1 seep and 9 shallow groundwater wells selected from more than 30 wells installed previously in WS3 (Detty and McGuire, 2010a; Detty and McGuire 2010b; Zimmer et al. 2012), and chosen for their tendency to provide water during midsummer and ease of repeated sampling. The wells included three wells in or within 2 m of the perennial channel of Paradise Brook and forming a transect across it (R12 on the east, R13 in-stream, and R14 on the west). The other six wells were ≥4 m away and upgradient from surface streamflow during July 2011, and included two wells located within a seasonally dry side-stream channel (JD29, JD30), one well ∼4 m from the perennial stream and generally upgradient from it (JD05), and three wells that form an upslope transect ∼3–29 m from a seasonally dry tributary (JD17, JD18, JD19). Well depth ranged from 30 cm (JD30) to 115 cm (JD05), with most wells accessing water from the lower mineral horizon. Three wells (JD05, JD17 and JD18), the seep, and seven sites on Paradise Brook and its tributaries were sampled on six dates on a near-daily basis (3, 4, 5, 7, 8, and 12 July 2011). The three near-stream wells were sampled up to nine times at 20–40 min intervals on 11 July 2011, along with concurrent sampling from Paradise Brook 0.5 m upstream of the wells. A mix of precipitation and throughfall was collected from a small clearing at the southern WS3 boundary. Solutions from weirs of other monitored watersheds (WS1 - WS9) were collected on two dates (9 and 14 July, 2011).
Field Collection Procedures
Precipitation was collected using a rinsed 10-L plastic collector. Stream samples were collected with a cleaned high-density polyethylene collector. Well samples were collected using a peristaltic pump, after purging.
Analytical Methods
Solution concentrations of nitrate, nitrite, and ammonium were measured using ion chromatography (Dionex ICS-2000; Dionex Corp.) after filtering in the lab to 0.45 μm. All nitrite concentrations were below the detection limit (0.1 μM). Total dissolved nitrogen (TDN) and dissolved organic carbon (DOC) were measured by combustion with a Shimadzu TOC-VCPN and TNM-1 chemiluminescent detector (Kyoto, Japan) after filtration to 0.7 μm. Samples for isotope analysis were frozen at −20 degreeC and later prepared at Cornell University using the denitrifier method (Sigman et al. 2001, Casciotti et al. 2002) to produce N2O gas, which was sent to the Stable Isotope Facility, University of California-Davis, Davis, CA, for analysis of δ18O and δ15N, with a precision of +/-0.3‰ for both isotopes.
Calculations
DON concentration was calculated by difference (DON = TDN – nitrate – ammonium).
REFERENCES
  • Detty, J.M., and McGuire, K.J. 2010. Topographic controls on shallow groundwater dynamics: Implications of hydrologic connectivity between hillslopes and riparian zones in a till mantled catchment. Hydrol. Processes 24(16):2222-2236.
  • Detty, J.M., and McGuire, K.J. 2010. Threshold changes in storm runoff generation at a till-mantled headwater catchment. Water Resour. Res. 46(7):.
  • Wexler, S.K., Goodale, C.L.., McGuire, K.J., Bailey, S.W., and Groffman, P.M. 2014. Isotopic signals of summer denitrification in a northern hardwood forested catchment. Proc. Nat. Acad. Sci. 411:16413-18.
  • Zimmer, M.A., Bailey, S.W., McGuire, K.J., and Bullen, T.D. 2013. Fine scale variations of surface water chemistry in an ephemeral to perennial drainage network. Hydrol. Processes 27(24):3438–3451.
DATA ACCESS GUIDELINES
Data Use Policy



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Citation example: Holmes, R.T. 2012. Bird Abundances at Hubbard Brook (1969-2010) and on three replicate plots (1986-2000) in the White Mountain National Forest. Durham, NH. Hubbard Brook Data Archive [Database]. http://hubbardbrook.org/data/dataset.php?id=81 (23 July 2012)

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Acknowledgment example: Data on [topic] were provided by [name of PI] on [date]. These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the USDA Forest Service, Northern Research Station, Newtown Square, PA. Significant funding for collection of these data was provided by [agency]-[grant number], [agency]-[grant number], etc.

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CONTACT PERSON

Information Manager, Hubbard Brook LTER
234 Mirror Lake Road
North Woodstock, NH 03262
USA

Phone: (603) 726-8902
Email: hbr-im@lternet.edu

Data file: w3_nitrate_isotope.txt
Description: Nitrogen and 15NNO3 and 18ONO3 isotope measurements in water samples from wells, streams, and precipitation on Watershed 3 and additional watershed weirs at Hubbard Brook Experimental Forest.
Notes on Data: Interested parties are free to use these data for informational purposes but they cannot be used in any publication without permission of the Principal Investigator. The data is physically located at the Goodale Lab, Cornell University.
ColumnVariableDescriptionUnitsCoded?Missing value label
1WshdWatershed number, 1-9noneynone
2TypeSolution typenoneynone
3SiteLocation of sampling by solution typenoneynone
4DateDate of sample collectionYYYY-MM-DDnnone
5TimeTime of sample collection, if more than one per date. HH:MMnnone
6d15NNitrate 15N composition in per mil relative to VSMOW standardperMiln
-999.9
7d18ONitrate 18O composition in per mil relative to air N2 standardperMiln
-999.9
8NO3Nitrate concentration in micromole per litermicromolePerLitern
-999.9
9NH4Ammonium concentration in micromole per litermicromolePerLitern
-999.9
10DONDissolved organic nitrogen concentration in micromole per litermicromolePerLitern
-999.9
11TDN Total dissolved nitrogen concentration in micromole per litermicromolePerLitern
-999.9
12DOCDissolved organic carbon concentration in micromole per litermicromolePerLitern
-999

CODES

Variable: Wshd
Description:
unique number for each HBEF watershed, numbered 1 - 9
Variable: Type
Code
Description
Precip
Precipitation sample
Seep
Seepwater sample
HillslopeWell
Hillslope well sample
RiparianWell
Riparian well sample
Streamwater
Streamwater sample
Variable: Site
Description:
A unique identifier for each sample point

MISSING VALUE CODES
Variable
Missing Value Code
Code Explanation
d15N
-999.9
Data missing or not taken at this time
d18O
-999.9
Data missing or not taken at this time
NO3
-999.9
Data missing or not taken at this time
NH4
-999.9
Data missing or not taken at this time
DON
-999.9
Data missing or not taken at this time
TDN
-999.9
Data missing or not taken at this time
DOC
-999
Data missing or not taken at this time