Journal of Hydrology 367 (2009) 177-187

The role of hillsope hydrology in controlling nutrient lossWillem J. van Versevelda *, Jeffrey J. McDonnellb, Kate Lajthac

a Oregon State University, Department of Forest Engineering, 204 Peavy Hall, Corvallis, OR 97331-5706. USAb Water Resources Section, Delft Technical University, Delft. The Netherlands, 204 Peavy Hall. Corvallis, OR97331-5706. USAcOregon State University. Department of Botany and Plant Pathology. 2070 Cordley Hall, Corvallis. OR97331-2902, USA

ARTICLE INFO

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Article history:Received 29 September 2007Received in revised form 10 August 2008Accepted 4 November 2008

This manuscript was handled byG. Vachaud, Editor-in-Chief

Keywords:Hillslope hydrologySmall watershedDissolved organic carbonDissolved organic nitrogenSpecific UV-absorbance

Introduction

SUMMARY

Hydrological controls on DOC and N transport at the catchment scale were studied for five storm eventsfrom the fall of 2004 through the spring of 2005 in WS10. H,J, Andrews Experimental Forest in the wes-tern Cascade Mountains of Oregon, This catchment is devoid of any riparian zone and characterized byhillslopes that issue directly into the stream, This enabled us to compare a trenched hillslope responseto the stream response without the influence of riparian zone mixing. DOC and N concentrations and dis-solved organic matter (DOM) quality (specific UV-absorbance (SUVA) and C:N of DOM) were investigatedat the plot scale. in lateral subsurface flow from the trenched hillslope and stream water at the catchmentoutlet at the annual and seasonal scale (transition vs. wet period) during baseflow and stormflow condi-tions, DON was the dominant form of total dissolved nitrogen (TDN) in all sampled solutions. except intransient groundwater. where DIN was the dominant form. Organic horizon leachate and transientgroundwater were characterized by high SUVA. and high DOC and total N concentrations. while SUVAand DOC and DON concentrations in lysimeters decreased with depth in the soil profile. This suggestsvertical preferential flow without much soil matrix interaction occurred at the site, Deep groundwater(from a spring at the base of the hillslope) was characterized by low SUVA and low DOC and N concen-nations. SUVA was always lower in lateral subsurface flow than in stream water at the seasonal scale.even during the wet period when other solutes were similar between lateral subsurface flow and streamwater. This suggested mixing of deep groundwater and shallow transient groundwater was different atthe hillslope scale compared to the catchment scale. DOC and DON sources were finite (production ofDOC and DON from the hillslope soils appeared to be limited) at the seasonal scale since DOC andDON concentrations were significantly lower during the wet period compared to the transition periodduring stormflow conditions. This was also reflected in the DOC and DON peak and flow weighted stormevent concentrations and antecedent soil moisture relationship where drier conditions (less prior flush-ing) resulted in the highest DOC and DON peak and flow weighted storm event concentrations. Resultsfrom this study showed the importance of the hills lope component in DOC and N transport at the catch-ment scale and underscore the importance of sampling solutes below the root zone (transient groundwa-ter) and the value of using SUVA to fingerprint DOC sources.

© 2008 Published by Elsevier B.V.

Controls on dissolved organic matter (DOM) losses at the catch-ment scale are poorly understood. and yet DOM fluxes may haveimportant consequences for both terrestrial and aquatic ecosystemfunction. DOM has the ability to form complexes with metals. andthus plays an important role in metal toxicity and transport(Leenheer et al., 1998). Dissolved organic nitrogen (DON) can rep-resent a significant loss of nitrogen (N) (Sollins et al., 1980; Hedinet al., 1995; Perakis and Hedin. 2002; Vanderbilt et al., 2003) inunpolluted forested ecosystems. and may be a critical factor inmaintaining N-limitation in these systems (Vitousek et al., 1998),

* Corresponding author. Tel.: +15417374149; fax: +15417374315.E-mail address: willem.vanverseveld@oregonstate.cdu (W.]. van Verseveld).

0022-1694/$ - see front matter © 2008 Published by Elsevier B.V.doi: 10.1 016/j.jhydroI.2008.11.002

In addition. dissolved organic carbon (DOC) is an important energysource to bacteria and some algae in streams (Kaplan andNewbold, 1993) and absorbs UV-radiation (Morris et al., 1995) thatcan damage aquatic organisms.

Recent research has focused on the hydrological controls onstream concentrations and quality of dissolved organic carbon(DOC) (McKnight et al., 2002; McGlynn and Mclionnell, 2003;Inamdar and Mitchell, 2006; Park et al., 2007). dissolved organicnitrogen (DON) (Hill et al., 1999; Hagedorn et al., 2000; Buffamet al., 2001; Bernal et al., 2005). and nitrate (NO3N) (McHale etal., 2002; Ocampo et al., 2006). While these studies have improvedour understanding of flushing and draining processes of nutrientsat the catchment scale (as described by Hornberger et al., 1994;Boyer et al., 1997; Creed et al., 1996). quantifying spatial sourcesof these nutrients during storm events and across seasons remain

178 WJ. van Verseveld et al. / Journal of Hydrology 367 (2009) 177-187

poorly understood. The main reason is that it is difficult to separatedifferent geomorphic units of the catchment. While hillslopesmake up the largest part of catchments, research has been mostlyfocused on the riparian zone (Cirmo and McDonnell, 1997), suchthat sources of nutrients from the hillslope component are morepoorly understood compared than those from the riparian zone.

One approach to increase our understanding of spatial sourcesof DOM and N at the catchment scale, is to isolate discrete land-scape units and to understand their individual hydro-biogeochem-ical dynamics. While some studies have done this for the riparianzone (e.g., Hill, 1993; McDowell et al., 1992; Vidon and Hill,2004) few studies have been able to isolate the hiIlslope hydro-bio-geochemical response (McGlynn and McDonnell, 2003). It is diffi-cult to observe hydro-biogeochemical expressions of hillslopes inthe stream (Hooper, 2001), due to chemical transformations inthe riparian zone (Hedin et al., 1998) or infrequent episodic flowinto the riparian zone (McGlynn and McDonnell, 2003). An ap-proach to quantify the hiIlslope response directly, without anyriparian zone modulation, is to trench experimental hillslopes. Afew trenched experimental hills lopes exist around the world(Woods and Rowe, 1996 (Maimai, New Zealand); Tromp-van Meer-veld and McDonnell, 2006 (Panola, USA); Uchida et al., 2003 (Fud-ojid, Japan) but these experiments have typically monitored onlya handful of storms to work with (Tromp-van Meerveld andMcDonnell, 2006), and often lack detailed biogeochemical data.

While isolating the hilIslope or riparian zone has led to new in-sights into spatial sources of nutrients, questions remain about thehydrological controls on DOM and N export from the hillslopecomponent at seasonal and storm event scales. It is especiallyimportant to understand the role of hillslopes in DOM and N exportacross different antecedent wetness conditions because severalstudies have suggested that seasonal variation in stream DOC,DON and NO3N is related to antecedent wetness conditions (Tris-ka et al., 1984; Vanderbilt et al., 2003; Bernal et al., 2005) andmany studies have reported significant increases in DON, DOCand N03-N during individual storm events (Creed et al., 1996;McHale et al., 2002; Boyer et al., 1997).

We report on work from a small well-studied hills lope trenchwithin a headwater catchment at the H. J. Andrews ExperimentalForest (HJA), Oregon. The catchment is well-suited for exploringquestions of how hillslope hydrological processes control streamDOC and N concentrations. This study site has a unique feature:hiIlslopes that issue directly into the headwater stream withoutany riparian zone modulation. Riparian zone water storage waseffectively removed from the site due to 1986 and 1996 debrisflows that evacuated the valley bottom. This setup made it possibleto isolate lateral subsurface flow from the hillslope trench andcompare the hydro-biogeochemical response from this hillslopeto the response of the whole array of hillslopes that make up thiswatershed. Furthermore, we explored the use of different indicesof DOM quality (specific UV-absorbance (SUVA) and DOC:DON)to fingerprint terrestrial sources of DOM. Recent studies have dem-onstrated that SUVA can be used as a surrogate for the aromaticcarbon content and molecular weight of DOC ( Chin et al., 1994;McKnight et al., 1997; Weishaar et al., 2003; Hood et al., 2005).The chemical character of DOM (DOC:DON, (SUVA)) has been usedto identify terrestrial sources of DOM at seasonal scales (Hoodet al., 2003, 2005; Mcknight et al., 1997, 2001) and during stormsat the catchment (Hagedorn et al., 2000; Hood et aI., 2006; Katsuy-ama and Ohte, 2002) and plot scale (Kaiser and Guggenberger,2005).

Our study builds upon a wealth of previous hydrological (Harr,1977; McGuire, 2004) and biogeochemical (SoIlins et al., 1980; Sol-!ins et al., 1981; Triska et al., 1984) research at the site. The H.J.An-drews Experimental Forest is characterized by dry summers, agradual wet up between October and December (transition period),

and from December through late spring the watershed is persis-tently wet. This steady and progressive shift from dry to verywet conditions allowed us to explore the role of antecedent wet-ness and flow conditions on DOC and nitrogen (N) patterns at sea-sonal and storm event scales. Monitoring and sampling of lateralsubsurface flow from the hilIslope trench, and stream water atthe catchment outlet between August 2004 and June 2005 duringand between storm events allowed us to compare DOC and N con-centrations and SUVAvalues between the transition period and thewet period at the hiIlslope and catchment scale, and between thesetwo scales, during baseflow and stormflow conditions. In addition,sampling storm events at the hillslope and catchments scale duringthe transition and wet period enabled us to examine the role ofantecedent wetness conditions on DOC and N concentrations andcompare export rates between these two scales.

We address the following questions to improve our understand-ing of the hydrological controls on DOM and N fluxes from hill-slopes in a small watershed: (1) What is the variation in DIN,DON and DOC concentrations and DOM quality (SUVA and DOC:-DON) among sources from the plot scale, lateral subsurface flowand stream water on an annual scale? (2) What is the influenceof flow conditions at the seasonal scale (transition vs. wet period)on the variation of solutes and DOM quality in lateral subsurfaceflow and stream water during baseflow and stormflow conditions?(3) What is the role of timing (transition vs. wet period) and underwhat flow conditions are the single gauged hillslope and catch-ment response similar with respect to the solutes and DOM qual-ity? (4) Do peak and flow averaged DOC and N concentrationsduring storms at the catchment and hilIslope scale increase witha decrease in antecedent soil moisture and antecedent precipita-tion conditions? (5) Is the total carbon and nitrogen export at thehillslope and catchment scale during storm events the same?

Site description

The study was conducted in Watershed 10 (WS10), a 10.2 haheadwater catchment located in the H.J. Andrews ExperimentalForest (HJA), in the western-central Cascade Mountains of Oregon,USA (44.2°N, 122.25DW) (Fig. 1). Elevations range from 470m atthe watershed flume to a maximum watershed elevation of680 m. HJA has a Mediterranean climate, with dry summers andwet winters characterized by long, low intensity storms. Averageannual rainfall is 2220 mm and about 80% falls between Octoberand April. Snow accumulation in WSl0 seldom exceeds 30 cm,and seldom persists for more than 2 weeks (Sollins et al., 1981).Atmospheric total bulk N deposition is low compared to other sitesin USA and averages 1.6-2 kg N ha-1 yr1 (Vanderbilt et al., 2003).The watershed was harvested in 1975 and is now dominated by anaturally regenerated second growth Douglas-fir (Pseudotsugamenziesii) stand. Seep areas along the stream have been observed(Harr, 1977; Triska et al., 1984), which are related to the localtopography of bedrock and/or saprolite, or to the presence of ver-tical, andesitic dikes approximately 5 m wide, located within thesouth-facing hillslope (Swanson and James, 1975; Harr, 1977).

The hillslope study area is located on the south aspect ofWS0,91 m upstream from the stream gauging station (Fig. 1). The 125 mlong stream-to-ridge slope has an average gradient of 37°, rangingfrom 27° near the ridge to 48° adjacent to the stream (McGuire,2004). Elevation at the hillslope ranges from 480 to 565 m. Thebedrock is of volcanic origin, including andesitic and dacitic tuffand coarse breccia (Swanson and James, 1975). The depth tounweathered bedrock ranges from 0.3 to 0.6 m at the stream-hill-slope interface and increases gradually toward the ridge to approx-imately 3-8 m. Soils are about 1 m deep, and formed either inresidual parent material or in colluvium originating from these

WJ.van Verseveld et al /Journal of Hydrology 367 (2009) 177-187 179

deposits. The soils are highly andic and vary across the landscapeas either Typic Hapludands or as Andie Dystrudepts (Yano et al.,2005) and are underlain by 1-8 m relatively low permeability sub-soil (saprolite), formed in the highly weathered coarse breccia(Ranken, 1974; Sollins et al., 1981). Soil textures range from grav-elly, silty clay loam to very gravelly clay loam. Surface soils are wellaggregated, but lower depths (70-11 0 cm) exhibit more massiveblocky structure with less aggregation than surface soils (Harr,1977).

Methods

Infrastructure

A 10m long trench was constructed to measure subsurface flowat a natural seepage face (McGuire, 2004). Intercepted subsurfacewater was routed to a calibrated 30° V-notch weir that recordedstage at 10-min time intervals using a l-mm resolution capaci-tance water-level recorder (TruFrack, Inc., model WT-HR). Rainfallwas measured with a tipping bucket and storage gauge located in asmall canopy opening on the hills lope. The drainage area of thehillslope was delineated topographically from a total station sur-vey of the entire hillslope (0.17 ha, round to 0.2 ha in all analyses)and verified by a water balance calculation (McGuire, 2004). Aspart of the long-term monitoring at the H.J.Andrews ExperimentalForest, discharge at the WSl0 outlet was measured with a trape-zoidal flume. During the summer a V-notch weir was used to mea-sure discharge at the WSl0 outlet. Stage was measured with aModel 2 Stevens Instruments Position Analog Transmitter (PAT)(0.001 ft resolution).

Soil water content (0) was measured with water content reflec-tometers (WCR) (CS615, Campbell Scientific, Inc.). The soil mois-ture probes were installed parallel to the slope at 3 depths (30,70 and 100 cm) in three soil pits in the lower portion of the hill-

slope. The nests were located 15, 20 and 25 m upslope from theslope base (McGuire, 2004).

We installed six plastic 10 x 10 em zero tension Iysimeters justbelow (0.5 cm) the organic layer (Fig. 1). Twenty seven superquartz(Prenart Equipment ApS) tension (0.5 bar) Iysimeters were in-stalled at shallow (20 cm), middle (30-40 cm) and deep (70-11cm) soil profile positions (Fig. 1) at a 30° angle following themethod described by Lajtha et al. (1999).

We installed 69 maximum cork rise wells (3.18 cm diameter),that were screened for the lower 25 cm, the maximum waterheight observed by Harr (1977). All wells were installed until refu-sal by a hand auger. We sampled five wells located outside theseepage area that showed transient saturation and one well(A01) located in the seepage area (Fig. 1).

Sampling and chemical analysis

Throughfall, lateral subsurface flow, WSl0 stream water, soilwater (zero tension and tension), transient and seepage groundwa-ter samples were collected between August 2004 and June 2005 at3-week intervals, and prior to, during, and after selected stormevents. Throughfall was captured using the technique of Keirnand Skaugset (2004). Grab samples at 3-week intervals and duringstorms were taken at the right fork during the transition period,and at the left fork during the whole study period. Prenart tensionlysirneters were evacuated to -50 kPa and allowed to collect waterfor 24 h. These samples were not filtered because initial experi-ments with filtered soil solutions demonstrated that tension Iysim-eter samples did not need to be filtered (Lajtha et al., 2005). Othersamples were filtered through combusted Whatman GF/F glass fi-ber filters (nominal pore size = 0.7 cm) and stored frozen untilanalysis. The gauged hills lope and watershed outlet were sampledwith ISCOsamplers during five storms, at 1-4 h intervals. Sampleswere analyzed for DOC, total dissolved nitrogen (TDN), nitrate,

180 WJ. van Verseveld et al ./ }ournal of Hydrology 367 (2009) 177-187

Resultsammonium and UV-absorbance at 254 nm (UV254).DOC and TDNwere measured with Pt-catalyzed high-temperature combustion(Shimadzu TOC-V CSH analyzer with TN unit). NO3-N was mea-sured with the hydrazine sulfate reduction method and NH4 - Nwas determined by the Berthelot reaction method with a an OrionScientific AC 100 continuous flow auto-analyzer (Westco ScientificInstruments, Inc., Danbury. CT). DON was calculated as the differ-ence between TDN and DIN (nitrate and ammonium). BecauseDON was calculated by difference. values sometimes fell slightlybelow 0 mg -1. Negative DON values were considered to beo mg 1-1. UV254 was measured with a Hitachi V-2001 spectropho-tometer and SUVA is UV254 normalized by DOC concentration.

Data analysis

We divided the dataset into two periods: a transition period(transition from dry to wet conditions) and a wet period in orderto investigate the influence of flow conditions on DIN. DOC.DONconcentrations and SUVA.The transition and wet periods were de-fined by measurable hillslope discharge. The transition period wasdefined as the period with hillslope baseflow (between stormevents) discharge <0.01 Ls-1. and the wet period was defined asthe period with hillslope baseflow discharges >0.01 LS

-1. We sub-

divided the runoff record of these two periods into two differentcatchment response modes; baseflow and stormflow conditions.Stormflow conditions were defined as flow during and 12 h afterstorm events. The remainder of the runoff dataset was defined asbaseflow conditions. A storm event was defined as a precipitationevent of more than 10 mm and separated by at least 12 h periodswith rainfall intensities smaller than 0.1 mm/h. We used the Wil-coxon ranksum to test for significant differences (p < 0.01) inDOC and N concentrations and SUVA for flow conditions withinand between the transition and wet period. Hillslope discharge be-fore 12-02-2004 was estimated from stream discharge with a sec-ond order polynomial because the hillslope discharge gauge failed.Average 95% confidence bounds of the second order polynomial onpredicted values during this period were ±7.95 x 10-4 mm h-1. Be-cause of these small confidence bounds uncertainty resulting fromcalculations with estimated hillslope discharge was not quantified.

Pearson correlation coefficients (r) were calculated betweenantecedent wetness conditions as antecedent soil moisture at30 cm depth and antecedent precipitation before a storm eventand DOC. DON and DIN peak and flow weighted concentrations.The correlation was considered significant when p < 0.1. We useda 7-day. 14-day and 30-day average of soil moisture before a stormevent as antecedent soil moisture indices (AMI7• AMI14and AM130)

and a 7-day. 14-day and 30-day total precipitation before a stormevent as antecedent precipitation indices (API7• APl14 and API30).

We used soil moisture data from the lower soil pit (Fig. 1). sincethat data was most reliable. All statistical and mathematical com-putations were made in MATLAB.

Variation in DOC and N

To investigate variation in DOC and N concentrations betweensources from the plot scale. lateral subsurface flow and streamwater we compared average DOC. DON and DIN concentrationsand average SUVA and DOC:DON values from throughfall, belowthe organic horizon. in shallow. middle and deep soil profile posi-tions, transient and seepage groundwater. lateral subsurface flowand stream water. Furthermore. through this analysis we are ableto identify sources that contributed to high DOC and N concentra-tions in lateral subsurface flow and stream water.

DOC and DON concentrations were low in through fall. highestfrom just below the organic layer and then progressively decreasedwith depth into the soil profile (Table 1). This suggests a net releaseof both DOC and DON from the organic layer. and net removal ofDON and DOC from solution below the organic layer. DOC andDON concentrations in transient groundwater were higher thansoil water DOC and DON concentrations observed at the deep soilprofile position. In addition, DOC and DON concentrations in tran-sient groundwater were higher than the groundwater seep concen-trations. DON and DOC concentrations of lateral subsurface flowand stream water were most similar to soil water DON and DOCconcentrations at the middle soil profile depth.

SUVAvalues showed a maximum in the organic layer and tran-sient groundwater solution (Table 1). SUVA values increased fromthroughfall to the organic horizon. and decreased from the organiclayer to the deep soil layer (70-110 cm). SUVAvalues in streamwater were most similar to SUVA values from deep soil water. Lat-eral subsurface flow showed a much lower SUVA value than WSl0stream water and was most similar to SUVA values from thegroundwater seep.

DOC:DONdid not show a clear trend with soil depth. Soil waterfrom the middle soil profile position had the greatest DOC:DON ra-tio. while organic horizon. shallow and deep soil water had lowerratios. DOC:DON increased from deep soil water to seepage andtransient groundwater. Lateral subsurface flow showed the great-est DOC:DON ratio. similar to the DOC:DON ratio ofthe groundwa-ter seep. Stream water showed the second highest DOC:DON ratio.The DOC:DON ratio of throughfall was similar to the shallow soilwater.

DIN was the dominant form of total dissolved nitrogen (TDN) intransient groundwater. while DON was the dominant form of TDNin the other solutions. DON as a fraction of TDN ranged from 0.71to 0.92 in all solutions, while the fraction was 0.37 in transientgroundwater. NO3N concentrations decreased with depth fromthe organic horizon to the deep soil profile position. but NH4-Nconcentrations did not show a trend with depth. Low NO3N andNH4-N concentrations (<0.01 mg 1-1) were found in throughfall,seep groundwater. lateral subsurface flow and WSl0 stream water.

WJ. van Verseveld et al./Jaurnal at Hydrology 367 (2009) 177-187 181

DOC and N during transition and wet period

First, we compared DOCconcentrations, DOCquality and N con-centrations between the transition and wet period during baseflowconditions in lateral subsurface flow and stream water. DIN, DOCquality (expressed as DOC:DON and SUVA) in lateral subsurfaceflow during baseflow conditions were not different between thetransition and wet period (Figs. 2 and 3a). Stream water duringbaseflow conditions followed the same pattern as lateral subsur-face flow, except that DIN was significantly higher during thewet period. DOC and DON concentrations during baseflow condi-tions in stream water were significantly lower during the wet per-iod, while in lateral subsurface flow only DOC was significantlylower during the wet period (p > 0.01 for hillslope water DON).

Secondly, we compared DOCconcentrations, DOC quality and Nconcentration between lateral subsurface flow and stream waterunder baseflow conditions, during the transition and wet period.During the wet period under non-driven conditions, SUVA valuesof lateral subsurface flow were significantly lower than SUVAval-ues of stream water, while DOC,DON and DIN concentrations andDOC:DON ratios of lateral subsurface flow and stream water weresimilar. During the transition period and baseflow conditions, DOMquality (DOC:DON and SUVA)between lateral subsurface flow andstream water was different; lateral subsurface flow had higherDOC:DON ratios and lower SUVA values. In addition, non-drivenDON concentrations in lateral subsurface flow were lower thanstream water during this period.

Thirdly, we compared DOC concentrations, DOC quality and Nconcentrations between the transition and wet period duringstormflow conditions in lateral subsurface flow and stream water.During stormflow conditions, stream water SUVAvalues were notdifferent between the transition and wet period, while DOC andDON concentrations were lower, DIN concentrations were higher,and DOC:DON ratios were higher during the wet period (Figs. 2and 3b). Lateral subsurface flow followed the same pattern as

stream water except for SUVAvalues that were significantly higherduring the wet period, and DOC:DON ratios that were not differentbetween the transition and wet period in lateral subsurface flow.

Finally, we compared DOC concentrations, DOC quality and Nconcentrations between lateral subsurface flow and stream waterunder stormflow conditions, during the transition and wet period.During the wet period and stormflow conditions SUVA values weresignificantly different between lateral subsurface flow and streamwater. Stormflow DOC and DON concentrations and SUVAvaluesduring the transition period were each lower in lateral subsurfaceflow than stream water, while DOC:DON ratios were higher in lat-eral subsurface flow.

Antecedent wetness conditions and stormflow DOC and N

We used five sampled storms (Fig. 2) to examine the influenceof antecedent wetness conditions on peak and flow weightedDOC,DON and DIN in stream water and lateral subsurface flow (Ta-bles 2 and 3). Three antecedent soil moisture indices were used:the 7-day, 14-day and 30-day antecedent soil moisture index(AMI7, AMI14 and AMI30), and three antecedent precipitation indi-ces were used: the 7-day, 14-day and 30-day antecedent precipita-tion index (API7,API14and API30).

The storm characteristics of these events as well as DOC. DON,DIN peak and flow weighted concentrations are summarized in Ta-ble 2. API7, API4 and API30 were not significantly related to peakand flow weighted DOC. DON and DIN concentrations. AMI7,AMI14 and AMI30 were similar (Table 2), and thus we calculatedpearson correlations between AMI7 and peak and flow weightedDOC, DON and DIN in stream water and lateral subsurface flow.The concentration of all solutes except DIN decreased with an in-crease in AMI7 at 30 cm depth. DOC,DON peak and flow weightedconcentrations in stream water were more weakly correlated toAMI7 than these solutes in lateral subsurface flow (Table 3). BothDIN peak and flow weighted DIN in stream water and lateralsubsurface flow were not significantly related to AMI7. Inaddition DON peak in stream water was not significantly relatedto AMI7.

Storm event export rates of C and N at hillslope and catchment scale

The export rates of DOC,DON and DIN for all five storms weresmaller at the hilIslope than watershed scale (Fig. 4a-c). DONwas the dominant form of total nitrogen export during all storms.The DON:TDN ratios during storms 4 and 5, both storms during thewet period, were 0.84 and 0.87 at the watershed scale for storm 4and 5, respectively, and 0.84 and 0.90 at the hillslope scale forstorm 4 and 5, respectively. In contrast DON:TDN ratios for stormsduring the transition period were >0.94.

The highest DOC,DON and DIN export rates were observed dur-ing storm 4. For the watershed and hillslope scale during storm 4,export rates of DOC were 4.4 and 3.0 kg/ha/storm respectively.Rates of DON export for the watershed and hillslope scale were0.11 and 0.08 kg/ha/storm, respectively. Rates of DIN export forthe watershed and hillslope scale were 0.020 and 0.014 kg/halstorm, respectively.

Discussion

Quantifying spatial sources of soluble nutrients at the catch-ments scale is one of the greatest challenges faced in hydro-bio-geochemical research. Isolating the hills lope component has beendifficult in past studies because so few trenched experimental hill-slopes exist around the world in catchments where active biogeo-chemical research is done. These trenched experimental hilIslopes

182 wJ. van Verseveld et al./Journal of Hydrology 367 (2009) 177-187

often lack detailed hydro-biogeochemical data across a wide rangeof antecedent wetness conditions. In addition, it is extremely diffi-cult to quantify the representativeness of a single experimentalhillslope for the whole array of hillslopes that make up a catch-ment. Single gauged hillslopes only sample part (tens of meters)of the hiIlslope component and, more importantly, most catch-ments have a riparian zone that transforms the biogeochemicalsignal of the hillslope component en route to the stream channel.

Our work exploits a rather unique experimental design where atrenched hillslope is compared to the stream response in a head-water catchment without a riparian zone. This natural experimen-tal design allowed us to study the hydrological controls on DOMand N export from the hillslope component at seasonal and stormevent scales unimpeded by riparian dynamics. Because of theinstallation of a 10m wide trench to capture lateral subsurfaceflow, we were also able to compare the hydro-biogeochemical sig-

nal from the single gauged hillslope to the overall hydro-biogeo-chemical response of the whole array of hillslopes that make upthe catchment, by examining when the study hillslope acted inconcert with the stream and when hiIlslope dynamics were differ-ent from the stream response.

The fundamental question of this study is whether the singlegauged hillslope is representative of the whole array of hillslopesthat make up the catchment. We state the null hypothesis thatthe chemistry (DOC, DON and DIN concentrations and DOM qual-ity) at the single gauged hillslope is the same as the chemistry atthe catchment outlet. First, we will discuss the acceptance or rejec-tion of the null hypothesis at the seasonal level (contrasting thewet seasons and the transition season), and then we will presenthydrological and biogeochemical explanations why the nullhypothesis was accepted or rejected. Finally, we will discuss stormresponses, in particular the difference in C and N storm export

WJ. van Verseveld et al./Journal of Hydrology 367 (2009) 177-187 183

rates between the hillslope and catchment scale and C and N stormevent concentrations during the study period. Again we will con-sider hydrological and biogeochemical controls that explain ourobservations.

Contrasting hillslope and catchment

Lateral subsurface flow during the transition period was charac-terized by lower DON concentrations and higher DOC:DON thanstream water during baseflow conditions, while DIN and DOC con-centrations showed no difference. Lateral subsurface flow DON andDOC concentrations were lower than stream water concentrationsduring stormflow conditions within the transition period. Thus, forthe transition period we reject the null hypothesis.

The single gauged hillslope response was not significantly dif-ferent from the catchment response (with respect to DOC, DONand DIN concentrations and DOC:DON ratios) during the wet per-iod, during both baseflow and stormflow conditions. However.SUVA in lateral subsurface flow was significantly lower than SUVAin stream water during the wet period during both baseflow and

stormflow conditions. Thus, for the wet period we also reject thenull hypothesis.

Hydrological explanations

The difference in SUVAwe observed in lateral subsurface flowand stream water suggests that sources of DOM at the singlegauged hillslope and catchment scale were not similar during thewet period and illustrates the value of using SUVAas a fingerprint-ing tool. Transient groundwater was high in DON, DOC and DINconcentrations, and was the only water source where DON wasnot the dominant form of TDN. Average values of DOC, DON andDOM quality (SUVA) in transient groundwater were similar to ob-served values in organic horizon and shallow soil water, suggestinga vertical preferential flow mechanism without much soil matrixinteraction. Jardine et al. (1989a) found that if preferential flowat the pedon scale was dominant, DOC was non-reactive with thesolid phase because it bypassed the soil matrix. DOM and DIN con-centrations and SUVA in deep soil water was lower than transientgroundwater, indicating that flow paths with significant soil ma-trix interaction undergo preferential retention of aromatic DOMand loss of DIN. Many other studies (Hagedorn et aI., 2000; Yanoet al., 2004; Kaiser and Guggenberger, 2005; Jardine et al.,1989b) have found preferential retention of aromatic DOM withdepth.

Transient groundwater represents one source of lateral subsur-face flow and stream water. Another source of lateral subsurfaceflow and stream water is seepage groundwater. Stream water dur-ing summer low flow conditions is sustained by different seeps inWSl0 (e.g., Triska et al. (1984) identified five different seeps inWSl0). These seepage areas are characterized by low SUVA valuesduring summer low flow conditions. During the transition periodthe contribution of transient groundwater will increase and SUVAvalues in stream water and lateral subsurface flow reflect the ratiobetween the two water sources. We argue that the ratio of seepgroundwater to transient groundwater from vertical preferentialflow at the single gauged hills lope was larger than the ratio atthe catchment scale during the whole study period. The differencein mixing at the hillslope scale compared to the catchment scale

184 wJ. van Verseveld et al./Journal of Hydrology 367 (2009) 177-187

inferred from SUVAmeasurements may have caused the differencein DOC and DON concentrations between lateral subsurface flowand stream water during the transition period. However, in thenext section we discuss why the difference in DOC and DON con-centrations between lateral subsurface flow and stream water dur-ing the transition period was likely largely biogeochemicalcontrolled.

Biogeochemical explanationsAn explanation could be that the lower observed SUVAduring

the wet period in lateral subsurface flow was caused by bio-avail-able carbon compounds with a low aromatic and nitrogen contentthat were rapidly processed in the WS10 stream. However, thisdoes not explain the observed low SUVAvalues in stream water(range: 0.17-0.88) and lateral subsurface flow (range: 0.14-0.41)before storm 1 during low flow conditions. During these conditionsflow paths are characterized by long residence times and thus pro-cessing of these bio-available carbon compounds within the hill-slope would occur.

Algae were observed in the bedrock channel of WS10, are char-acterized by low C:N ratios, and higher stream DON concentrationsduring the transition period may have been caused by by-productsof in-stream production. In addition, N2-fixing alder was present in

the right fork of watershed 10, which can influence stream N con-centrations (Compton et al., 2003; Cairns and Lajtha, 2005). Duringlow flow conditions, when in-stream processes as by-products ofalgae production (autochthonous input) and leaching of leaf litter(allochthonous input) (Meyer et al., 1998) are likely more impor-tant than transport of DOM from the terrestrial to the stream envi-ronment (Mulholland and Hill, 1997; Hagedorn et al., 2000) thesesources likely contributed significantly to elevated stream DONconcentrations during the transition period, While there was a dif-ference in mixing of sources between the hillslope and catchmentscale during the transition period inferred from SUVA measure-ments, we argue that in-stream processes were the dominant con-trol on higher DOC and DON concentrations in stream water. Grabsamples during stormflow conditions from the hillslope trench,right fork and left fork during the transition period showed thataverage DOCand DON concentrations were higher at the right fork(DOC: 9.7; DON: 0.28 mg 1-1, n=4) and left fork (DOC: 7.3; DON:0.17mg l-1 ,n=4) compared to the hillslope trench (DOC: 5.0;DON: 0.11 mg l-1, n=4). This indicates that sources more up-stream in the catchment caused the higher DOC and DON concen-trations during stormflow conditions at the WS10 stream outletcompared to lateral subsurface flow. Average SUVA values fromthe right and left fork were 3.54 and 2.13, respectively, and averageDOC:DON ratios were 37 and 44, respectively, suggesting a terres-trial source of DOM. Grab samples (n=58, during the wet period)were taken from the left fork (DOC: 3.8 mg 1-1; DON: 0.10 mg l-1,n=58) during the whole study period that were not different inDOC and DON concentrations compared to lateral subsurface flow(DOC: 4.1 mg 1-1; DON: 0.09 mg 1-1, n= 56) and stream water(DOC: 3.9 mg 1-1; DON: 0.10 mg 1-1, n=56) at the catchment out-let during the wet period. This lack of difference indicates thatDOC and N concentrations were controlled mainly by lateral sub-surface flow and not in-stream processes during the wet periodwith storms characterized by high runoff ratios (Table 2). consis-tent with the findings of Mulholland and Hill (1997). During thetransition period storms were characterized by small runoff coeffi-cients (Table 2) and a lower discharge regime (Fig. 2b). In addition,DOCand DON concentrations were higher at the left, right fork andcatchment outlet compared to lateral subsurface flow during thetransition period. The hydrological observations as runoff coeffi-cients and discharge regime and spatial sampling within the catch-ment indicated that in-stream processes were the dominantcontrol during the transition period.

Storm responses

C and N export rates at the hillslope and catchment scaleWe used mean flow weighted DOC, DON and DIN concentra-

tions and storm flow totals to assess what caused lower C and Nexport rates during storm events at the hillslope scale comparedto the catchment scale. The difference in DOC export for storm 5was caused by a storm difference since flow weighted mean DOCconcentration were lower in stream water and thus hydrologicalcontrolled. The difference in DOC export during storms 1-4 andthe difference in DON export during all storms between the catch-ment and hillslope was an effect oflower mean flow weighted con-centrations in lateral subsurface flow and lower storm totals at thehillslope scale. These differences were both hydrological (lowerstorm totals) and biogeochemical controlled since higher DOCand DON concentrations in stream water at the WS10 outlet likelyresulted from in-stream processes.

McGlynn and McDonnell (2003) found that hillslope DOC ex-port accounted for 22-36% of total catchment DOCexport in a Mai-mai catchment, New Zealand. The remaining 64-78% originated inriparian and channel zones. At our site we observed that hillslopeDOC export during five storm events accounted for a range of 56-

WJ. van Verseveld et al./Journal of Hydrology 367 (2009) 177-187

82% of total WSl0 catchment DOC export. In WSl0 hillslopes issuewater directly into the stream without significant riparian zonemodulation and this caused very likely higher hills lope DOCexportcontributions to total catchment DOC export than reported byMcGlynn and McDonnell (2003).

Antecedent wetness conditions control C and N storm eventconcentrations

High solute (DOC, DON) concentrations during storms after dryantecedent wetness conditions have been reported by others(Grieve, 1991; Vanderbilt et al., 2003; Cooper et al., 2007; Inamdaret al., 2006). Storm DOC and DON peak and flow weighted concen-trations at the hillslope and catchment scale generally decreasedduring the sequence of storms (Table 2), with an increase in ante-cedent soil moisture. The AMI7 at 30 cm soil depth can be consid-ered as an index of how much flushing in the soil profile occurredprior to a storm event. AMI14 and AMI30 were not different fromAMI7. Furthermore, API7, API14 and API30 were not significantly re-lated to DOC and N peak and flow weighted concentrations. Thissuggests that solute concentrations during storm events were notcontrolled by rainfall events and thus flushing in the soil profileup to a month prior to these events. Rather, our results indicatethat DOCand DON in the soil profile were exhausted rapidly duringthe transition period and stayed 'constant' during the wet period asa result of long-term precipitation patterns reflected in the soilmoisture pattern at 30 cm depth. We calculated the Pearson corre-lation coefficient between AMI7 and total precipitation before eachstorm event since August 1, 2004 (long-term precipitation) to eval-uate if the soil moisture pattern at 30 cm reflected long-term pre-cipitation during the transition period. During the transition periodthe Pearson correlation was 0.95 (p < 0.1) and this result supportsthe explanation that AMI7 reflected long-term precipitation pat-terns. Thus the observed pattern of storm DOC and DON peakand flow weighted concentrations was largely hydrological con-trolled and was likely caused by rapid exhaustion of DOC andDON during the transition period as a response to flushing duringstorm events over time. We did not find a significant relationshipbetween AMI7 and DIN concentrations during storms. This is likelycaused by the high biological demand of nitrate and ammonium inthis environment.

Biogeochemical control on DIN in transient groundwater?It is not likely that the high DIN concentrations in transient

groundwater were caused exclusively by a preferential flow mech-anism. Transient groundwater DIN concentrations were higherthan organic horizon DIN concentrations, indicating net productionof nitrate and ammonium in transient groundwater. Sollins et al,(1981) also found higher nitrate concentrations in suction Iysime-ters at 2 m depth than at 0.3 m depth at the same location in WS10.They hypothesized that this difference may have been caused by adecrease in bio-available C compounds below the rooting zonesuch that nitrifiers were able to compete for reduced N with het-erotrophic bacteria. We observed high DOC and DON concentra-tions in transient groundwater 'below' the rooting zone,suggesting that a significant C source was available below the root-ing zone, although the lability of this DOM is unknown. Transientgroundwater was only sampled frequently from one well (E04)during storm 5 within the study period. NO3N increased until 5/17/05 during the rising limb of the storm, while NH4- N decreaseduntil 5/17/05 (Fig. 5). This indicates that during this period of thestorm autotrophic and/or heterotrophic nitrification occurred.After 5/17/05, NO3N began to decrease during the remainder ofthe storm while at the same time NH4-N increased until 5/20/05.The increase in NH4N may have been caused by dissimilatory ni-trate reduction to ammonium (DNRA). DNRA is an anaerobicmicrobial pathway that transforms NO3N to NH4-N and has been

documented in soils and sediments (Buresh and Patrick, 1978;Tiedje, 1988; Silver et al., 2001). Conditions that favor DNRA areavailable N03-N, (labile) C and a low redox potential and couldhave occurred after 5/17/05 during the storm. Thus, the highNO3N and NH4-N concentrations in transient groundwater weobserved could be the result of the co-occurrence of nitrificationand DNRA.Another explanation we cannot rule out is that the pat-tern of NO3N andNH4-N in transient groundwater was simplycaused by ammonification and nitrification. Since groundwaterwas transient, and thus the soil at this depth was likely not suffi-cient anoxic to favor DNRA, the observed NOrN and NH4-N con-centration patterns were most likely caused by ammonificationand subsequent nitrification of organic N.

Conclusions

The high observed DOC and N concentrations in transientgroundwater underscore the importance of measuring DOC andN concentrations at different depths within the soil profile. HighDOC and N concentrations and SUVAvalues in transient ground-water did indicate the occurrence of vertical preferential flow atour site. During the wet period the chemistry with respect to con-centrations at the hillslope and catchment scale was the same.However, SUVAshowed that mixing of water sources may be dif-ferent between the hillslope and catchment scale. This resultunderscores that similar results (nutrient concentrations duringthe wet period) can be the outcome of different processes, and itshows the value of using multiple lines of measurements to inves-tigate the complex hydrologic and solute response of a steep for-ested catchment.

We explained the differences in chemistry between the catch-ment and hillslope scale by a dominant hydrological and a domi-nant biogeochemical control (in-stream processes) during thewet and transition period, respectively. Small runoff coefficientsand a low discharge regime suggested a dominant biogeochemicalcontrol during the transition period. However, we acknowledgethat the role of in-stream processes remains somewhat uncertainsince the hillslope was not representative of the catchment duringthe transition and wet period. Only a few studies have attemptedto separate hydrological controls from biogeochemical controls asin-stream processes across seasons, and more research is neededto fully clarify when and how these controls drive solute responsesin forested catchments.

Antecedent wetness conditions controlled DOC and DON con-centrations in lateral subsurface flow and stream water duringstorm events; more prior flushing (expressed as AMI7 resulted in

185

186 WJ. van Verseveld et al. / Journal of Hydrology 367 (2009) 177-187

lower DOC and DON peak and flow weighted concentrations dur-ing storms. In addition, DOC and DON concentrations in lateralsubsurface flow and in stream water during stormflow conditionswere lower during the wet period compared to the transition per-iod. Both of these results suggest that the production of DOC andDON in soils lagged behind the flushing of these nutrients. If DOCand DON production is seasonally limited, this has important con-sequences for the interpretation of soil solution concentrations inend-member mixing analysis (EMMA) and annual calculations ofsolute losses from limited soil solution data. Since EMMA requiresconservative behavior of tracers and time invariance of end-mem-ber compositions, limitation of DOC and DON on a seasonal scaleviolates the EMMA assumptions.

Acknowledgements

This work was supported through funding from the National Sci-ence Foundation (Grant DEB021-8088 to the Long-Term EcologicalResearch Program at the H.J. Andrews Experimental Forest) andDepartment of Forest Engineering at Oregon State University. Wethank Marloes Bakker and John Moreau for providing field assis-tance. We also thank RD. Harr and D.Ranken for initiating the hill-slope studies at WS10, and K. J McGuire for re-initiating this site.

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