factors influencing the release of dissolved organic carbon and dissolved forms of nitrogen from a...

HYDROLOGICAL PROCESSESHydrol. Process. 21, 622–633 (2007)Published online 31 October 2006 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.6261

Factors influencing the release of dissolved organic carbonand dissolved forms of nitrogen from a small upland

headwater during autumn runoff events

Richard Cooper,* Vera Thoss and Helen WatsonThe Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, Scotland, UK

Abstract:

We identify and assess the relative importance of the principal factors influencing the release of dissolved organic carbon(DOC) and dissolved forms of nitrogen (N) from a small upland headwater dominated by podzolic soils during a sequenceof autumn runoff events. We achieve this by subjecting high-resolution hydrometeorological and hydrochemical data to anR-mode principal component factor analysis and a stepwise multivariate regression analysis. We find that the release of DOCand N is influenced by four principal factors, namely event magnitude, soil water flow through the Bs horizon, the length oftime since the soil profile was last flushed, and rewetting of the H horizon. The release of DOC and dissolved organic nitrogen(DON) is most strongly influenced by the combination of event magnitude and soil water flow through the Bs horizon, and to alesser extent by the length of time since the soil profile was last flushed. Rewetting of the H horizon also influences the releaseof DOC, but this is not the case for DON. The release of nitrate (NO3-N) is most strongly influenced by the combinationof the length of time since the soil profile was last flushed and rewetting of the H horizon, and to a lesser extent by eventmagnitude. Soil water flow through the Bs horizon does not influence the release of NO3-N. We argue that the mechanismsby which the above factors influence the release of DOC and N are probably strongly associated with moisture-dependentbiological activity, which governs the turnover of organic matter in the soil and limits the availability of NO3-N in the soilfor leaching. We conclude that the release of DOC and N from upland headwaters dominated by podzolic soils is largelycontrolled by the variable interaction of hydrometeorological factors and moisture-dependent biological processes, and that ashift in climate towards drier summers and wetter winters may result in the release of DOC and N becoming increasinglyvariable and more episodic in the future. Copyright 2006 John Wiley & Sons, Ltd.

KEY WORDS dissolved organic carbon (DOC); nitrate (NO3-N); dissolved organic nitrogen (DON); upland headwaters; podzolicsoils; runoff events

Received 17 March 2005; Accepted 8 November 2005

INTRODUCTION

The release of dissolved organic carbon (DOC) and dis-solved forms of nitrogen (N) from upland headwatersin the UK has attracted much attention recently, due toconcerns about the potential impacts of climate change(Freeman et al., 2001) and continuing deposition of highlevels of anthropogenically derived nitrogen from theatmosphere (Chapman et al., 2001). In the absence ofdisturbance, upland headwaters typically serve as netsinks of carbon and nitrogen, due to slow and incom-plete decomposition of organic matter (Haslam et al.,1998) and tightly controlled cycling of nitrogen betweensoils and biota (Batey, 1982). It is feared that increas-ing temperatures (Freeman et al., 2001; Worrall et al.,2003), changing rainfall distributions (Evans et al., 2002)and increasing atmospheric CO2 content (Freeman et al.,2004) may stimulate turnover rates and destabilize thecarbon stored in these systems, causing it to be releasedin both gaseous and dissolved forms. It is also feared thatcontinuing deposition of high levels of anthropogenically

* Correspondence to: Richard Cooper, The Macaulay Institute,Craigiebuckler, Aberdeen AB15 8QH, Scotland, UK.E-mail: [email protected]

derived nitrogen from the atmosphere may exceed theretention capacity of upland soils and biota, resultingin disruption of the nitrogen cycle and enhanced leach-ing of nitrate (NO3-N; Black et al., 1993) and dissolvedorganic nitrogen (DON; Yesmin et al., 1995). Both ofthese effects could lead to the eutrophication and acidifi-cation of receiving waters, with potentially serious conse-quences for the health and functioning of aquatic ecosys-tems (reviewed in Wetzel (2001)) and the utility of suchwaters for domestic supplies (Worrall and Burt, 2004).

Previous studies examining the release of DOC and Nfrom upland headwaters have generally employed rou-tine sampling at monthly, weekly or daily intervals,and have tended to focus on the identification of sea-sonal, interannual and long-term trends or the calcula-tion of annual fluxes (reviewed in Hope et al. (1994);reviewed in Reynolds and Edwards (1995); Adamsonet al., 1998; Scott et al., 1998; Chapman et al., 2001;Dawson et al., 2002; Worrall et al., 2003). Such studieshave revealed distinct seasonal patterns of DOC and Nrelease, with concentrations of DOC and DON displayingsummer maxima and winter minima, and vice versa forNO3-N. The patterns for DOC and DON are generallyexplained by enhanced turnover and release of soluble

Copyright 2006 John Wiley & Sons, Ltd.

DISSOLVED ORGANIC CARBON AND NITROGEN RELEASE 623

organic matter in soils (Grieve, 1984) and surface waters(Chapman et al., 2001) respectively during the warmersummer months, whereas the pattern for NO3-N reflectsthe seasonal availability of NO3-N in the soil for leach-ing, which is significantly reduced during the summer bybiological uptake (Edwards et al., 1985). However, soiltype and drainage status can influence these trends, sinceDOC may be removed from solution by adsorption insesquioxide-rich mineral horizons (Jardine et al., 1989;Kennedy et al., 1996), and NO3-N may not be producedin waterlogged profiles, due to the effects of anaero-bia (Rangely and Knowles, 1988; Adamson et al., 1998).Such studies have also demonstrated that the release ofDOC and N from upland headwaters is strongly influ-enced by antecedent hydrological conditions, and, in par-ticular, by periods of drought (Grieve, 1991; Reynoldset al., 1992). With regard to N, long periods of droughtare often followed by flushes of mineralization and nitri-fication, which can disrupt the seasonal NO3-N cycle forup to several years thereafter (Reynolds et al., 1992).

Comparatively few previous studies have attemptedto investigate the release of DOC and N from uplandheadwaters over time-scales consistent with event-basedresponses, i.e. hours rather than days; and of those thathave, only a handful have examined sequences of runoffevents (Grieve, 1984, 1990; Chapman et al., 1993; Wor-rall et al., 2002) as opposed to individual storm events(Reid et al., 1981; Edwards et al., 1984; Muscutt et al.,1990). Virtually all of these event-based studies havedemonstrated a significant positive correlation betweenthe release of DOC and stream discharge in upland head-waters, with maximum DOC concentrations occurringduring storm events. Hysteresis effects often occur duringrunoff events, giving rise to DOC concentrations that areusually higher, at a given discharge, on the falling limbof the hydrograph (Grieve, 1984; Edwards and Cresser,1987). Significant changes in soil water flowpaths duringrunoff events may enhance the release of DOC, partic-ularly if lateral flows through shallow organic horizonsbecome dominant, enabling DOC-rich water to bypasspotential adsorption sites within deeper mineral horizons(Edwards and Cresser, 1987; Grieve, 1990). Antecedenthydrological conditions can also exert a significant influ-ence on the release of DOC during runoff events, withunexpectedly high concentrations of DOC often occurringafter long periods of dry weather, in response to flush-ing of accumulated soluble organic matter (Edwards andCresser, 1987; Grieve, 1991). In comparison with DOC,the dynamics of N during runoff events have received rel-atively little attention, but are seemingly more complex,reflecting the variable interaction of seasonally deter-mined biological processes and antecedent hydrologicalconditions (Foster et al., 1983).

Virtually all of the studies of DOC and N dynamicsidentified above have been mutually exclusive withregard to DOC and N, which is surprising given thatthe biogeochemical cycling of carbon and nitrogen isintrinsically linked. Moreover, no previous study that weare aware of has examined a sequence of runoff events

in an upland headwater and applied a rigorous statisticalmethodology in order to identify and assess the relativeimportance of the principal factors influencing the releaseof DOC and N. This is equally surprising, given thatDOC and N dynamics appear to be strongly influencedby a wide range of hydrometeorological factors. Withthese issues in mind, the specific aims of the presentstudy are to (i) identify and assess the relative importanceof the principal factors influencing the release of DOCand N from a small upland headwater dominated bypodzolic soils during a sequence of runoff events, and (ii)elucidate the mechanisms by which such factors influencethe release of DOC and N. In order to fulfil these aims,we chose to sample a sequence of runoff events occurringduring the autumn, as this period spans major transitionalphases in seasonal patterns of DOC and N release fromupland headwaters, when many influencing factors maybe at work. It should be noted that the year in which thepresent study was undertaken was a relatively dry year:only 30% of mean annual precipitation had been receivedby the time the study commenced in early September,and only 78 mm of precipitation had fallen over theprevious 3 months. Therefore, in this instance, the periodof study also represents a transitional period of rewettingfollowing prolonged drought.

METHODS

Site description

The headwater of the Birnie Burn is situated withinthe boundaries of the Macaulay Institute’s research sta-tion at Glensaugh, on the eastern flank of the GrampianMountains in northeast Scotland, approximately 39 kmsouthwest of the city of Aberdeen (Figure 1). The areawas extensively glaciated in the past, leaving a landscapeof rounded hills that have since been dissected by a net-work of first- and second-order streams. The headwater,which serves as a combined terrestrial-freshwater site inthe UK Environmental Change Network (ECN), has atotal area of 0Ð8 km2, lies between 220 and 435 m a.s.l.and drains from north to south. Mean annual air temper-ature and mean annual precipitation, recorded approxi-mately 0Ð8 km southeast of the headwater outflow at anelevation of 200 m a.s.l., are 7Ð8 °C and 1090 mm respec-tively (1996–2003). Mean annual runoff from the head-water, recorded 60 m upstream of the outflow, equates to819 mm (1994–2003). The site, which lies to the northof the Highland Boundary Fault, is underlain by quartzmica schists of Dalradian age. The depth of the overlyingglacial till varies with topography, with greater depthsfound in the valley floor and thinner, stonier depositsfound on the adjacent hill slopes.

The soils (Strichen Association) and vegetation in theheadwater can be broadly classified into three zones,based on altitude and slope. The upper zone (>400 ma.s.l.) accounts for 17% of the watershed by area andfeatures hill peat (>50 cm) developed on gentle slopes

Copyright 2006 John Wiley & Sons, Ltd. Hydrol. Process. 21, 622–633 (2007)DOI: 10.1002/hyp

624 R. COOPER, V. THOSS AND H. WATSON

400

250

350

300

Streams50 m contours10 m contoursWatershedInstrumentation

Flume

0 500Metres

Soil moisture probes

Birnie BurnHeadwater

B

A

i

i

i

Figure 1. Location (A) and topographic (B) maps of the headwater of theBirnie Burn

covered by Sphagnum spp. (peat mosses) and Eriopho-rum vaginatum (hair’s-tail cottongrass). The intermediatezone (350–400 m a.s.l.) accounts for 26% of the water-shed by area and features freely drained peaty podzols(average depth of the organic horizons: 25 cm) developedin thin glacial till on steeper slopes covered by Cal-luna vulgaris (heather) and to a lesser extent Vacciniummyrtillus (blaeberry), Deschampsia flexuosa (wavy hair-grass) and Nardus stricta (mat-grass). The lower zone(220–350 m a.s.l.) accounts for 57% of the watershedby area and features freely drained humus–iron podzols(average depth of the organic horizons: 15 cm) devel-oped in thin glacial till on steep slopes covered by C.vulgaris and V. myrtillus. Along the length of the valleyfloor are narrow, poorly drained areas immediately adja-cent to the stream that feature flush gleys. Land use atthe site is limited to rough grazing by sheep (stock den-sity approximately 100 km�2), with heather regenerationbeing encouraged by occasional controlled burning.

The hydrology of the Birnie Burn is typical of uplandheadwaters in general, in that the combination of steepslopes and thin soils allows precipitation to be deliveredrapidly to the stream, resulting in a very flashy runoffregime. Runoff responses are normally observed withina couple of hours of the onset of precipitation irrespectiveof antecedent soil moisture content, and the rise topeak discharge and subsequent fall to previous valuesusually takes no longer than a couple of days, even forstorm events. The stream is fed by numerous springsthat emerge on the lower sections of the adjoining hillslopes, some of which flow perennially, whereas othersare more intermittent, appearing only after periods of wetweather. The proportion of streamflow originating fromthese springs ranges from 59% under baseflow conditionsto 10% during storm events, which is higher than mightbe expected for an upland headwater of this type (Dunn

et al., 2006). Streamflow at any given point along thelength of the stream is almost exactly proportional to thearea of the watershed upsteam, meaning that around 83%of precipitation reaching the stream passes through freelydraining podzolic soils en route. The freely drainingnature of these soils allows infiltrating precipitation topass rapidly through the soil profile and flow laterally tothe stream at depth, rather than via surface or near-surfaceflowpaths. This is especially true following long periodsof dry weather, when a wetting front moves progressivelyupwards from the base of the soil profile in response toaccumulative precipitation (Miller et al., 1997).

Instrumentation and data collection

In line with standard ECN measurement protocols(Sykes and Lane, 1996; Sykes et al., 1999), streamflow,precipitation and soil moisture content were monitoredcontinuously at the site during the 3-month period 3September–3 December 2003. Streamflow was moni-tored 60 m upstream of the headwater outflow, usinga precalibrated Forth River Purification Board 60 cmV-notch glass-fibre flume and stilling well, originallyinstalled in the summer of 1993. Water level in thestilling well was measured every 30 s by float actua-tion and recorded as an average every 15 min using aNewlog datalogger. The record of water level was subse-quently converted into a record of discharge using ratingequations supplied with the flume, which were periodi-cally checked for accuracy in the year leading up to theperiod of study by salt dilution gauging. The precisionof the water level measurements was š0Ð001 m, whichequates to š0Ð1 l s�1 at Q95 and š0Ð5 l s�1 at Q5 whenconverted into units of discharge.

Rainfall was monitored using a Campbell ScientificARG100 tipping bucket rain gauge connected to a Didcotautomatic weather station (AWS), sited approximately0Ð8 km southeast of the headwater outflow at an elevationof 200 m a.s.l. Rainfall was measured in increments of0Ð2 mm and recorded as a total every hour. Volumetricsoil moisture content was monitored at two depths onthe eastern flank of the headwater, at an elevation ofapproximately 275 m a.s.l., using Delta-T Devices ML2xThetaProbes connected to a Campbell Scientific CR10datalogger. The monitored depths, 10 cm and 45 cm,correspond respectively to the base of the H and Bshorizons of the humus iron podzol present (note that,under the soil classification scheme used, an H horizondenotes an organic horizon formed under freely drainingaerobic conditions). The output of each probe wasmeasured and recorded every 20 min, and subsequentlyconverted into a record of volumetric water contentusing generalized calibration equations supplied with theprobes.

Sample collection and analysis

Streamwater samples were collected 50 m upstreamof the headwater outflow every 2 h during the periodof study, using a Rock and Taylor sampling unit, which



was restocked with clean bottles twice weekly. Two grabsamples were collected every time the sampling unit wasrestocked. One was left at the site until the next restock-ing visit, in order to serve as a duplicate for determiningsampling error, and the other was returned to the lab-oratory for immediate analysis, in order to serve as acontrol for determining error associated with holding timeon site. Mean sampling error for all analytes was subse-quently determined to be below the limits of analyticalprecision, whereas errors associated with holding timeswere only found to be significant for total dissolvednitrogen (TDN; š7Ð8%) and NO3-N (š5Ð2%). In linewith standard ECN measurement protocols, samples ofbulk precipitation (dry deposition plus wet deposition)were collected at the site on a weekly basis during theperiod of study. Samples were collected 20 m west of theflume, using a polypropylene bulk collector fitted with a152 mm diameter polyethylene funnel, mounted 1Ð75 mabove the ground.

Upon return to the laboratory, all samples were filteredthrough pre-washed Whatman 0Ð45 µm cellulose nitratefilter papers. Whenever possible, further analysis wasconducted immediately. When delays were unavoidable,samples were temporarily stored in the dark at 4 °C,in the longest case for 4 days. Total organic carbonand total nitrogen (in this case equivalent to DOCand TDN respectively, due to the filtration step) weredetermined by the combustion catalytic oxidation/NDIRmethod, using a Shimadzu TOC-VCSH/TNM-1 analyser.The mean precision of these analyses was š6Ð6% andš1Ð6% for DOC and TDN respectively, and the limit ofdetection was 0Ð1 mg l�1 and 0Ð01 mg l�1 for DOC andTDN respectively. NO3-N and ammonium (NH4-N) weredetermined colorimetrically by the cadmium reductionand salicylate oxidation methods respectively, using aSkalar SANCC segmented flow analyser. The meanprecision of these analyses was š2Ð4% and š9Ð5%for NO3-N and NH4-N respectively, and the limit ofdetection was 0Ð0125 mg l�1 for both analytes. DON wasdetermined by difference, i.e. DON D TDN � �NO3-N CNH4-N�.

Data preparation and statistical analysis

The record of discharge derived at the flume wasinitially examined for the occurrence of runoff events,which were defined as any sudden increases in discharge>1Ð0 l s�1. For each runoff event that was identified,11 descriptive variables were extracted from the mete-orological and hydrological datasets: time since the lastprecipitation event; total precipitation; maximum precip-itation intensity; mean precipitation intensity; time sincethe last runoff event; peak discharge; the change in dis-charge; maximum soil moisture contents at 10 and 45 cm;and the changes in soil moisture content at 10 and 45 cm.For those variables measuring change, the amount ofchange during each event was determined by subtractingthe value immediately prior to the onset of the event fromthe maximum difference in the value during the event (a

positive result thus indicates that the value increased dur-ing the event, whereas a negative result indicates that thevalue decreased). In addition, for the same runoff events,four response variables were extracted from the hydro-chemical dataset: the change in concentration of DOC;the change in concentration of NO3-N; the change in con-centration of NH4-N; and the change in concentration ofDON. For each of these analytes, the change in concentra-tion during each event was determined by subtracting theconcentration immediately prior to the onset of the eventfrom the maximum difference in concentration during theevent (a positive result thus indicates that the concentra-tion increased during the event, whereas a negative resultindicates that the concentration decreased).

The subset of hydrometeorological descriptive vari-ables was then subjected to an R-mode principal com-ponent factor analysis, in order to combine groups ofinterdependent variables and minimize the effect of multi-collinearity prior to regression analysis with the responsevariables (Shaw and Wheeler, 1994). Factors were ini-tially identified through an eigenanalysis of the cor-relation matrix, the mathematics of which have beendescribed by Davis (1973). Factors with eigenvaluesgreater than 1Ð0 (i.e. explaining at least as much ofthe total variance as one of the original variables) wereretained for further analysis, as is normally the case insuch studies (e.g. Evans et al., 1996; Bernal et al., 2002).The retained factors were then rotated using the vari-max method (Johnston, 1978), in order to maximize thesum of the variance of the squared factor loadings on thehydrometeorological descriptive variables, and thus sim-plify the overall factor loading structure. Factor scoreswere then determined for each runoff event by standard-izing the hydrometeorological descriptive variables andthen multiplying them by the factor score coefficientsand summing the products. The extracted factors werethen used as independent terms in a series of univariateand stepwise multivariate regression analyses, in order toexamine the extent to which the individual factors andcombinations of factors could explain the hydrochemi-cal responses to the runoff events that occurred duringthe period of study. The standardized residuals derivedfrom the best regression model for each response vari-able were finally analysed for autocorrelation, using theDurbin–Watson test and extended tables for small samplesizes (Savin and White, 1977).

RESULTS

Hydrometeorology

A summary of total precipitation, the number of pre-cipitation events, total runoff, the number of runoffresponses to precipitation, the number of runoff eventsand the change in soil moisture content at depths of10 and 45 cm during the period of study is given in



Table I. Precipitation events were defined as continu-ous periods of positive hourly precipitation totals sep-arated from preceding and succeeding periods of pre-cipitation by at least 1 h in which no precipitation wasrecorded. Runoff responses were defined as any sud-den increases in discharge >0Ð1 l s�1, whereas runoffevents were defined as any sudden increases in dis-charge >1Ð0 l s�1. Total precipitation during the periodof study amounted to just 64% of the long-term meanfor the same period (364Ð2 mm, 1996–2003). Time-series of precipitation, soil moisture content at depthsof 10 and 45 cm, and discharge during the period ofstudy are presented in Figure 2. Total precipitation duringSeptember (43Ð8 mm), October (68Ð0 mm) and December(114Ð4 mm) amounted to 59%, 45% and 85% of the long-term means for the same months (74Ð4 mm, 151Ð7 mmand 134Ð7 mm respectively, 1996–2003). Over half ofthe precipitation received in October was received inthree consecutive precipitation events on 22 October2003. No soil moisture content data were acquired duringthe period 16–30 October 2003, due to a malfunctioningdatalogger. A summary of hydrometeorological variablesdescribing the runoff events that occurred during theperiod of study is given in Table II. No soil moisturecontent data are given for runoff event 5, which occurredon the 22 October 2003, due to the problem with thedatalogger mentioned above.

Hydrochemistry

Time-series of concentrations of DOC, NO3-N andDON during the period of study are presented in Figure 3.NH4-N was only detected at trace concentrations in asmall number of samples, and thus the data are notpresented. Short breaks in the time-series presented inFigure 3 reflect sample collection failures. On mostoccasions this was due to a malfunctioning timer on thesampling unit, with the exception of the 24 November2003, when the sample collection tube froze, and theperiod 29 November–1 December 2003, when the samplecollection tube was lifted out of the stream duringvery high flow conditions. Fortunately, these samplecollection failures rarely coincided with any of the runoffevents, the exception being runoff event 14, which

Table I. Summary of total precipitation P, the number ofprecipitation events PE, total runoff R, the number of runoffresponses to precipitation RR, the number of significantrunoff events RE and the change in soil moisture content atdepths of 10 cm and 45 cm, �v10 and �v45 respectively,

during the period of study

Period 3 September–3 December 2003

P (mm) 232Ð2PE 122R (mm) 59Ð9RR 30RE 14�v10 �m3 m�3� 0Ð143�v45 �m3 m�3� 0Ð295

Date03/09 16/09 29/09 12/10 25/10 07/11 20/11 03/12

Lo

g 1

0Q (

L s−1

)

1

10

100

1000

v (m

θ3

m−3

)

0.00.20.40.60.81.0

10 cm 45 cm

12 34

5 6 7 8

910

1112

13

14

P (

mm

)

02468

10

Figure 2. Time-series of precipitation P, soil moisture content �v at depthsof 10 and 45 cm, and discharge Q during the period of study. Individualrunoff events (i.e. when flow increased by >1Ð0 l s�1) are labelled in

sequence on the discharge plot

Date03/09 16/09 29/09 12/10 25/10 07/11 20/11 03/12

DO

N (

mg

L−1)

0.0

0.2

0.4

0.6

0.8

NO

3-N

(m

g L−1

)

0.0

0.3

0.6

0.9

1.2

DO

C (

mg

L−1)

0

4

8

12

16

123

4

5 67 8

9 10

11

12

13

Figure 3. Time-series of concentrations of DOC, NO3-N and DON duringthe period of study. Individual runoff events (i.e. when flow increased by>1Ð0 l s�1) are labelled in sequence on the DOC plot (with the exception

of runoff event 14, for which no hydrochemical data were acquired)

occurred on the 29 November 2003, and for whichno hydrochemical data were consequently acquired. Asummary of hydrochemical responses (i.e. changes inconcentrations of DOC, NO3-N and DON) to the runoffevents that occurred during the period of study is givenin Table III. No data are given for runoff event 14and no data are given for NH4-N, for the reasonsoutlined above. A summary of weekly bulk precipitationchemistries during the period of study is given inTable IV. Contamination by bird droppings was evidentin the samples collected on the 17 September 2003 andthe 13 November 2003. Localized controlled burningwas carried out in the vicinity of the site during thefirst two weeks of October, and contamination by soot



Table II. Summary of hydrometeorological variables describing the runoff events RE that occurred during the period of study,including the date on which peak discharge was recorded, time since the last precipitation event tPE�1, total precipitation P,maximum precipitation intensity PImax, mean precipitation intensity PImean, time since the last runoff event tRE�1, peak dischargeQmax, the change in discharge Q, maximum soil moisture content at 10 cm and 45 cm, �v10max and �v45max respectively, and the

changes in soil moisture content at 10 cm and 45 cm, �v10 and �v45 respectively

RE Date(2003)

tPE�1

(h)P

(mm)PImax

�mm h�1�PImean

�mm h�1�tRE�1

(h)Qmax

�l s�1�Q

�l s�1��v10max

�m3 m�3��v10

�m3 m�3��v45max

�m3 m�3��v45

�m3 m�3�

1 6 Sep 15 3Ð6 2Ð0 0Ð7 438 4Ð3 2Ð1 0Ð267 0Ð001 0Ð333 0Ð1382 8 Sep 12 6Ð4 3Ð4 0Ð7 4 4Ð4 2Ð1 0Ð270 0Ð002 0Ð440 0Ð1773 22 Sep 4 12Ð2 3Ð4 1Ð4 299 5Ð3 3Ð0 0Ð270 0Ð002 0Ð585 0Ð3834 12 Oct 26 5Ð0 1Ð4 0Ð6 448 3Ð2 1Ð1 0Ð258 0Ð003 0Ð193 0Ð0025 22 Oct 17 35Ð4 8Ð0 2Ð4 184 12Ð1 9Ð9 — — — —6 2 Nov 20 8Ð2 1Ð8 1Ð2 181 8Ð9 6Ð4 0Ð312 0Ð025 0Ð672 0Ð3617 5 Nov 7 6Ð0 2Ð8 1Ð5 1 12Ð1 8Ð8 0Ð330 0Ð018 0Ð685 0Ð2158 9 Nov 2 5Ð4 2Ð4 0Ð8 30 10Ð0 6Ð3 0Ð346 0Ð015 0Ð678 0Ð2389 12 Nov 56 11Ð8 3Ð0 1Ð1 5 35Ð5 31Ð9 0Ð394 0Ð048 0Ð674 0Ð209

10 14 Nov 17 14Ð2 2Ð8 0Ð8 0 45Ð7 40Ð9 0Ð439 0Ð046 0Ð679 0Ð19711 19 Nov 16 4Ð0 1Ð6 1Ð0 43 5Ð9 2Ð0 0Ð408 �0Ð004 0Ð469 0Ð00112 25 Nov 8 9Ð0 2Ð8 0Ð9 88 15Ð0 11Ð6 0Ð423 0Ð019 0Ð673 0Ð21613 26 Nov 5 10Ð6 2Ð4 1Ð5 0 77Ð9 70Ð4 0Ð443 0Ð020 0Ð646 0Ð13214 29 Nov 4 28Ð8 4Ð0 1Ð0 11 287Ð7 281Ð3 0Ð476 0Ð052 0Ð669 0Ð174

Table III. Summary of hydrochemical responses to the runoffevents RE that occurred during the period of study, including thedate on which peak discharge was recorded and the associated

changes in concentrations of DOC, NO3-N and DON

RE Date(2003)

DOC�mg l�1�

NO3-N�mg l�1�

DON�mg l�1�

1 6 Sep 1Ð8 0Ð01 0Ð052 8 Sep 2Ð6 0Ð02 0Ð103 22 Sep 3Ð8 �0Ð05 0Ð154 12 Oct 1Ð5 0Ð04 0Ð065 22 Oct 5Ð6 0Ð07 0Ð216 2 Nov 4Ð5 0Ð08 0Ð157 5 Nov 5Ð6 0Ð35 0Ð218 9 Nov 4Ð7 0Ð37 0Ð129 12 Nov 9Ð1 0Ð77 0Ð20

10 14 Nov 7Ð9 0Ð58 0Ð2111 19 Nov 1Ð0 �0Ð03 0Ð0112 25 Nov 6Ð5 0Ð32 0Ð2413 26 Nov 6Ð9 0Ð39 0Ð2714 29 Nov — — —

was evident in the sample collected on the 16 October2003.

Statistical analysis

Eigenanalysis of the correlation matrix of the hydrom-eteorological variables describing the runoff events thatoccurred during the period of study identified three under-lying factors with eigenvalues >1Ð0 and a fourth under-lying factor that accounted for a further significant frac-tion of the total variance (8Ð2%) and exhibited a distinctloading structure, despite having an eigenvalue of <1Ð0.All four factors were thus retained for rotational anal-ysis. A summary of varimax rotated factor loadings onthe hydrometeorological descriptive variables is given inTable V. In this instance, rotation had the desired effectof simplifying the overall factor loading structure, witheach factor subsequently demonstrating a distinct loading

Table IV. Summary of weekly bulk precipitation chemistriesduring the period of study, including the date on which eachsample was collected and associated concentrations of DOC,

NO3-N, NH4-N and DON

Date(2003)

DOC(mg l�1)

NO3-N�mg l�1�

NH4-N�mg l�1�

DON(mg l�1)

10 Sep — — — —17 Sepa — 0Ð63 1Ð00 —24 Sep 1Ð9 0Ð14 0Ð08 0Ð151 Oct — — — —9 Oct 2Ð0 0Ð00 0Ð00 0Ð1316 Octb 2Ð4 1Ð39 0Ð41 0Ð0722 Oct 1Ð2 0Ð18 0Ð18 0Ð1530 Oct 0Ð4 0Ð09 0Ð05 0Ð095 Nov 0Ð6 0Ð39 0Ð63 0Ð0013 Nova 4Ð4 1Ð46 3Ð35 0Ð1319 Nov 0Ð8 0Ð46 0Ð39 0Ð0027 Nov 0Ð8 0Ð57 0Ð57 0Ð004 Nov 0Ð3 0Ð35 0Ð20 0Ð00

Meanc 1Ð0 0Ð27 0Ð26 0Ð06

a Contamination by bird droppings.b Contamination by soot.c Mean values are exclusive of contaminated data.

structure with respect to the original variables. Factor 1was characterized by high positive loadings (>0Ð75) onmaximum discharge and the change in discharge duringeach runoff event, and thus may be considered to repre-sent event magnitude. Factor 2 was characterized by highnegative loadings on the change in soil moisture contentat a depth of 45 cm and maximum precipitation inten-sity, which suggests that it may represent soil water flowthrough the Bs horizon. Factor 3 was characterized by ahigh positive loading on time since last runoff event andmoderate negative loadings on maximum soil moisturecontent at 10 and 45 cm, and thus may be considered torepresent the length of time since the soil profile was lastflushed. Factor 4 was characterized by a high negative



Table V. Summary of varimax rotated factor loadings for thehydrometeorological variables presented in Table II. High load-ings (>0Ð75) and moderate loadings (0Ð40–0Ð75, in parentheses)are presented; low loadings (<0Ð40) are excluded. The eigen-value, proportion of total variance explained and effect on cumu-

lative variance explained are also shown for each factor

Variable Factor 1 Factor 2 Factor 3 Factor 4

tPE�1 — — — �0Ð90P (0Ð59) (�0Ð62) — —PImax — �0Ð76 — —PImean (0Ð52) (�0Ð49) — —tRE�1 — — 0Ð91 —Qmax 0Ð93 — — —Q 0Ð93 — — —�v10max (0Ð59) — (�0Ð72) —�v10 (0Ð46) — — (�0Ð64)�v45max — (�0Ð65) (�0Ð62) —�v45 — �0Ð95 — —

Eigenvalue 3Ð07 2Ð71 2Ð13 1Ð54Variance explained 0Ð28 0Ð25 0Ð19 0Ð14Cumulative variance 0Ð28 0Ð53 0Ð72 0Ð86

loading on time since the last precipitation event and amoderate negative loading on the change in soil moisturecontent at a depth of 10 cm, which suggests that it mayrepresent rewetting of the H horizon.

A summary of univariate and stepwise multivariateregression models examining the extent to which theextracted factors explain the changes in concentrationsof DOC, NO3-N and DON that occurred during runoffevents in the period of study is given in Table VI. None ofthe univariate models demonstrated a significant degreeof explanation, with the exception of Factor 1 (eventmagnitude) in relation to DON, although the amount ofvariance explained was relatively low (40%). In contrast,all of the multivariate models demonstrated a significantdegree of explanation, albeit using different combinationsof factors for each response variable. For DOC and DON,the initial combination of Factor 1 and Factor 2 (soilwater flow through the Bs horizon) was most significant,explaining 46% and 60% of the variance of DOC andDON respectively. The inclusion of Factor 3 (the length

of time since the soil profile was last flushed) and thenFactor 4 (rewetting of the H horizon) was also significantfor DOC, explaining a further 22% and 19% of thevariance respectively. Although the inclusion of Factor3 was also significant for DON, explaining a further 9%of the variance, the inclusion of Factor 4 was not. ForNO3-N, the initial combination of Factor 3 and Factor4 was most significant, explaining 48% of the variance.The inclusion of Factor 1 was also significant, explaininga further 23% of the variance.

Time series of the standardized residuals of DOC,NO3-N and DON, derived from the final multivariateregression models presented in Table VI, are given inFigure 4. In each case, the Durbin–Watson test for auto-correlation was indeterminate at the 0Ð05 significancelevel, although a degree of pattern does appear to bepresent in the time series, with strong positive and nega-tive residuals associated with particular runoff events thatoccurred during the latter half of the period of study. Forall three response variables, large negative residuals inexcess of one standard error were generated for runoffevent 11, which occurred on the 19 November 2003.These results reflect the limited scale of hydrochemi-cal responses to runoff event 11 (see Figure 3), whichoccurred towards the end of the flow recession associ-ated with runoff event 10, the magnitude of which wassignificantly greater. For DOC and DON, a large positiveresidual in excess of one standard error was generatedfor runoff event 12, which occurred on the 25 November2003, following several days of severe frost. For NO3-N,large positive residuals in excess of one standard errorwere generated for runoff events 7 and 8, which occurredon the 5 November and the 9 November 2003 respec-tively, at the onset of the period of rewetting.

DISCUSSION

Factors influencing the release of dissolved organiccarbon during autumn runoff events

In the headwater of the Birnie Burn, the most sig-nificant combination of factors influencing the release of

Table VI. Summary of univariate and stepwise multivariate regression models examining the extent to which the extracted factorsexplain the changes in concentrations of DOC, NO3-N and DON that occurred during runoff events in the period of study. Note thatthe r2 values for the multivariate models have been adjusted for the number of independent terms and the p values are presented

using standard notation (n.s. not significant, Ł � 0Ð05, ŁŁ � 0Ð01, Ł Ł Ł � 0Ð001)

Model DOC �mg l�1� NO3-N �mg l�1� DON �mg l�1�

r2 p se r2 p se r2 p se

Factor 1 (F1) 0Ð32 n.s. 2Ð3 0Ð21 n.s. 0Ð25 0Ð40 Ł 0Ð07Factor 2 (F2) 0Ð25 n.s. 2Ð4 0Ð04 n.s. 0Ð27 0Ð28 n.s. 0Ð07Factor 3 (F3) 0Ð20 n.s. 2Ð5 0Ð30 n.s. 0Ð23 0Ð10 n.s. 0Ð08Factor 4 (F4) 0Ð15 n.s. 2Ð5 0Ð28 n.s. 0Ð24 0Ð0 n.s. 0Ð09F1 C F2 0Ð46 Ł 1Ð9 — — — 0Ð60 ŁŁ 0Ð05F1 C F2 C F3 0Ð68 ŁŁ 1Ð5 — — — 0Ð69 ŁŁ 0Ð05F1 C F2 C F3 C F4 0Ð87 Ł Ł Ł 1Ð0 — — — — — —F3 C F4 — — — 0Ð48 Ł 0Ð19 — — —F3 C F4 C F1 — — — 0Ð71 ŁŁ 0Ð14 — — —



Figure 4. Time-series of the standardized residuals of DOC, NO3-N and DON derived from the final multivariate regression models presented inTable VI. In each case, the residuals are plotted against the corresponding runoff events, but over time as opposed to a category scale. Residuals inexcess of one standard error are labelled with reference to the runoff event to which they relate. The Durbin–Watson d statistic is given in the topleft corner of each plot (in italics where autocorrelation is present at the 0Ð05 significance level). The time-series of discharge during the period of

study is also shown in the background of each plot, in order to aid interpretation

DOC during autumn runoff events is event magnitude andsoil water flow through the Bs horizon, which translatesin real terms to the magnitude of the water flux passingthrough the soil to the stream. Put simply, the greater thewater flux passing through the soil, the greater the releaseof DOC. This is in agreement with findings from manyprevious studies of DOC dynamics in upland headwa-ters that have demonstrated a significant positive corre-lation between the release of DOC and stream discharge(Reid et al., 1981; Edwards et al., 1984; Grieve, 1984,1990; Muscutt et al., 1990; Chapman et al., 1993; Wor-rall et al., 2002). The influence of soil water flow throughthe Bs horizon may be anomalous in this instance, simplyreflecting the dominant soil water flowpath during runoffevents in the period of study, rather than any direct influ-ence on the release of DOC. Alternatively, the influenceof soil water flow through the Bs horizon may be real,since mineral soil horizons frequently contain apprecia-ble amounts of translocated organic matter, which may,under certain physico-chemical conditions, be releasedinto solution (Kennedy et al., 1996). Although soil waterflow through the Bs horizon was the dominant soil waterflowpath during runoff events in the period of study, con-centrations of DOC were consistently high, suggestingthat adsorption was limited. This may have been becauserunoff responses to precipitation were driven mainly bypreferential flow through macropores in the soil, whichare characterized by low surface areas and rapid through-flow velocities, which together limit the potential for

adsorption. This challenges the commonly held view thatfluxes of DOC are only likely to be significant where lat-eral flows through shallow organic soil horizons dominate(e.g. McDowell and Wood, 1984; Aitkenhead-Petersonet al., 2003), and suggests that care should be takenwhen extrapolating catchment fluxes from soil data alone.In addition to the above, both the length of time sincethe soil profile was last flushed and rewetting of the Hhorizon also significantly influenced the release of DOCduring runoff events in the period of study. The influenceof these factors is intuitive, reflecting the time availablefor biologically derived soluble organic matter to accu-mulate in the soil between runoff events and the potentialfor that store to be subsequently dissolved and flushedfrom the soil (Edwards and Cresser, 1987; Grieve, 1991;Scott et al., 1998).

Factors influencing the release of nitrate and dissolvedorganic nitrogen during autumn runoff events

The most significant combination of factors influencingthe release of NO3-N during autumn runoff events inthe headwater of the Birnie Burn is the length of timesince the soil profile was last flushed and rewetting ofthe H horizon. This suggests that the release of NO3-Nis supply limited, and thus dependent on the state ofbalance between rates of mineralization and nitrificationby microbial activity and rates of removal by biologicaluptake and leaching. For example, little NO3-N may bereleased during a runoff event following a long period



since the soil profile was last flushed if soil moisturecontent in the H horizon in the intervening period becameso low as to limit rates of mineralization and nitrificationby microbial activity (Birch, 1960; Stanford and Epstein,1974; Stark and Firestone, 1995). Equally, little NO3-Nmay be released in a runoff event occurring shortly afteranother in which the soil profile was flushed, even ifsoil moisture content in the H horizon remained optimalfor microbial activity. In addition to the above, eventmagnitude also significantly influenced the release ofNO3-N during runoff events in the period of study. Theinfluence of event magnitude on the release of NO3-N isobvious, given that NO3-N is highly mobile and readilyleached from soils. Put simply, provided that sufficientNO3-N is available for leaching, then the greater thewater flux passing through the soil, the greater the releaseof NO3-N. The combination of factors influencing therelease of DON during autumn runoff events is essentiallythe same as that influencing the release of DOC; thisis not surprising, given that both DOC and DON arederived from the same store of soluble organic matter inthe soil. However, the exception to this rule is rewettingof the H horizon, which did not significantly influencethe release of DON during runoff events in the periodof study. This may be because, as rewetting of the Hhorizon occurs, microbial communities gradually regainnormal functionality and mineralize increasing quantitiesof organic matter, converting potential sources of DONfirst into NH4-N and then NO3-N, via the processesof ammonification and nitrification respectively (Birch,1960; Stanford and Epstein, 1974). Thus, as rewettingoccurs, the response of DON to runoff events maybecome increasingly limited by the effects of microbialactivity, and thus demonstrate a weaker relationship withsoil moisture content in the H horizon.

Were any factors missed that might explain some of thevariance in dissolved organic carbon and nitrogen?

Although DOC and DON responses to runoff eventswere relatively consistent throughout the period of study,two runoff events were identified during the regres-sion analysis as being unusual, on the basis of havinglarge positive and negative residuals associated with thechanges in concentrations of DOC and DON. In the caseof runoff event 11, which occurred on the 19 November2003, the predicted changes in concentrations of DOCand DON were significantly greater than those that wheremeasured. This may have been due to temporary deple-tion of the store of soluble organic matter in the soil as aresult of excessive flushing during the preceding runoffevent, causing the release of DOC and DON to becomesupply limited. However, this seems unlikely, given thatsubsequent runoff events of greater magnitude and occur-ring shortly thereafter were characterized by significantreleases of DOC and DON. A more likely explanationis that the magnitude of the previous runoff event wassufficiently great that the high concentrations of DOCand DON that persisted through the flow recession effec-tively limited the changes in concentrations of DOC and

DON determined for runoff event 11. In the case ofrunoff event 12, which occurred on the 25 November2003, following several days of severe frost, the pre-dicted changes in concentrations of DOC and DON weresignificantly lower than those that where measured. Thisindicates that infiltrating precipitation dissolved more sol-uble organic matter than would normally be the case. Thiswas probably due to freezing of the litter layer at the soilsurface, which has the potential to disrupt soil structureand microbial tissues, resulting in the exposure of previ-ously stabilized organic matter and cell lysis (reviewedin Kalbitz et al. (2000)). Both of these effects may havetemporarily increased the availability of highly solubleorganic matter for dissolution and leaching during runoffevent 12.

In contrast to DOC and DON, the response of NO3-Nto runoff events during the period of study varied consid-erably, from being virtually unresponsive during the firsthalf of the period of study to being highly positive duringthe period associated with rewetting of the H horizon. Ingeneral, most of this variation was adequately explainedby the regression analysis, but three events were identi-fied as being unusual, again on the basis of having largepositive and negative residuals associated with changesin the concentration of NO3-N. In the case of runoffevents 7 and 8, which occurred on the 5 November 2003and 9 November 2003 respectively, the predicted changein the concentration of NO3-N was significantly lowerthan that which was measured. This may have been dueto enhanced diffusion of NO3-N from micropores con-taining immobile soil water into mesopores containingmobile soil water as rewetting occurred, thus increasingthe availability of NO3-N for leaching. However, thisexplanation would require a plentiful store of NO3-Nto have been present in the micropore network at theonset of the period of rewetting. Given that the rate ofNO3-N production by microbial activity over the preced-ing months would have probably become increasinglyinhibited as a consequence of increasing moisture stress(Stanford and Epstein, 1974; Stark and Firestone, 1995),and that plant uptake would have continued to deplete thestore of NO3-N until all available soil water was drawnup, then it seems unlikely that such a store would havebeen present. An alternative explanation for the resid-ual excess of NO3-N associated with runoff events 7and 8 is enhanced delivery of anthropogenically derivednitrogen from the atmosphere, via bulk precipitation. Themain source of NO3-N in most upland headwaters inthe UK is atmospheric deposition, and it is not uncom-mon for inputs in bulk precipitation to exceed streamflowlosses significantly (Reynolds and Edwards, 1995). In thisinstance, inputs of NO3-N in bulk precipitation exceededstreamflow losses by >500% over the period of study(63Ð9 kg versus 10Ð5 kg), and the concentration of NO3-Nin bulk precipitation increased significantly over time.However, at no point during the period of rewetting wasthe concentration of NO3-N in bulk precipitation suffi-cient to account for the peak concentrations of NO3-Nmeasured in the stream. Moreover, if the residual excess



of NO3-N associated with events 7 and 8 was caused byenhanced delivery of anthropogenically derived nitrogenfrom the atmosphere, then such a residual excess shouldalso have been generated for subsequent runoff events,when the concentration of NO3-N in bulk precipitationwas even greater.

A more likely explanation for the residual excess ofNO3-N associated with runoff events 7 and 8 is thata flush of mineralization and nitrification occurred inresponse to rewetting of the H horizon, as rehydratedmicrobial communities regained normal functionality andrates of extracellular enzyme activity increased (Birch,1960; Stark and Firestone, 1995). If the rate of uptakeof newly formed substrates did not keep pace with therate of mineralization by extracellular enzyme activity,then the availability of NO3-N in the soil for leachingwould have gradually increased, until such time as it waseither removed by soil water throughflow or the rate ofbiological uptake increased. In this instance the formeris more probable, since the growing season was virtuallyover by the onset of the period of rewetting and plantuptake would have been increasingly limited. In additionto the effects of rewetting, runoff events 7 and 8 alsooccurred shortly after the main period of litterfall in theheadwater, which would have provided an abundant sup-ply of metabolically favourable substrates for the rehy-drated microbial communities, thus potentially enhancingrates of mineralization and nitrification. Similar flushes ofmineralization and nitrification have been observed pre-viously in upland headwaters following long periods ofdrought (Reynolds et al., 1992). In the case of runoffevent 11, which occurred on the 19 November 2003, thepredicted change in the concentration of NO3-N was sig-nificantly greater than that which was measured, as wasthe case for both DOC and DON. This may have beendue to temporary exhaustion of available NO3-N in thesoil for leaching, in response to excessive flushing duringthe preceding runoff events. Alternatively, it may havebeen because the magnitude of the previous event wassufficiently great that the high concentration of NO3-Nthat persisted through the flow recession effectively lim-ited the change in concentration of NO3-N determinedfor runoff event 11.

A further factor that may have influenced the releaseof DOC and N during runoff events in the period ofstudy is the narrow zone of flush gleys immediately adja-cent to the stream. A number of previous studies havedemonstrated that soils in riparian settings can serve asmajor sources of DOC during storm events (Hemond,1990; Hinton et al., 1998), and that the relative impor-tance of hydrological flowpaths through such zones mayvary considerably over time, producing changes in boththe quantity and quality of DOC released to the stream(Fiebig et al., 1990; Easthouse et al., 1992). Thus, it isconceivable that this zone of flush gleys may be a sig-nificant contributor of DOC to the stream during runoffevents, and may also serve as a locus for biogeochemi-cal processing of both DOC and N (Fiebig et al., 1990).

However, the magnitude of the former effect may be rela-tively limited, since hill slope exports of DOC to streamsare typically much greater than riparian contributionsin headwaters where soil water flowpaths are predom-inantly through lower B horizons, as is the case here(Hinton et al., 1998). Moreover, analysis of soil waterthroughflow samples acquired upslope of the flush gleysduring more recent autumn runoff events has revealedDOC concentrations of >50 mg l�1 in the Bs horizon,supporting the argument that the podzolic soils in theheadwater are capable of releasing significant quantitiesof DOC (Stutter, unpublished data). Notwithstanding thisfinding, examination of the dynamics of DOC and N dur-ing individual runoff events in the period of study revealsa consistent pattern of NO3-N depletion during the earlystages of each event, which may be indicative of therelease of a sizeable volume of water from anaerobic set-tings in the zone of flush gleys (Cooper et al., submitted).Given that we currently have insufficient data with whichto quantify the influence of the zone of flush gleys on therelease of DOC and N, this issue remains the subject ofongoing investigation.

CONCLUSIONS

The statistical methodology employed in this study hasenabled us to identify and assess the relative importanceof the principal factors influencing the release of DOCand N from the headwater of the Birnie Burn during asequence of autumn runoff events. Four principal factorswere identified, namely event magnitude, soil water flowthrough the Bs horizon, the length of time since thesoil profile was last flushed, and rewetting of the Hhorizon. The release of DOC and DON was most stronglyinfluenced by the combination of event magnitude andsoil water flow through the Bs horizon, and to a lesserextent by the length of time since the soil profile waslast flushed. Rewetting of the H horizon also influencedthe release of DOC, but this was not the case for DON,probably due to the effects of microbial activity. Therelease of NO3-N was most strongly influenced by thecombination of the length of time since the soil profilewas last flushed and rewetting of the H horizon, and to alesser extent by event magnitude. Soil water flow throughthe Bs horizon did not influence the release of NO3-N.

Although the principal factors identified above areessentially physical in nature, being derived from hydro-meteorological data alone, we have argued that the mech-anisms by which they influence the release of DOC andN are strongly associated with moisture-dependent bio-logical activity. Thus, at the watershed scale, it wouldappear that the release of DOC and N from upland head-waters dominated by podzolic soils is largely controlledby the variable interaction of hydrometeorological fac-tors and moisture-dependent biological processes, andthat physico-chemical controls established either at theplot scale (reviewed in Kalbitz et al. (2000)) or for otherupland soil types (e.g. Clark et al., 2005) may be less



important. If this is indeed the case, then the likely shiftin climate (towards drier summers characterized by morefrequent droughts and wetter winters characterized bymore frequent extreme events) predicted for the UK undercurrent emissions scenarios (Hulme et al., 2002) mayresult in the release of DOC and N from upland headwa-ters dominated by podzolic soils becoming increasinglyvariable and more episodic in the future.

ACKNOWLEDGEMENTS

We gratefully acknowledge assistance provided by FrankMilne and David Hamilton in the field, and Susan McIn-tyre, Emma Lyon and Yvonne Cook in the laboratory.We also gratefully acknowledge financial support pro-vided by the Scottish Executive Environment and RuralAffairs Department (MLU/493/96 and MLU/917/03).

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