alternative explanations for rising dissolved organic carbon export from organic soils

O P I N I O N

Alternative explanations for rising dissolved organiccarbon export from organic soils

C H R I S T O P H E R D . E VA N S , * P I P PA J . C H A P M A N , w J O A N N A M . C L A R K , wD O N T . M O N T E I T H z and M A L C O L M S . C R E S S E R §

*Centre for Ecology and Hydrology, Bangor LL57 2UP, UK, wEarth and Biosphere Institute, School of Geography,

University of Leeds, Leeds LS2 9JT, UK, zEnvironmental Change Research Centre, University College

London, London WC1H 0AP, UK, §Environment Department, University of York, York YO10 5DD, UK

Abstract

Since 1988, there has been, on average, a 91% increase in dissolved organic carbon (DOC)

concentrations of UK lakes and streams in the Acid Waters Monitoring Network

(AWMN). Similar DOC increases have been observed in surface waters across much of

Europe and North America. Much of the debate about the causes of rising DOC has, as in

other studies relating to the carbon cycle, focused on factors related to climate change.

Data from our peat-core experiments support an influence of climate on DOC, notably an

increase in production with temperature under aerobic, and to a lesser extent anaerobic,

conditions. However, we argue that climatic factors may not be the dominant drivers of

DOC change. DOC solubility is suppressed by high soil water acidity and ionic strength,

both of which have decreased as a result of declining sulphur deposition since the 1980s,

augmented during the 1990s in the United Kingdom by a cyclical decline in sea-salt

deposition. Our observational and experimental data demonstrate a clear, inverse and

quantitatively important link between DOC and sulphate concentrations in soil solution.

Statistical analysis of 11 AWMN lakes suggests that rising temperature, declining

sulphur deposition and changing sea-salt loading can account for the majority of the

observed DOC trend. This combination of evidence points to the changing chemical

composition of atmospheric deposition, particularly the substantial reduction in anthro-

pogenic sulphur emissions during the last 20 years, as a key cause of rising DOC. The

implications of rising DOC export for the carbon cycle will be very different if linked

primarily to decreasing acid deposition, rather than to changes in climate, suggesting that

these systems may be recovering rather than destabilising.

Nomenclature:

AWMN 5 UK Acid Waters Monitoring Network;

CET 5 Central England Temperature Record;

DOC 5 dissolved organic carbon;

SAA 5 sum of acid anions;

xSO4 5 nonmarine sulphate

Keywords: acidification, climate change, CO2, dissolved organic carbon, sulphate

Received 18 March 2005; revised version received 4 October 2005 and accepted 19 December 2005

Introduction

During the last two decades, increases in dissolved or-

ganic carbon (DOC) concentrations have been observed in

freshwaters across large areas of Europe and North

America (e.g. Driscoll et al., 2003; Hejzlar et al., 2003;

Evans et al., 2005; Skjelkvale et al., 2005). Some of the

earliest evidence of widespread DOC increases was pro-

vided by the UK Acid Waters Monitoring Network

(AWMN; Monteith & Evans, 2005), where a set of 11 lakesCorrespondence: Christopher Evans, e-mail: [email protected]

Global Change Biology (2006) 12, 2044–2053, doi: 10.1111/j.1365-2486.2006.01241.x

r 2006 The Authors2044 Journal compilation r 2006 Blackwell Publishing Ltd

and 11 streams have shown consistent, statistically sig-

nificant DOC increases as monitoring began in 1988

(Monteith & Evans, 2000; Freeman et al., 2001a; Evans

et al., 2005; Fig. 1a and c). Median DOC concentrations

were 31–140% higher during 1998–2003 than during the

first 5 years of monitoring at each site, with average

increases similar at lakes (63%) and streams (71%) (Ta-

ble 1). Increases are significant (nonparametric Mann–

Whitney test) at all 22 sites. Additional monitoring data

suggest that increases in DOC are near-ubiquitous across

the United Kingdom uplands (Worrall et al., 2004b), and

similar rising trends have occurred across large parts of

Scandinavia and North America (Skjelkvale et al., 2005).

The potential environmental implications of such trends

are wide ranging, from local effects on water transpar-

ency, acidity and metal transport, through to effects on

drinking water quality. Riverine DOC exports comprise

an important component of the carbon balance of peat-

lands (Billett et al., 2004), and rising surface water DOC

concentrations could therefore indicate depletion of ter-

restrial carbon stores, increasing fluxes into more reactive

(riverine, marine and ultimately atmospheric) pools. A

range of hypotheses have been put forward as potential

driving mechanisms for the increase in DOC, many of

which have been linked to climate change. Here, we

reexamine some of the ‘climate change’ hypotheses pro-

posed and, based on recent experimental work and

reanalysis of the AWMN dataset, consider the extent to

which an alternative driver, changes in atmospheric

deposition, could explain observed DOC trends.

Some recent explanations for increased DOC

in freshwaters: changes in temperature, rainfall

and CO2

When DOC trends in the AWMN dataset were first

noted, we speculated that they might reflect increased

organic matter decomposition rates due to warming, and

associated drying, of the peaty soils that dominate the

UK uplands (Freeman et al., 2001a; Evans et al., 2002). The

temperature hypothesis was consistent with data from

the Central England Temperature Record (CET, Parker

et al., 1992), which showed that mean summer tempera-

tures across England were 0.66 1C higher during the

1990s than in the preceding 30 years. In addition, peat

core warming experiments presented by Freeman et al.

(2001a) showed a positive DOC response to temperature.

Although the magnitude of the warming response was

0

1

2

3

4

5

6

7

1988 1990 1991 1993 1995 1997 1998 2000 2002 2003

1988 1990 1991 1993 1995 1997 1998 2000 2002 2003

1988 1990 1991 1993 1995 1997 1998 2000 2002 2003

1988 1990 1991 1993 1995 1997 1998 2000 2002 2003

mg

L–1

mg

L–1

mg

L–1

mg

L–1

DOC, lakes

0

2

4

6

8

10

12

14

16

18

DOC, streams

0

1

2

3

4

5

0

1

2

3

4

5

6

7

8

9

(a)

(c)

(b)

(d)

Fig. 1 Trends in dissolved organic carbon (DOC) and sulphate (SO4) in the UK Acid Waters Monitoring Network, 1988–2003. Plots

show median DOC and SO4 concentrations for 10 lakes sampled quarterly since 1988, and eight streams sampled monthly since 1988.

Bars show 25th and 75th percentile concentrations at each sampling interval. Gaps in the time series occur when samples were not

obtained for all sites, notably for a period of access closure during the 2001 Foot and Mouth disease outbreak.

A LT E R N AT I V E E X P L A N AT I O N S F O R R I S I N G D I S S O LV E D O R G A N I C C A R B O N 2045

r 2006 The AuthorsJournal compilation r 2006 Blackwell Publishing Ltd, Global Change Biology, 12, 2044–2053

only moderate, with a Q10 (rise in DOC production rate

over 10 1C) of 1.33, the proportion of recalcitrant phenolic

compounds in the DOC was also found to increase. It

was, therefore, argued that temperature-driven increases

in DOC production could be amplified in runoff due to

reduced levels of subsequent degradation. Taking the

Table 1 Changes in DOC and total SO4 concentration between the first and last 5 years of monitoring, all AWMN lakes and streams

Site (longitude, latitude)

DOC median

first 5 years

(mg L�1)

DOC median

last 5 years

(mg L�1)

% change,

Mann–Whitney

P

SO4 median

first 5 years

(mg L�1)

SO4 median

last 5 years

(mg L�1)

% change,

Mann–Whitney

P

Loch Coire nan Arr

(51390W, 571250N)

1.3 3.1 1 35*** 2.0 1.7 �15

Allt a’Mharcaidh

(31500W, 57170N)

1.7 2.3 1 35** 2.2 2.0 �9***

Allt na Coire nan Con

(51360W, 561450N)

2.9 4.6 1 59*** 2.9 2.6 �10**

Lochnagar

(31130W, 561570N)

0.8 1.5 1 88*** 2.8 2.2 �21***

Loch Chon

(41320W, 561120N)

2.6 5.0 1 92*** 3.4 2.7 �21***

Loch Tinker

(41300W, 561130N)

3.7 6.6 1 78** 2.6 1.7 �35***

Round Loch of Glenhead

(41250W, 55150N)

2.8 4.3 1 54*** 3.3 2.1 �36***

Loch Grannoch

(41160W, 55100N)

3.9 5.4 1 38*** 4.7 3.1 �34***

Dargall Lane

(41250W, 55140N)

1.2 2.0 1 67*** 3.9 3.0 �23***

Scoat Tarn

(31170W, 541280N)

0.5 1.2 1 140*** 3.0 2.5 �17***

Burnmoor Tarn

(31150W, 541250N)

1.4 2.7 1 96*** 3.9 3.0 �23***

River Etherow

(11490W, 531290N)

3.5 8.1 1 133*** 13.8 9.8 �29***

Old Lodge

(0140E, 51120N)

3.2 7.5 1 135*** 13.6 8.2 �40***

Narrator Brook*

(4110W, 501300N)

1.2 1.8 1 53** 3.5 3.5 0

Llyn Llagi

(4100W, 53100N)

2.1 3.0 1 41** 3.1 2.2 �31***

Llyn Cwm Mynach

(31570W, 521470N)

2.0 2.7 1 37* 4.0 3.4 �16***

Afon Hafren

(31420W, 521280N)

1.4 2.4 1 71*** 3.8 3.3 �13***

Afon Gwyw

(31420W, 521270N)

1.8 2.4 1 32*** 3.0 2.5 �17***

Beaghs Burn

(6190W, 55160N)

8.4 12.9 1 54*** 3.2 1.9 �41***

Bencrom River

(6100W, 54190N)

2.9 4.1 1 41*** 4.5 3.4 �24***

Blue Loughz

(51580W, 54190N)

3.2 4.2 1 31*** 4.8 3.3 �31***

Coneyglen Burnz

(7100W, 541440N)

6.4 11.0 1 72*** 2.3 1.8 �22***

Monitoring sites are located in acid-sensitive upland areas across the UK, and are described further in Monteith & Evans (2005).

First 5 years 1988–1993 (spring to spring) except at w1991–1996 (sampling began 1991) and z1990–1995 (sampling began 1990). Last 5

years 1998–2003 (spring to spring) at all sites. White backgrounds indicate streams, grey backgrounds lakes. Significance of change

assessed using nonparametric Mann–Whitney test. *Po0.05. **Po0.01. ***Po0.001.

DOC, dissolved organic carbon; SO4, sulphate; AWMN, Acid Waters Monitoring Network.

2046 C . D . E VA N S et al.


original Q10 value, however, Freeman et al. (2004) argued

that a temperature rise in excess of 10 1C would be

needed to explain observed DOC trends: much greater

than the 0.66 1C warming recorded.

The Q10 value used by Freeman et al. (2001a) was

derived from analysis of a Welsh ‘flush’ peat (a relatively

nutrient-rich system receiving lateral water inputs from

the hillslope). Ombotrophic ‘blanket’ peats (systems

receiving water and nutrients primarily from precipi-

tation) are more extensive in the UK uplands and else-

where, and few data are available on the temperature

sensitivity of DOC production in these systems. How-

ever, recent experiments on blanket peat cores from

Moor House National Nature Reserve, Northern Eng-

land (54170N, 2140W), found DOC production to be

more sensitive to temperature than the Welsh peat. In

these experiments, a Q10 of 2.13 was observed under

anaerobic conditions over a 10–20 1C range, which in-

creased to 3.66 under aerobic conditions (Clark, 2005).

Aerobic production was not only more sensitive than

anaerobic production to changes in temperature, it was

also 3–5.5 times greater than the corresponding anaero-

bic production rate at 10 1C (Clark, 2005). While water-

logged deep peats are anaerobic, most AWMN sites at

which DOC is rising are dominated by (commonly

aerobic) organo-mineral soils, and by blanket peats with

seasonally aerobic upper horizons. On the basis of these

data, it appears that a 0.66 1C temperature increase

could have generated a DOC increase of around 10–20%,

depending on the degree of soil aeration. This contribution

could be higher if there has been a net shift towards

increased soil aeration, particularly in peat soils. These

results suggest that temperature is a quantitatively

significant driver of long-term DOC trends, but is unlikely

to explain the full magnitude of observed DOC increases.

Hydrological factors may influence DOC production

and export through a number of mechanisms (e.g. Evans

et al., 2005). DOC concentrations in streams draining

organo-mineral soils typically increase following rainfall,

as the dominant flowpath shifts from the lower mineral

horizons, which adsorb DOC, to the organic horizons

that produce DOC (McDowell & Likens, 1988). In parts

of Scandinavia and North America, high lake DOC

concentrations have been associated with periods of

elevated rainfall via this mechanism (e.g. Hongve et al.,

2004) and also due to shorter residence times that restrict

in-lake turnover of DOC (e.g. Schindler et al., 1997;

Tranvik & Janssen, 2002). Pastor et al. (2003) observed

reduced DOC export from experimentally warmed peat

mesocosms as increased evapotranspiration led to

reduced discharge, and hence, reduced DOC transport,

out of the soil. Droughts may also affect the decomposi-

tion processes that produce DOC by aerating normally

anaerobic peats, thereby increasing the activity of the

phenol oxidase enzyme, which degrades the phenolic

compounds that inhibit decomposition (Freeman et al.,

2001b). This ‘enzymatic latch’ mechanism was pro-

posed as a possible driver of DOC increases by Worrall

et al. (2004a), although as yet there are no quantitative,

process-based data to show that this mechanism affects

DOC production. A semiempirical model based on the

enzymatic-latch concept (Worrall & Burt, 2005) cap-

tured the approximate trend in DOC export from the

Trout Beck catchment at Moor House, but not the short-

term dynamics. Other studies provide varying results,

with some suggesting that lowered water tables in-

crease peat DOC production (e.g. Tipping et al., 1999),

while others suggest a decrease (e.g. Freeman et al.,

2004) or no significant change (e.g. Blodau et al., 2004).

If hydrology has been the principal driver of in-

creased freshwater DOC concentrations, a concurrent

monotonic hydrological trend would be expected.

There is some evidence of hydrological change in the

United Kingdom since the 1960s, with increased pre-

cipitation during winters and less precipitation during

summers (Green & Marsh, 1997; Osborn & Hulme,

2002). Corresponding trends in runoff over the same

period have been identified for some rivers in Western

Scotland (Werritty, 2002). However, available discharge

data for AWMN sites showed no clear changes in flow

since 1988 (Evans et al., 2005). Rainfall data from mon-

itoring stations located close to the AWMN catchments

(Fig. 2) also indicate that: (i) few sites exhibit systematic

trends in either annual rainfall or winter/summer rain-

fall ratios; (ii) temporal patterns vary considerably

among sites; and (iii) averaged across all sites, no trends

are evident. It is, therefore, difficult to identify a hydro-

logical driver that could explain the monotonic rise in

DOC that has been observed. In addition, for two sites

with long-term discharge data, there has been a clear

increase in DOC under all flow conditions (Fig. 3). If

DOC increases were driven principally by changing

runoff, DOC–discharge relationships would be consis-

tent over the whole period. The increase in DOC con-

centrations at all flow levels implies that there has been

an increase in the available pool of DOC within the soil

system, and therefore that long-term DOC changes

cannot be attributed solely to increased leaching and/or

changes in water flowpath. Overall, there is little doubt

that periods of drought or high flow can influence the

year-to-year pattern of DOC variations at individual

sites, and in principle a long-term change in hydro-

logical conditions could cause a long-term change

in DOC. However, in the apparent absence of clear

hydrological trends since 1988, we argue that while

hydrological factors may account for short-term (1–4

years) DOC fluctuations, they cannot explain long-term

trends in DOC (over 15 years) at all 22 sites of the



AWMN, or indeed those observed more widely across

Europe and North America.

A further climate-related mechanism, proposed by

Freeman et al. (2004), was that rising atmospheric CO2

may be driving increased DOC. Their hypothesis was

based on results from a CO2-enrichment experiment, in

which atmospheric CO2 levels were raised by 235 ppm.

Peat cores taken from an upland blanket bog, a lowland

fen and a nutrient-rich riparian wetland showed in-

creases in leachate DOC concentration of 14%, 49%

and 61%, respectively. Freeman et al. (2004) suggested

that this was due to elevated net primary productivity

(NPP), and increased root exudation of DOC, possibly

linked to increased abundance of vascular plants over

mosses. Larger responses to elevated CO2 in wetlands

with greater nutrient availability appear consistent with

the hypothesis. Since 1988, however, atmospheric CO2

has risen by only around 20 ppm. The data presented by

Freeman et al. (2004), assuming a linear DOC–CO2

relationship, would only account for a 1.2%, 4.2% and

5.2% increase in DOC release from bog, fen and riparian

peat, respectively. Bog peat, the least responsive wetland

type, is the most extensive in the UK uplands repre-

sented by the AWMN. Therefore, while the mechanism

appears reasonable, we question whether it makes a

quantitatively important contribution to observed DOC

increases. We are also unaware of any data suggesting

the large-scale UK vegetation changes that would be

associated with a CO2-induced increase in NPP.

Alternative explanation for increased DOC in

freshwaters: changes in deposition chemistry

From the available evidence, we suggest that none of the

climate-change-related drivers so far proposed can fully

explain the magnitude of DOC increases in the United

Kingdom. Following recent experimental work and a

reassessment of time series for the period 1988–2003, we

argue that a progressive change in deposition chemistry

over this period, specifically a decline in anthropogenic

sulphur deposition and marine-derived sea-salt deposi-

tion, have been major drivers of increased DOC con-

centrations in freshwaters.

Over the last 20 years, there has been a major reduc-

tion in pollutant sulphur deposition, amounting to a

50% fall since 1986 (Fowler et al., 2005). This decrease

was not immediately reflected in reduced freshwater

concentrations of sulphate (SO4) or nonmarine SO4

0

0.5

1

1.5

2

2.5

3

3.5

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Win

ter/

sum

mer

rai

nfa

ll ra

tio

Northeast Scotland Central Scotland Northern England Mid Wales Northern Ireland Average of 16 sites

0

500

1000

1500

2000

2500

3000

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

An

nu

al r

ain

fall

(mm

)

Northeast Scotland Central Scotland Northern England Mid Wales Northern Ireland Average of 16 sites

Fig. 2 Annual rainfall and winter/summer rainfall ratio at UK Meteorological Office monitoring stations located close to Acid Waters

Monitoring Network (AWMN) catchments. Sites shown are: Northeast Scotland – Aviemore (nearby AWMN sites Allt a’Mharcaidh,

Lochnagar); Central Scotland – Loch Venachar (Loch Chon, Loch Tinker); Northern England – Emley Moor (River Etherow); Mid Wales –

Cwmystwyth (Afon Hafren, Afon Gwy); Northern Ireland – Lough Feagh (Coneyglen Burn). A further 11 UK Meteorological Office sites

located near the remaining AWMN sites were used to calculate the network-wide means shown, but for clarity are not plotted individually.

2048 C . D . E VA N S et al.


(xSO4, which normally represents the pollutant-derived

component of SO4), due to large climatic fluctuations

during the early years of monitoring (Monteith &

Evans, 2000). However, subsequent data have provided

clear evidence of decreasing xSO4 (and total SO4) con-

centrations in almost all AWMN lakes and streams

(Davies et al., 2005; Table 1). Based on the trend analyses

of Davies et al. the average 15-year decline in xSO4,

relative to 1988�1993 means, is 41%. This has led to a

corresponding decrease in the acidity of runoff at many

sites, and by inference in the acidity of the catchment

soils. Similar trends towards recovery from acidification

are occurring in other areas of Europe and North

America, where DOC is also increasing (Driscoll et al.,

2003; Stoddard et al., 2003; Skjelkvale et al., 2005).

Additionally in the United Kingdom, marine ion

deposition peaked during the early 1990s. As marine

ions raise soil solution ionic strength and cause transi-

ent acidification (e.g. Evans et al., 2001), the subsequent

decrease in marine ion loading may have contributed to

declining acidity and ionic strength, and hence, DOC

trends, particularly during the first 10 years of AWMN

monitoring when xSO4 decreases were less apparent.

An inverse relationship between mineral and organic

acid export to surface waters was first proposed in the

1980s (Krug & Frink, 1983). Increases in both acidity and

ionic strength (associated with a high SO4 loading) have

been shown to reduce soil solution DOC in a range of

laboratory experiments (Kalbitz et al., 2000). The impact

of acid deposition on DOC solubility is greatest in the

pH 4–5 range (Thurman, 1985; Peterson, 1990), typical

of upland organic soils in Britain. Mechanistic modelling

of humic substances suggested that acidified UK organic

soils were finely poised with regard to DOC (Tipping

& Hurley, 1988), and titration experiments indicated that

an increase in soil water pH of 0.5 U could cause a 50%

increase in DOC (Tipping & Woof, 1990). Liming has

also been shown to increase DOC (Andersson &

Nilsson, 2001), although as increased soil water pH also

resulted in increased microbial activity, it is difficult to

disentangle the relative role of chemical solubility from

microbial production. Several experimental studies have

reported that high levels of acidic deposition can reduce

litter decomposition rates (Killham & Wainwright, 1981;

Brown, 1985; Sanger et al., 1994).

Recent analysis of long-term data from Moor House

(Chapman et al., 2005; Clark et al., 2005) suggests a strong

influence of SO4 concentrations on DOC mobility. Ob-

servations were based on the release of SO4 from peat

Dargall lane

Discharge quartile

Discharge quartile

1st 2nd 3rd 4th

DO

C (

mg

L−1)

DO

C (

mg

L−1)

0

1

2

3

41988–19941995–2003

Afon Gwy

1st 2nd 3rd 4th0

1

2

3

4

51985–19941995–2004

(a)

(b)

Fig. 3 Increases in dissolved organic carbon (DOC) under all

flow conditions at two flow-gauged Acid Waters Monitoring

Network (AWMN) sites. Data are from AWMN monitoring sites

at Dargall Lane (Southwest Scotland), and from longer-term

monitoring by the Centre for Ecology and Hydrology at the

Afon Gwy (Mid-Wales). See Table 1 for site locations.

SO4 (mg L−1)

0 10 20 30 40 50

'Sup

pres

sed'

DO

C (

mg

L−1)

−10

0

10

20

30

40

50

Fig. 4 Relationship between sulphate (SO4) and dissolved

organic carbon (DOC) suppression in peat soil water under

simulated drought. DOC and SO4 measured at 10 cm depth in

peat cores at a controlled temperature of 10 1C. DOC suppression

is calculated as the reduction in concentration under drought

conditions relative to expected concentrations at that tempera-

ture (Clark, 2005).



during droughts, which occurs as reduced sulphur stored

within the peat is reoxidized under aerobic conditions

(Dillon et al., 1997; Adamson et al., 2001; Bottrell et al.,

2004; Eimers et al., 2004). This reoxidation was found, in

both monitoring data and laboratory manipulations, to

raise soil solution acidity and ionic strength, which

strongly suppressed DOC mobilization from the peat

(e.g. Figs 4 and 5). Although this process affected short-

term DOC dynamics during droughts, the same solubi-

lity controls on DOC should also operate over longer time

scales. Assuming that this is the case, the implication is

that reductions in soil solution SO4 due to decreasing

sulphur deposition should cause significant increases in

soil solution and surface water DOC concentrations.

The SO4–DOC relationship observed in Fig. 4 shows

that, for every twofold increase in SO4, DOC suppres-

sion (i.e. the reduction in DOC concentrations) increased

by a factor of 1.4. A similar relationship was observed in

field data at this site (Clark et al., 2005). The relationship

in Fig. 4 suggests that a decrease in SO4 from 3.35 to

2.65 mg L�1 (the median change in stream xSO4 between

1988–1993 and 1998–2003, Table 1) could cause DOC to

increase by 1.2 mg L�1. This is close to the median DOC

increase of 1.25 mg L�1 observed at AWMN sites (Table

1). However, results should be interpreted with caution.

Firstly, the experimental relationship is based on soil

solution data from one site (Moor House), soil type

(blanket peat), soil depth (10 cm) and temperature

(10 1C). The degree to which this relationship holds

for other sites, soils or environmental conditions is

unknown. Secondly, Moor House soil solution DOC

concentrations are relatively high, and therefore in

proportional terms the predicted change above is rela-

tively small. However, large changes in DOC have been

observed at the site: a 60% reduction in DOC concentra-

tions associated with a large SO4 pulse during the 1995

drought (Clark et al., 2005), and a 30% DOC reduction

associated with a smaller SO4 pulse during 1999 (Fig. 5).

There are also several reasons why stream water DOC

may be more sensitive to SO4 change than the soil

solution data suggest. Firstly, surface waters typically

contain runoff from several sources (deep mineral soil

and groundwater, as well as shallow organic soil water),

and a small change in runoff solute concentrations could

reflect larger changes in organic soil solution. Geological

sulphur sources, if present in groundwater, may provide

a consistent ‘background’ SO4, which, by contributing to

calculated xSO4, could mask a larger percentage de-

crease in pollutant-derived SO4 in soil water. Changes

in soil water acidity associated with drought-released

SO4 and deposited SO4 may also differ; SO4 released

during droughts at Moor House was substantially buf-

fered by accompanying pulses of base cations (Adamson

et al., 2001), whereas long-term changes in deposited SO4

are associated with proportionally greater changes in

acidity. Overall, the field and experimental data from

Moor House demonstrate a clear inverse link between

SO4 and DOC, but further research is required to quan-

0

10

20

30

40

50

60

70

80

90

µS c

m–1

Conductivity

0

1

2

3

4

5

6

7

8

9

Mar

Apr

May Jun

Jul

Aug

Sep Oct

Nov

Dec Jan

Feb

Mar

Mar

Apr

May Jun

Jul

Aug

Sep Oct

Nov

Dec Jan

Feb

Mar

Mar

Apr

May Jun

Jul

Aug

Sep Oct

Nov

Dec Jan

Feb

Mar

Mar

Apr

May Jun

Jul

Aug

Sep Oct

Nov

Dec Jan

Feb

Mar

mg

L–1

mg

L–1

Sulphate

3.8

3.9

4

4.1

4.2

4.3

4.4

4.5

4.6

pH

pH

0

5

10

15

20

25

30

35

40

45

50

DOC

(a)

(b)

(c)

(d)

Fig. 5 Example of a summer drought-induced sulphate flush

causing acidification and suppression of dissolved organic

carbon (DOC) in peat soil solution, Moor House (Northern

England). Bold lines show observed chemistry during the 1999

drought year. Narrow lines and shaded areas show median and

range of observed chemistry during the 7 years of monitoring in

which droughts have not occurred. Data was provided by the

Environmental Change Network.

2050 C . D . E VA N S et al.


tify the magnitude of change that could result from a

decline in sulphur deposition over different sites.

To examine the extent to which temperature and

deposition-related drivers could explain the observed

increases in DOC, we analysed data from the 11 AWMN

lakes (lakes were selected as they have greater short-

term chemical stability, making long-term patterns

easier to detect and analyse). After removing seasonal

variation (which is strongly temperature correlated, e.g.

Clark et al., 2005), we undertook a linear regression of

DOC against (i) xSO4, as the major influence on soil

solution acidity; (ii) the sum of acid anions (SAA,

representative of soil solution ionic strength); and (iii)

a simple temperature variable (mean CET for the pre-

ceding 1–3 years, based on stepwise selection for each

site). Regressions were significant in all cases (Po0.05,

most Po0.001) and in general both temperature and

deposition-related variables contributed to the total

variance explained (Fig. 6a). On average over the 11

lakes, temperature could account for 11% of (seasonally

detrended) DOC variance, while SAA and xSO4 to-

gether accounted for 27% of variance. There is, how-

ever, some variability in the relative and overall

strength of correlations between sites. Reductions in

sea-salt deposition may help to explain rising DOC

trends at relatively unpolluted sites such as Loch Coire

nan Arr (NW Scotland), where xSO4 changes are small

(this is also the one site where temperature accounts for

more of the DOC variance than the deposition-related

variables). However, it is also worth noting that during

our experiments, the greatest DOC response was seen at

lower SO4 concentrations (Fig. 4) and, therefore, less

acidified sites may in fact be more responsive to

changes in SO4 deposition than those that are still

chronically acidified. In general, it appears that much

of the unexplained variance is short term (e.g. related to

hydrological conditions at the time of sampling) and

consequently the proportion of observed DOC trend

explained is markedly higher (Fig. 6b), on average 66%.

Conclusions

DOC concentrations have risen at the 22 UK AWMN

lakes and streams by an average of 91% since 1988. Of

the mechanisms that have been put forward to explain

these increases, we estimate that increases in tempera-

ture alone could account for about a 10–20% increase,

and rising atmospheric CO2 could account for a 1–5%

increase. The extent to which various hydrological

factors could influence DOC production and transport

remains difficult to determine, but the absence of con-

sistent hydrological changes across the United King-

dom (and other parts of Europe and North America)

during the monitoring period argues against this as the

main driver of the general DOC increase that has been

observed. In contrast, the approximate halving of sul-

phur loading to UK upland ecosystems represents by

far the largest chemical change in these systems during

the last two decades, and has been replicated across

much of Europe and North America. From the results

described, it appears that decreases in soil solution SO4

could explain a large part of observed DOC increases,

although a general quantitative relationship cannot yet

be defined. Additional reductions in ionic strength and

acidity associated with falling sea-salt deposition over

the monitoring period could have contributed further to

DOC increases.

On the basis of the available data, we conclude that

changing atmospheric deposition and rising tempera-

tures are currently the most convincing mechanisms for

rising DOC in the United Kingdom. The dominant

driver, based on both experimental and time-series

data, appears to be the decrease in atmospheric sulphur

deposition. The same mechanism may explain the DOC

increases now being observed in other Northern Hemi-

0%

10%

20%

30%

40%

50%

Coire

nan

Arr

Loch

naga

r

Loch

Cho

n

Loch

Tink

er

Loch

Gra

nnoc

h

Round

Loch

Scoat

Tar

n

Burnm

oor T

arn

Coire

nan

Arr

Loch

naga

r

Loch

Cho

n

Loch

Tink

er

Loch

Gra

nnoc

h

Round

Loch

Scoat

Tar

n

Burnm

oor T

arn

Llyn

Llagi

Llyn

Llagi

Cwm M

ynac

h

Cwm M

ynac

h

BlueLo

ugh

BlueLo

ugh

Per

cen

tag

e o

f va

rian

ce

Temperature

0%

20%

40%

60%

80%

100%

Per

cen

tag

e o

f tr

end

(a)

(b)

Fig. 6 (a) Percentage of dissolved organic carbon (DOC) var-

iance (following seasonal de-trending) that can be reproduced

using temperature, nonmarine sulphate (xSO4) and sum of acid

anions (SAA) as explanatory variables, UK Acid Waters Mon-

itoring Network (AWMN) lakes. (b) Percentage of observed

DOC trend that can be reproduced using temperature, xSO4

and SAA as explanatory variables, UK AWMN lakes.



sphere regions that have been subjected to acid deposi-

tion. While we recognize that global-scale changes in

climate and atmospheric CO2 are likely to have signifi-

cant impacts on carbon cycling, we believe it is essential

that other factors, potentially operating at smaller scales

and in response to different anthropogenic drivers,

must also be considered in any assessment of observed

biogeochemical change. In the case of freshwater DOC,

the alternative explanation for observed increases ap-

pears to be decreasing acid loading; if correct, this

hypothesis has very different consequences for the

global carbon cycle, as it implies that these systems

may simply be returning to their preindustrial condi-

tions. Thus, rising DOC in freshwaters may to a large

extent reflect recovery from the effects of acid deposi-

tion, rather than ecosystem degradation in response to

climate change, and future predictions of dramatic

intensification of carbon export from global peatlands

may prove overly pessimistic. However, further work is

required before the major changes that have occurred

can be unambiguously attributed to particular mechan-

isms. Until this has been completed, predictions of the

future trajectory of DOC change, and its subsequent

impact on the carbon cycle, freshwater biota, and water

supply, will remain highly uncertain.

Acknowledgements

This work was supported by the UK Department of the Envir-onment, Food and Rural Affairs (Contract No. RMP 2036), theScottish Executive Environment and Rural Affairs Department/Welsh Assembly Government (Contract No. FF/03/08) and theEuropean Union Framework Programme 6 Eurolimpacs project(GOCE-CT-2003-505540). Experimental work was supported bya NERC-CASE Studentship with CEH Merlewood/Lancaster(NER/S/A/2000/03431). We thank Stuart Lane, Rachel Gasior,David Ashley, John Adamson and Louise Heathwaite for theirsupport; English Nature for use of the Moor House Reserve; theEnvironmental Change Network for use of their data; and thereviewers for their helpful comments.

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alternative explanations for rising dissolved organic carbon export from organic soils

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