The influence of pedology and changes in soil - School of

The Influence of Pedology and Changes in Soil MoistureStatus on Manganese Release from Upland Catchments: SoilCore Laboratory Experiments

A. M. Hardie & K. V. Heal & A. Lilly

Received: 26 June 2006 /Accepted: 14 January 2007# Springer Science + Business Media B.V. 2007

Abstract Manganese (Mn) contamination of drinkingwater may cause aesthetic and human health problemswhen concentrations exceed 50 and 500 μg l−1,respectively. In the UK, the majority of Mn-relateddrinking water supply failures originate from unpol-luted upland catchments. The source of Mn istherefore soil, but the exact mechanisms by which itis mobilised into surface waters remain unknown.Elevated Mn concentrations in surface waters havebeen associated with the rewetting of dried uplandsoils and with conifer afforestation. We investigatedthese hypotheses in a laboratory experiment involvingthe drying and rewetting of intact soil cores(1,900 cm3 volume) of horizons of four representativesoil type-land use combinations from an upland watersupply catchment in southwest Scotland. Although no

statistically significant effect of land use or soil typewas detected on Mn concentrations in soil water, Mnrelease occurred from three soil horizons uponrewetting. Soil water Mn concentrations in themoorland histosol H2 (10–30 cm), the histic podzolH and Eh horizons increased from means of 5.8, 6.2and 0.6 μg l−1 prior to rewetting to maxima of 90, 76and 174 μg l−1 after rewetting, respectively. Theproperties of these three horizons indicate that Mnrelease is favoured from soil horizons containing amixture of organic and mineral material. Mineralmaterial provides a source of Mn, but relatively highsoil organic matter content is required to facilitatemobilisation. The results can be used alongside soilinformation to identify catchments at risk of elevatedMn concentrations in water supplies.

Keywords Drinking water . Intact soil cores .

Land use . Manganese . Mobilisation . Soil horizon .

Soil water . Upland catchments

1 Introduction

The trace metal manganese (Mn) is the 12th mostabundant element within the earth’s crust (Manahan,1994). Like other transition metals, Mn can exist indifferent oxidation states, but the most widely occur-ring forms in the environment are soluble Mn (II)when reduced and insoluble Mn (IV) when oxidised.

Water Air Soil PollutDOI 10.1007/s11270-007-9348-6

A. M. HardieScottish Environment Protection Agency,5 Redwood Crescent, Peel Park,East Kilbride, G74 5PP Scotland, UK

K. V. Heal (*)School of GeoSciences, The University of Edinburgh,Crew Building, West Mains Road,Edinburgh, EH9 3JN Scotland, UKe-mail: [email protected]

A. LillyThe Macaulay Institute,Craigiebuckler, Aberdeen, AB15 8QH Scotland, UK

Where rocks containing high concentrations of Mnoccur near the earth’s surface, Mn-rich rock fragmentscan be incorporated into soils providing a pool of Mn,which may be released into the soil by chemical andphysical weathering. Indeed, this is thought to be themain source of Mn mobilised into surface waters inBritish upland catchments (Heal, 2001).

Although Mn is an essential trace element for allliving organisms, elevated concentrations in water canhave detrimental effects on human health and aquaticecology (particularly fish stocks (Nyberg et al., 1995;Stubblefield et al., 1997)), and also cause aestheticand structural problems in water supply systems. HighMn concentrations in drinking water can result in Mnprecipitation within supply pipes, hence reducingflow rates (Gounot, 1994). Such precipitates mayalso slough away from pipe surfaces causing dis-coloured, metallic-tasting drinking water. The toxico-logical effects on humans exposed to Mn in drinkingwater are well documented (Hudnell, 1999; Iwami etal., 1994; Kawamura et al., 1941; Kondakis et al.,1989; Santos-Burgoa et al., 2001) and the principalsource of non-occupational exposure is considered bysome to be drinking water supplies contaminated bynaturally high Mn (Mergler & Baldwin, 1997).

Consequently, the World Health Organisation (1996)recommended that the Mn concentration in drinkingwater should not exceed 500 μg l−1 in order to protectpublic health. Nevertheless, standards are often setmuch lower than this, owing to consumer objectionsarising from aesthetic Mn-related problems. Forexample, the current European Community (EC)maximum admissible concentration for drinking wateris 50 μg l−1 (EC, 1998). In the UK, the majority ofdrinking water supplies that fail to meet this standardoriginate from upland catchments and these failures arethought to arise from reducing conditions prevalent inacidic soils favouring the mobilisation of naturallyoccurring Mn (Heal, 2001). Once mobilised, Mn canbe transported in solution to water supply reservoirs.As a consequence of the adverse effects resulting fromhigh Mn concentrations in drinking water, there isconsiderable ongoing effort to develop methods forreducing the soluble Mn concentration once it hasentered the water supply system (Casale et al., 2002).However, to minimise water supply costs, there is alsoa need to understand how Mn is mobilised andtransported into surface water supplies, in order topredict where and when elevated Mn concentrations

may occur, so that water supply sources and treatmentregimes can be adjusted accordingly.

Field observations suggest that Mn may be releasedfrom certain soils, such as near-neutral soils uponrewetting following drought conditions (Grasmanis &Leeper, 1966) and that land use may also affect Mnrelease (Heal, 2001). Water chemistry data forScottish upland reservoirs show increased Mn con-centrations after the onset of autumn rainfall follow-ing unusually dry summers (Heal, 2001). ElevatedMn concentrations have also been measured in uplandstreams in southeast Scotland and northern Englandfollowing rewetting after prolonged dry conditions(Abesser et al., 2006; Heal et al., 2002). Therefore,the principal aim of the research reported here was toexamine the effect of an extended drying period,followed by saturated moisture conditions, on releaseof Mn from soils typical of UK upland water supplycatchments. By selecting soils beneath moorland andconifer plantation, it was also possible to investigatethe effect of these land use types on the Mn releasefrom soils. As an extended drying period could not beguaranteed in the field, a laboratory study with anartificially controlled soil water regime was conductedusing intact soil cores, following similar procedures toHolden and Burt’s (2002) investigation into the effectof drought on peat hydrology in northern England.

2 Materials and Methods

2.1 Site Description

The study site was located in the catchment of LochBradan, a drinking water supply reservoir managedby Scottish Water and situated in the Galloway Hillsin southwest Scotland (55° 13′ N, 4° 29′ W). TheLoch Bradan catchment covers an area of 15.5 km2

and is predominantly underlain by Mn-rich Ordovi-cian greywackes of the Blackcraig and Kirkcolmformations (British Geological Survey, 1994). Catch-ment soils are predominantly organic (histosols) andorgano-mineral (histic podzols and histic gleysols),while the land use is primarily commercial forestplantation, mainly Sitka spruce (Picea sitchensis) andopen moorland vegetation dominated by dwarfshrubs, such as Calluna vulgaris and Vacciniummyrtillus, as well as by grasses, such as Molinia

Water Air Soil Pollut

caerulea and Scirpus cespitosus. During the 1990s,there were three extended periods when watersupplied from Loch Bradan to the nearby treatmentworks contained Mn concentrations considerably inexcess of the 50 μg l−1 EC drinking water standard,with peak concentrations of 358–1040 μg l−1 (Fig. 1).High Mn concentrations occurred mainly during theautumn months following extended dry summers. Forexample, the Mn peak in autumn 1995 followed avery dry summer in which the total June–Augustrainfall for Loch Bradan was only 153 mm comparedto the mean (1980–1999) total rainfall for thesemonths of 319 mm. Repeated Mn failures of thisnature were a major consideration in the extensiveupgrading of the Loch Bradan treatment works at acost of £16 million (Little & McFadzean, 1991).

In a preliminary investigation to identify possiblesources of Mn in the Loch Bradan catchment,elevated Mn concentrations were identified in severalof the feeder streams. Out of these, BallochbeattiesBurn was selected for this investigation as itscatchment area offered the combination of soil typesand land use which is most representative of UKupland catchments. This sub-catchment has an areaof 0.86 km2 and ranges in elevation from 320 to480 m a.s.l. The upper reaches are covered bymoorland vegetation, underlain by peaty podzols(histic podzols) and some minor occurrences of peatyrankers (epileptic histosols) in shedding sites as well

as by peaty gleys (histic gleysols) and peat soils(dystric histosols) in receiving sites (Bown, 1973;Bown et al., 1968, 1982; IUSS, 2006). The histosolsclose to the burn often contain mineral alluvialmaterial interspersed throughout the soil profile. Thedeepest histosols are found in the comparativelylevel receiving sites in the lower reaches of the sub-catchment, where near-permanently saturated condi-tions at shallow depths have allowed organicmaterial accumulation to depths of over 2 m.Commercial forest, planted in 1964, covers approx-imately 50% of the sub-catchment and is mainlylocated on these areas of deep histosols. A smallarea of the forest plantation (approximately0.04 km2) was clearfelled in the 1990s prior to thisstudy. The remainder of the sub-catchment area iscovered by moorland vegetation.

2.2 Soil Core Collection

Soil cores for laboratory experiments were taken inApril 2000 from the major horizons of the mostcommon soils (histosol, histic gleysol, histic podzol)within the Ballochbeatties Burn sub-catchment. Coreswere collected from three horizons of four soil-landuse combinations that are typical of many uplandwater supply catchments in the UK. A total of 12horizons were investigated as detailed in Table 1.Since only the histosols occur in both moorland and

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Fig. 1 Mn concentrations(unfiltered acidified sam-ples) in water enteringtreatment works from LochBradan, southwest Scotland,January 1988–December2000 (data supplied byScottish Water). The dashedhorizontal line indicates theEC drinking water standardof 50 μg Mn l−1


forest plantation, this was the only soil type where theeffect of land use could be investigated.

To obtain the cores, the required soil type was firstlocated in the field. The overlying vegetation and soil(where necessary) were removed to expose the top ofthe uppermost soil horizon to be sampled. Intact soilcores (11 cm diameter, 20 cm deep, volume1,900 cm3) were collected with minimal disturbanceby gently pressing rings cut from PVC pipingvertically into the exposed horizon and carefullyexcavating the surrounding soil. The PVC ringscontaining the soil cores were then removed andwrapped in PARAFILM®M to minimise moistureloss. The cores were transported to the laboratory,where they were stored in darkness at 4°C untilrequired. Since the histic podzol Eh horizon was lessthan 20 cm thick, relatively undisturbed cores wereobtained by removing the partly filled ring at thelower horizon boundary, moving it to another part ofthe exposed horizon and repeating the sampling

procedure until 20 cm thickness of core was collected.Where the soil horizons were too stony to permitcollection of intact cores, as for example in the histicgleysol Cg and the histic podzol Bs horizons, coreswere collected by carefully repacking the soil into thering at a bulk density approximate to the in situmaterial (see Table 1). Six replicate cores werecollected from each horizon, with the exception ofthe histic podzol Eh horizon, from which only threecores were obtained, as it was too thin to provide sixreplicates without excavating a large area and dis-turbing existing field monitoring sites.

2.3 Soil Core Laboratory Experiment

The collected soil cores were used in a laboratoryexperiment to determine whether Mn was releasedinto the soil solution on rewetting the dried soil. Theexperiment was conducted in four separate runsbecause of equipment and time constraints. Each run

Table 1 Characteristics of soil horizons in the Ballochbeatties sub-catchment, southwest Scotland, from which cores were collected

Soil type Horizon Land use Elevation(m a.s.l.)

Soildepthcored(cm)

SoilpHa

(in H2O)

Soil Mn contentb,c

(μg g−1 dryweight)

Organic mattercontent (by losson ignition)b

(%)

Gravimetricwatercontentb

(%)

Bulkdensityb

(g cm−3)

Histosol H2 Forest 338 10–30 3.74 2.94±0.195 98.6±0.09 89.7±0.50 0.096Histosol H3 Forest 338 30–50 3.75 1.94±0.147 98.8±0.09 89.0±0.69 0.103Histosol H3 Forest 338 70–90 3.57 1.43±0.120 99.2±0.06 88.0±0.23 0.115Histosol H2 Moorland 390 10–30 4.11 9.35±1.30 94.6±0.55 88.6±0.31 0.107Histosol H2 Moorland 390 30–50 4.68 8.69±2.68 92.7±1.54 83.5±0.39 0.161Histosol H2 Moorland 390 70–90 5.38 16.3±5.72 86.5±3.62 79.3±1.71 0.211Histicgleysol

H2 Moorland 392 10–30 4.25 70.0±11.3 46.4±6.20 67.6±0.79 0.353

Histicgleysol

Bg Moorland 392 35–55 5.42 116±3.36 6.09±0.63 30.6±1.39 0.920

Histicgleysol

Cg Moorland 392 70–90 5.88 115±4.36 2.30±0.22 18.2±1.08 1.332

Histicpodzol

H Moorland 394 5–25 4.05 36.0±5.65 59.6±4.51 66.8±2.45 0.347

Histicpodzol

Eh Moorland 394 25–35 4.58 39.2±2.87 46.4±11.2 59.6±7.31 0.338

Histicpodzol

Bs Moorland 394 40–60 5.13 102±8.33 7.13±1.34 25.8±2.93 0.887

a Determined in a bulk sample from the same horizon collected in April 1999.b Determined on soil removed from each soil core to fit two microtensiometers and a small suction probe for the experiment. Valuesare means of 3 replicates±standard error (where shown).cMeasured by flame atomic absorption spectrophotometry in digests of ashed samples prepared using hydrochloric and nitric acids.

Analyses were carried out using standard procedures described in Allen et al. (1974) and detailed in Hardie (2002). Dashed linesseparate the different soil type-land use combination.


contained all cores from one of the four soil type-landuse combinations. At the start of each experimentalrun, the six replicate cores from each horizon underinvestigation were randomly divided into two groupsof three: ‘treatment cores’ and ‘control cores’. Thebases of the cores (contained within the same PVCrings used for collection) were sealed with a PVC trayand silicone sealant and each core was instrumentedto measure soil matric potential and to collect soilwater samples for chemical analysis during theexperiment as detailed in Section 2.4.

During each experimental run (at least 6 weeksduration) the soil cores under investigation wererandomly arranged on a laboratory bench to randomiseany positional effects and were exposed to a nearconstant temperature of ∼20°C. Throughout the exper-iment, the soil moisture content in the control cores waskept constant, equivalent to the field value at the time ofcollection, by the daily addition of deionised water (pH∼6.4) to achieve constant weight. For the first 2 weeksof the experiment, the treatment cores underwent an“acclimatisation” phase in which they were treated inthe same way as the control cores. This period wasdesigned to allow the identification of any Mn releaseinto soil water resulting from disturbance during corecollection or changes in environmental conditions (e.g.,temperature) during transfer to the laboratory andstorage. Following the acclimatisation phase, the treat-ment cores were allowed to air dry within the PVC ringsfor a period of two weeks. The length of the dryingphase was chosen to be consistent with the longestperiods of no rainfall recorded in the Bradan catchmentduring the summer of 1995 (a year in which anautumnal Mn peak in reservoir water was recorded).After the drying phase, the treatment cores wererewetted to saturation, by adding as much deionisedwater as the cores could physically hold, to simulate thereturn to saturated soil conditions that are commonlyexperienced in the study area. Saturated conditions weremaintained during the rewetting phase by the dailyaddition of deionised water to achieve constant weight.

The rewetting phase for the treatment cores wasoriginally set to two weeks, corresponding to the timeinterval in 1995 between the onset of wet autumnconditions in the Bradan catchment and the occur-rence of peak Mn concentration in reservoir water.However, findings from the first experimental runusing the moorland histosol cores indicated that Mnrelease into soil water only began to occur in the final

days of this two-week period. Therefore, the durationof the rewetting phase in later experimental runs wasincreased by one week for the forest histosol and thehistic gleysol cores and by 3 weeks for the histicpodzol cores to encompass the time duration of Mnrelease into soil water from the cores.

2.4 Soil Core Measurements

In order to determine the extent of drying in each core,soil matric potential was measured with two micro-tensiometers comprising 1 cm diameter porous ceramiccups filled with deionised de-gassed water. These wereinserted horizontally, one at 5 cm from the top and theother at 5 cm from the bottom of the core. Themicrotensiometers were read regularly (normally dai-ly) by connecting them to a pressure gauge via a shortlength of plastic tubing, also filled with deionised de-gassed water. To investigate Mn release, soil waterfrom each core was also sampled regularly (normallydaily) via a small suction probe inserted horizontally atthe midpoint of the core The probe consisted of a2.5 cm diameter porous ceramic cup attached to a50 ml polycarbonate sample chamber. The suctionprobes were pre-conditioned prior to installationfollowing the procedures of Grover and Lamborn(1979) and Creasey and Dreiss (1988) by flushingwith 1 M hydrochloric acid, followed by deionisedwater, until the pH of the extracted solution remainedconstant. Soil water samples were collected from thecores by creating a vacuum of approximately 60–70 kPa in the suction probe with a hand-held pumpand leaving the probe sealed for 24 h. There wereperiods during the drying phase when insufficient soilwater was collected from the treatment cores foranalysis, either due to the matric potential in the drysoil being greater than the suction applied to theprobe, or due to the entry of air into the suction probeas the soil surrounding it cracked and shrank.

2.5 Analytical Methods

Soil water samples for Mn determination were stored(unacidified) in sterile polycarbonate vials in darknessat 4°C until analysis by graphite furnace atomicabsorption spectrophotometry (Pye-Unicam SOLAARseries M5 set for absorption at 279.5 nm). The limit ofdetection, calculated from measurement of blanks,was 0.12±0.01 Mn μg l−1 and Mn concentrations


were measured as the mean of triplicate sampleinjections. Soil water samples were particle-free,having passed through the ceramic suction cup, andwere therefore not filtered after collection. To indicatewhether any increases in soil water Mn concentrationwere associated with increased acidification or organiccomplexation, soil water samples were also analysedfor pH (using a Hanna Instruments HI9025 hand-held

meter) and colour (measured as absorbance m−1 at400 nm using a Helios Gamma spectrophotometer)immediately after collection.

2.6 Data Analysis

The soil water chemistry results were analysed todetermine whether there was a statistically significant

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Fig. 2 Matric potentialsmeasured during the experi-ment in the upper and lowerparts of the control andtreatment cores of selectedsoil horizons: a Forest his-tosol H3 (70–90 cm),b Moorland histosol H2(10–30 cm), c Histic gleysolCg (70–90 cm), d Histicpodzol Eh (25–35 cm),e Histic podzol H (5–25 cm). Values are means±SE of samples from 3 repli-cate cores. There were nocontrol cores for the histicpodzol Eh horizon becauseof insufficient soil avail-ability. Dashed vertical linesseparate the different phasesof the experiment (acclima-tisation (ACCLIM.), drying,rewetting). Note the differ-ent x-axis scales due to thedifferent durations of therewetting phase in the fourexperimental runs


release of Mn into the soil water when the driedtreatment cores were rewetted and to ascertain theeffects of soil water pH, colour, soil horizon, soil typeand land use on Mn release. All statistical analyseswere conducted using the Minitab v.12.1 programunless stated otherwise. Prior to statistical analysis,the normality of the soil water chemistry datasetswere tested using the Anderson–Darling test, and theMn concentration and colour data were log10 trans-formed to achieve normality. Differences betweencontrol and treatment cores in each phase of theexperiment were analysed using 1-way ANOVA.Repeated measures analysis of variance (rm-ANOVA)was used (Genstat 5 program) to determine if therewas a statistically significant difference in soil waterchemistry between the different treatments (dryingand rewetting), soil horizons, soil types, land usetypes and over time. rm-ANOVA compares the meansof repeated measurements of the same variable atdifferent times in contrast to 1-way ANOVA whichonly compares measurements of the same variable forone point in time. One distinct advantage repeatedmeasures analysis has over other statistical methodsfor analysing single observations from several occa-sions is that it recognises that there is likely to be agreater correlation between observations made atadjacent time points than those further apart. Theuse of rm-ANOVA was regarded as the mostappropriate statistical test for these data because thesame variable was measured on a number ofoccasions within a relatively short period of time (T.Hunter, personal communication). Data were pooledby treatment phase for 1-way ANOVA and by soiltype for the rm-ANOVA test of the effect of soil type.Pearson’s correlation coefficients (r) were calculatedfor the relationships between the soil water Mnconcentrations, colour and pH to determine the effectof soil water pH and colour on Mn concentrations.The significance level chosen for all statistical analyseswas 0.05, apart from for the rm-ANOVA. Here it wasdecided to adopt a lower threshold of P<0.1 since thepower of the hypothesis test was diminished due tothe small number of replicates.

3 Results

Figure 2 shows the time series of matric potentialsmeasured during the experiment in treatment and

control cores from selected soil horizons. Theincrease in matric potential in the treatment coresduring the drying phase is clearly evident, whilematric potential remained relatively constant in thecontrol cores throughout the experiment. Data fromthe soil horizons not shown also followed this pattern.In general, the difference in matric potential betweenupper and lower microtensiometers in the treatmentcores was greatest in the organic horizons, indicatingthat there were greater vertical differences in theextent of drying in these cores compared to the moremineral-rich horizons.

The time series of Mn concentrations and colourmeasured during the experiment in the soil watersamples from cores of the same horizons depicted inFig. 2 are shown in Figs. 3 and 4, respectively. In nineof the 12 horizons investigated, soil water Mnconcentrations were generally low throughout theexperiment (<50 μg l−1), and there was no evidenceof Mn release from treatment cores upon rewetting.However mean soil water Mn concentrations in-creased upon rewetting from treatment cores of themoorland histosol H2 (10–30 cm) (Fig. 3b) and histicpodzol H horizons compared to the control cores(Fig. 3e). A similar pattern occurred on rewetting incores of the histic podzol Eh horizon compared to theacclimatisation and drying phases, since no controlcores were available for reasons previously mentioned(Fig. 3d). Treatment cores from the three horizonswith evidence of Mn release also showed the greatestchange in soil water colour upon rewetting. In soilwater treatment cores of the moorland histosol H2(10–30 cm) horizon, colour decreased during thedrying phase before increasing to pre-drying valuesduring the rewetting phase (Fig. 4b). Soil water colourfrom the histic podzol Eh (Fig. 4d) and H (Fig. 4e)horizon cores increased during the rewetting phase.

In treatment cores from histic podzol H and Ehhorizons, soil water Mn concentrations were signifi-cantly higher during rewetting compared to the accli-matisation phase (1-way ANOVA P<0.05, P<0.001,for each soil horizon respectively). However, althoughthere was graphical evidence of the release of Mn intosoil water after drying and rewetting of three of the soilhorizons investigated, the results of the statisticalanalyses of the soil water Mn concentration data fromthe experiment using 1-way ANOVA and rm-ANOVAwere mainly inconclusive. Several factors may haveconfounded the statistical analyses so that, even if Mn


release occurred into soil water as the result of soildrying and rewetting, the resultant increase in Mnconcentrations in soil water might not be identified asstatistically significant. Possible confounding factorsincluded the small number of replicate cores, hetero-geneity of cores from the same horizon, missing datafrom the drying phase as insufficient soil water couldbe collected for analysis, and masking of the effects of

treatment and soil type due to data pooling required toconduct some statistical analyses. Furthermore, mostof the statistically significant variation in soil waterMn concentrations identified by rm-ANOVA occurreddue to changes over time during the experiment ratherthan as the result of the drying and rewetting treatments.

Results from rm-ANOVA showed that during therewetting phase, there were no statistically significant

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Fig. 3 Mn concentrationsmeasured during the experi-ment in soil water samplesfrom control and treatmentcores of selected soil hori-zons: a Forest histosol H3(70–90 cm), b Moorlandhistosol H2 (10–30 cm),c Histic gleysol Cg (70–90 cm), d Histic podzol Eh(25–35 cm), e Histic podzolH (5–25 cm). Values aremeans±SE of samples from3 replicate cores. Note thedifferent y-axis scales.There were no control coresfor the histic podzol Ehhorizon because of insuffi-cient soil availability.Dashed vertical lines sepa-rate the different phases ofthe experiment (acclimatisa-tion (ACCLIM.), drying,rewetting). Note the differ-ent x-axis scales due to thedifferent durations of therewetting phase in the fourexperimental runs


differences in soil water Mn concentrations betweencores of different land uses (forest histosols vs.moorland histosols) or soil types (moorland histosol,histic podzol, and histic gleysols) arising from thetreatment. No statistically significant effect of soiltype or soil horizon on soil water Mn concentrations

was found during the rewetting phase, apart from asignificant (rm-ANOVA P=0.068, n=66) interactionbetween time and treatment effects for the moorlandhistosol H2 (10–30 cm) horizon which corresponds tothe increase in soil water Mn concentrations observedtowards the end of the rewetting phase (see Fig. 3b).

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Fig. 4 Colour measuredduring the experiment insoil water samples fromcontrol and treatment coresof selected soil hori-zons: a Forest histosol H3(70–90 cm), b Moorlandhistosol H2 (10–30 cm),c Histic gleysol Cg (70–90 cm), d Histic podzol Eh(25–35 cm), e Histic podzolH (5–25 cm). Values aremeans±SE of samples from3 replicate cores. Therewere no control cores forthe histic podzol Eh horizonbecause of insufficient soilavailability. Dashed verticallines separate the differentphases of the experiment(acclimatisation(ACCLIM.), drying, rewet-ting). Note the different x-axis scales due to the dif-ferent durations of therewetting phase in the fourexperimental runs


The effect of rewetting on the Mn concentrations inthe soil water from the histic podzol Eh horizon couldnot be tested statistically using rm-ANOVA becauseof the absence of control cores due to the lack ofavailable soil.

Pearson’s correlation coefficients (r) (Table 2)showed the expected negative correlation betweenpH and Mn concentration in soil water (i.e., higherMn concentrations were correlated with lower pHs) tobe statistically significant in treatment cores for nineout of the 12 horizons, apart from the histic gleysolH2, histic podzol H and Bs horizons. For colour,significant negative correlations with Mn (P<0.005)were identified in treatment and control cores from allhorizons of the forest histosol, apart from the H3 (30–50 cm) treatment cores. In contrast, significantpositive correlations between Mn and colour wereidentified in some horizons of the moorland soils,namely in the histic podzol H and Eh treatment cores(P<0.005) and the histosol H2 (10–30 cm) treatmentand control cores (P<0.01 and P<0.005, respectively).Some of the strongest correlations between soil waterMn, pH and colour were found in the treatment cores

of the histic podzol Eh horizon. In this horizon,increases in soil water Mn were significantly correlatedwith decreases in pH (r=−0.657, P<0.005) andincreases in colour (r=0.693, P<0.005), which mayindicate that complexation with organic (humic orfulvic) acids facilitates the mobilisation of Mn.However, since there were no control cores, the extentto which this correlation resulted from drying andrewetting cannot be ascertained.

4 Discussion

The soil core experiment showed that Mn releasefrom dried soils upon rewetting only occurred in threehorizons: the moorland histosol H2 (10–30 cm) andthe histic podzol H and Eh horizons. From examina-tion of the properties of these horizons it is suggestedthat most of the Mn released upon the rewetting ofdried soils is from individual soil horizons whichcontain both organic and mineral material. The histicpodzol H and Eh horizons contain a mixture oforganic and mineral material with organic matter

Table 2 Pearson’s correlation coefficients and significance levels calculated between Mn concentrations, colour and pH measured insoil water from the control and treatment soil cores during the experiment

Soil type Horizon Land use Soil depth cored(cm)

n Control cores Treatment cores

Mn vs.colour

Mn vs. pH colour vs.pH

Mn vs.colour

Mn vs. pH colour vs.pH

Histosol H2 Forest 10–30 102 −0.402*** 0.229* 0.075 −0.301*** −0.220* −0.677***Histosol H3 Forest 30–50 102 −0.509*** 0.452*** −0.067 0.122 −0.376*** 0.030Histosol H3 Forest 70–90 102 −0.304*** −0.422*** 0.202* −0.456*** −0.759*** 0.329***Histosol H2 Moorland 10–30 84 0.316*** −0.407*** −0.885*** 0.298** −0.238* −0.574***Histosol H2 Moorland 30–50 84 0.190 −0.299** 0.184 −0.091 −0.234* 0.545***Histosol H2 Moorland 70–90 84 0.143 −0.460*** −0.005 −0.028 −0.287* 0.353***Histicgleysol

H2 Moorland 10–30 93 −0.025 −0.205* −0.591*** 0.034 0.134 −0.228*

Histicgleysol

Bg Moorland 35–55 93 0.026 −0.104 0.104 0.058 −0.503*** −0.146

Histicgleysol

Cg Moorland 70–90 93 0.205* −0.287** −0.181 0.160 −0.308*** 0.122

Histicpodzol

H Moorland 5–25 117 0.029 −0.748*** 0.111 0.698*** 0.008 −0.088

Histicpodzol

Eh Moorland 25–35 117 a a a 0.693*** −0.657*** −0.934***

Histicpodzol

Bs Moorland 40–60 117 0.165 0.046 0.379*** −0.045 −0.134 0.746***

* P<0.05, ** P<0.01, ***P<0.005a No values as no control cores.


contents of ∼50% (see Table 1). Although themoorland histosol H2 (10–30 cm) horizon appearsto have a high organic matter content, and byinference a low mineral content, thin horizontal bandsof mineral grains were identified in soil cores fromthis horizon at the end of the experiment. Thismaterial was probably derived from periodic influxof sediment during flood flows in the BallochbeattiesBurn and could provide a source of Mn formobilisation. The soil total Mn content alone is nota reliable indicator of potential Mn release into soilwater. In this experiment the highest soil total Mnconcentrations were measured in the histic gleysol Bgand Cg and the histic podzol Bs horizons (see Table 1)yet no release of Mn into the soil water was observedfrom these horizons, perhaps because they also hadthe lowest organic matter contents of the horizonsinvestigated (see Table 1). The soil mineral materialprovides a source of Mn, expected to occur in thesesoils mainly as the Mn-containing mineral birnessiteand amorphous Mn oxides (Post, 1999; M. J. Wilson,personal communication). In addition a relativelyhigh soil organic matter content, in the order of∼50% (from the soils studied here), is required toprovide the conditions for Mn mobilisation.

The soil organic matter may contribute to Mnmobilisation either by the depression of soil redoxpotential and the reduction of Mn (IV) to mobile Mn(II) and/or by complexation of organic ligands withMn. The operation of the latter mechanism issupported by the highly significant positive correla-tions (P<0.01) between Mn concentrations and colourin soil water which only occurred in the treatmentcores of the three soil horizons from which Mnrelease was observed. Further evidence in support ofthis mechanism is provided by Graham et al. (2002)who reported that up to 50% of the Mn in watersamples from the Ballochbeatties Burn was humic-complexed.

The relationships between soil water Mn concen-trations and colour appeared to differ between forestand moorland soils. Significant negative correlationsbetween Mn and colour were identified in the foresthistosols, while significant positive correlations werefound in some horizons of the moorland soils. Onepossible explanation for this is that decompositionproducts may differ between vegetation types and couldtherefore interact in different ways with Mn in soilwater. For example, Kaiser et al. (2001) reported

differing proportions of hydrophilic organic carbon inseepage waters from organic soil layers beneathdifferent forest types. In a similar manner, the organiccarbon compounds released from decomposing moor-land vegetation could be more hydrophilic and mayhave a greater tendency to form soluble complexes withsoil Mn than those released from decomposing forestvegetation.

Manganese concentrations observed in the soilwater samples during the experiment were often anorder of magnitude lower than those measured in thereceiving watercourse, the Ballochbeatties Burn.Manganese concentrations at the outflow of the burninto Loch Bradan were reported to be 154±17.7(mean±SE, n=21, acidified unfiltered water samples,Hardie (2002)) and 180±60 μg l−1 (mean±1 S.D, n=4,acidified filtered (<1.2 μm) water samples, Graham etal. (2002)). The most likely explanation for this is thatthe soils in riparian areas have the greatest influence onstream water chemistry, whilst the soil cores for theexperiments were collected from outside riparian areas.Indeed, elevated Mn concentrations were measured insoil water sampled in situ with suction probes inriparian areas of the Ballochbeatties Burn sub-catch-ment. Soil water in peaty alluvial soils in the moorlandand clearfelled areas had Mn concentrations in acidifiedfiltered (<0.45 μm) samples of 209±50.7 and 5,210±1,260 μg l−1, respectively, (mean±SE, n=17). Incommon with the horizons that showed evidence ofMn release during the soil core experiment, these soilsalso contained a mixture of mineral and organicmaterial, with mineral material from flood flowsdispersed throughout the soil profile.

The exact mechanisms by which increased Mnmobilisation occurs from dried soils upon rewettingwere not identified in this study although thesignificant correlations between soil water Mn con-centrations and colour suggest that organic complex-ation may play a role. Soil pH is unlikely to be animportant factor for Mn release here since significantnegative correlations were found between Mn con-centrations and pH in soil water for most of the soilhorizons investigated, yet Mn release only occurredfrom three soil cores. Whilst the observation of no Mnrelease from the forest plantation soil cores could beattributed to the higher pH of the deionised wateradded in the experiments (pH ∼6.4) compared to thethroughfall pH expected under field conditions, it ismore likely to be due to the low mineral content and,


consequently, a low Mn content of the forest histosolhorizons (<3 μg Mn g−1 dry weight).

Although no field-based studies have been con-ducted which elucidate directly the processesresponsible for Mn mobilisation from soil, labora-tory studies of bacterial metabolism or of factorsaffecting Mn concentrations and forms in soil andsoil water suggest that microbial activity, inorganicsoil chemical reactions and physical and chemicalweathering may all be involved in Mn release.Numerous authors (e.g., Ghiorse, 1988) havereported that in anaerobic conditions bacteria able touse Mn as an electron acceptor in their metabolicpathways instead of oxygen switch their metabolismand anaerobically reduce Mn. Hence the observedincrease in soil water Mn concentrations after soildrying and rewetting could be linked to increasingsoil microbial populations during favourable aerobicconditions in the drying phase which produce largequantities of soluble Mn when the soil rewets andbecomes saturated. In contrast, Pohlman and McColl(1989) suggested from laboratory experiments thatabiotic oxidation of certain organic compounds insoil, accompanied by Mn (IV) reduction, may beimportant in the mobilisation of Mn from forest soils.Another hypothesis is suggested from a study byShuman (1980) which showed that levels of ex-changeable soil Mn increased upon air-drying. It istherefore possible that mobile Mn accumulates in thesoil during the drying phase and is flushed out byrewetting.

The result from the experiment that Mn release intosoil water appears to be associated with specific soilproperties, namely a mixture of mineral and organicmaterial, can be used in conjunction with soilmapping to identify catchments where Mn mobilisa-tion may cause problems for surface water quality.The accuracy of such predictions could be improvedby greater understanding of the processes leading toMn mobilisation from upland soils. This could beachieved by further soil core experiments (with morereplicates to enhance statistical power) in whichexamination of the mineralogy of mineral matter anddetermination of redox potential, microbial popula-tions and activity and the forms of Mn in soil watersamples would enable the role of abiotic and bioticfactors to be identified.

Given that the experimental results showed thatMn release occurred from some soil horizons after

drying and rewetting, the prediction of more frequenthot summers in the UK as a result of climate change(e.g., Hulme et al., 2002) means that the occurrence ofelevated Mn concentrations in upland surface waterscould increase in the future. Indeed, elevated Mnconcentrations have been reported in water suppliesfrom UK upland catchments since the 1980s. Forexample, a seasonal deterioration of Mn has occurredin raw waters from the Elan Valley, in upland Wales,which supplies water to Birmingham (Schofield et al.,1991), the second largest city in the UK bypopulation. A further factor which may exacerbateMn mobilisation into surface waters in the future isthe suggestion from the experimental results that Mnrelease from soil is associated with colour (anindicator of dissolved organic carbon (DOC)). In-creased DOC concentrations in UK upland surfacewaters have been reported by some studies (e.g.,Freeman et al., 2004; Worrall et al., 2003), andattributed to increasing atmospheric carbon dioxideconcentrations. Thus, if Mn mobilisation fromupland soils is associated with organic complexa-tion, any increase in DOC concentrations in surfacewaters could be associated with increased Mnconcentrations as an indirect effect of climatechange.

5 Conclusion

Soil core experiments showed that Mn can be releasedfrom soils typical of upland catchments as a result ofchanges in the soil moisture regime. The releaseappears to be mainly from particular soil horizonscontaining a mixture of organic and mineral matter,with land use and soil type not being majorinfluences. A period of extended drying, as seenduring very dry summers, followed by rewettingresults in an increase of mobile Mn in soil water ofsome soil horizons. The mobilised Mn can be trans-ported in soil and surface water and, if it enters thewater supply system, the quality of drinking watersupplies may be compromised. Although the mecha-nisms of Mn mobilisation could not be identified fromthese experiments, the results suggested that Mnrelease is associated with organic complexation.Further investigation of the processes responsible forMn mobilisation is required.


Acknowledgements We thank Scottish Water (formerly Westof Scotland Water) and The University of Edinburgh forfunding a studentship for Alasdair Hardie. We also thankForest Enterprise for permission to conduct fieldwork in theLoch Bradan catchment, Andrew Gray (The University ofEdinburgh) for assistance and advice with the laboratoryanalysis, Tony Hunter (Biomathematics and Statistics Scotland)for advice with the statistical analysis and the BritishAtmospheric Data Centre for rainfall data. Allan Lilly wasfunded by the Scottish Executive Environment and RuralAffairs Department.

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