the effect of lake-water infiltration on the acidity and base cation status of forest soil

THE EFFECT OF LAKE-WATER INFILTRATION ON THE ACIDITYAND BASE CATION STATUS OF FOREST SOIL

ANTTI-JUSSI LINDROOS1∗, JOHN DEROME2, LAURA PAAVOLAINEN1 andHELJÄ-SISKO HELMISAARI1

1 Vantaa Research Centre, Finnish Forest Research Institute, P.O. Box 18, FIN-01301 Vantaa,Finland; 2 Rovaniemi Research Station, Finnish Forest Research Institute, P.O. Box 16, FIN-96301

Rovaniemi, Finland (∗ author for correspondence, e-mail: antti.lindroos@metla.fi)

(Received 9 March 2000; accepted 6 September 2000)

Abstract. In this article, the effects of the artificial recharging of groundwater by infiltrating surfacewater through forest soil, i.e. sprinkling infiltration, on the acidity and base cation status of the soilare described. The study was carried out in the Ahvenisto esker area, Hämeenlinna, southern Finland,during 1996–1998. The sample plots were located in a 110- to 160-yr-old Scots pine (Pinus sylvestrisL.) stand. The site was classified as the Oxalis-Maianthemum type. The soil consisted of a mixtureof till and glaciofluvial sediments. The pH of the organic layer increased from 4.7 to >6.5 soonafter the start of irrigation on the infiltration plot. The pH of the 0–10 cm mineral soil layer alsoincreased from 4.9 to 6.4 as a result of infiltration. Sprinkling infiltration increased the exchangeableCa and Mg concentrations in the organic and uppermost mineral soil layers. The output of Ca andMg in percolation water from the 0–100 cm thick layer was lower than the input to the soil surfacevia irrigation in 1996. The retention of Ca and Mg on cation exchange sites took place within arelatively short period of time, since retention was observed only in 1996 but no longer in 1997 or1998 indicating saturation of the cation exchange sites by base cations. Lake water infiltration leadsto the neutralisation of forest soil acidity, and increases the capacity of the soil to withstand acidicinputs by increasing the concentrations of exchangeable base cations on cation exchange sites in thesoil.

Keywords: acidity, base cations, forest soil, percolation water, sprinkling infiltration

1. Introduction

Changes in the chemical properties of forest soil are dependent on many factors in-cluding land-use practices. The artificial recharging of groundwater by infiltratingsurface water through forest soil, i.e. sprinkling infiltration, on forested eskers isan example of a land-use practice that alters the environmental conditions in forestsoil, as well as in the whole ecosystem (Helmisaari et al., 1998). Groundwater isproduced artificially at groundwater plants because it can usually be used as house-hold water without any chemical treatment (Hatva, 1996). Artificial recharging ofgroundwater using infiltration basins has been a common practice in water works inthe Nordic countries already for decades. The new method, sprinkling infiltration,has been developed to improve the quality of artificially recharged groundwater,

Water, Air, and Soil Pollution 131: 153–167, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

154 A.-J. LINDROOS ET AL.

as well as to minimise the environmental or aesthetic impacts that the location ofinfiltration basins on eskers may cause.

Sprinkling infiltration differs from the infiltration methods used earlier in thatsurface water is sprinkled directly onto the forest floor via a network of pipes.The forest soil, as well as the whole ecosystem, is subjected to extremely highinputs of water. Furthermore, although the concentration of nutrients (e.g. Ca, Mg,NO3) in the surface water may be relatively low, the total input of nutrients willbe high. The changes caused by sprinkling infiltration in the species compositionof the understorey vegetation, as well as in the nitrogen transformation processestaking place in forest soil, have already been reported (Helmisaari et al., 1998;Lindroos et al., 1998; Paavolainen, 1999; Paavolainen et al., 2000). These changesare not only interesting from the point of view of the technical implementation ofsprinkling infiltration, but they also provide a possibility to study soil processesunder extreme conditions.

The exchange of base cations (primarily Ca2+ and Mg2+), bound at cation ex-change sites on the soil particles, with protons in the soil solution is an importantbuffering mechanism in forest soil against acidic deposition (Schwertmann et al.,1987). Soil acidification, or the accumulation of heavy metals derived from depos-ition, may lead to losses of base cations from the cation exchange sites (Freedmanand Hutchinson, 1980; Abrahamsen, 1983; Løbersli and Steinnes, 1988; Deromeand Lindroos, 1998; Derome et al., 1998). On the other hand, the concentrations ofexchangeable base cations in forest soil can also increase under certain conditions.Liming, for example, increases the base saturation of forest soil (Derome et al.,1986).

A number of rather extensive studies have been carried out on the acidity andbase cation status of forest soils in relation to acidifying sulphur and nitrogendeposition during the last two decades in Finland (e.g. Tamminen and Starr, 1990;Tamminen, 1998), as well as on the European scale (e.g. De Vries et al., 1999). Theeffects of fertilisers and forest management on the acidity parameters of forest soilshave also been studied extensively (Derome et al., 2000; Mälkönen et al., 2000).Although a considerable amount of information is available about the current acid-ity and base cation status of forest soils, our knowledge of the changes taking placein these properties under different environmental conditions is still rather deficient.The aim of this study was to investigate the effects of sprinkling infiltration onthe acidity and base cation status of forest soil. Such information is extremelyimportant in estimating the environmental impacts of this new infiltration techniqueon forested eskers.

2. Materials and Methods

The study was carried out in the Ahvenisto esker area located near Hämeenlinna,southern Finland (61◦01′N, 24◦47′E). Sprinkling infiltration was performed on

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TABLE I

Sample plots and the amount of water applied in sprinklinginfiltration during 1996–1998

Plot/treatment Irrigation period Amount applied

(mm)

Infiltration 25 June–30 October 1996 1 118 000

16 May–6 October 1997 1 258 000

18 May–29 October 1998 1 452 000

Control – –

TABLE II

General characteristics of the sample plots

Control plot Infiltration plot

Dominant tree species (stand basal area, m2 ha−1)

Scots pine 33.6 24.7

Norway spruce 1.7 8.8

Soil typea Carbic podzol Carbic podzol

Organic layer Moder Moder

Thickness (cm) 6–8 6–8

LOI-%b 39 26

Soil texture Sand/gravel/sandy till Sand/gravel/sandy till

a FAO.b Mean for the control plot during 1996–1998; mean for the infiltrationplot before infiltration in 1996.

a relatively steep, stony slope (20–25 ◦ to the east). The altitude of the area is100 m a.s.l. The mean annual temperature is 4.5 ◦C and mean annual precipitation630 mm. Lake water (infiltration water) was pumped from a nearby lake, and wassprinkled onto the forest floor by means of irrigation pipes. Sprinkling infiltrationwas carried out during the 1996–1998 growing seasons. The amounts of wateradded were equivalent to 1 118 000 mm of rainfall in 1996, 1 258 000 mm in 1997,and 1 452 000 mm in 1998, which is approximately equivalent to 2000× the meanannual precipitation (ca. 600 mm) in the area. A control plot without irrigationwas located adjacent to the infiltration plot. As artificial groundwater is used tosupplement the water supply of the city of Hämeenlinna, the actual amounts ofwater applied were regulated by the city waterworks (Table I).

156 A.-J. LINDROOS ET AL.

Figure 1. The pH of the organic (a) and 0–10 cm mineral soil (b) layers on the control and infiltrationplots during 1996–1998. Infiltration was started on 25th June 1996. Mean and standard error of themean are indicated (n = 3).

The infiltration and control plots were located within a stand of 110- to 160-yr-old Scots pines (Pinus sylvestris L.). The site was rather fertile and was classified asthe Oxalis-Maianthemum forest site type (Cajander, 1949). Blueberry (Vacciniummyrtillus), lingonberry (Vaccinium vitis-idaea) and some herbs, grasses and mosseswere typical understorey species. The soil consisted of a mixture of till and glacio-fluvial sediments. The percolation water zone of the esker was >10 m thick. Thestand and soil characteristics are given in Table II.

Soil samples were taken on 9 occasions from the organic layer during 1996–1998, and on two occasions from the 0–10 cm mineral soil layer (see Figure 1 for

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sampling dates). Each plot was divided into three sections, and between 7 to 10sub-samples taken of the organic layer and mineral soil in each section. The sub-samples from each section were bulked to give one composite sample per section.This meant that there were three replicate samples for each plot on each samplingoccasion. The diameter of the soil auger used in sampling was 25 mm. The organicmatter content was determined as loss in weight on ignition in a muffle furnace at550 ◦C (LOI %). The pH(H2O) was measured in a soil/water (3/5, v/v) slurry. Theexchangeable Ca, Mg and K concentrations were determined by extraction with1 M ammonium acetate (pH 4.65) for 2 hr on a shaker. After filtration, the extractwas analysed by inductively coupled plasma atomic emission spectrophotometry(ICP/AES). The results were calculated on the basis of the organic matter contentin the organic layer, and on dry matter in the mineral soil.

Samples were taken of the infiltration water and of the percolation water belowthe organic layer and at depths of 40 and 100 cm from the ground surface. Per-colation water was collected below the organic layer using plate lysimeters, andat deeper depths using suction cup lysimeters (pore size 1–2 µm). There were 6lysimeters at each depth on the infiltration and control plots. Water samples weretaken daily at the beginning of the study, but weekly during 1997 and 1998. Thewater samples from the same depth on the same plot were bulked. The pH of thewater samples was measured. The samples were preserved by adding 65% HNO3

(0.25 mL/50 mL sample) and the Ca, Mg and K concentrations determined byICP/AES.

The Ca and Mg fluxes for 1996 in the percolation water at a depth of 100 cmwere calculated by multiplying the mean concentrations of Ca and Mg by theamount of infiltration water. It was assumed that the possible losses of water throughe.g. evapo-transpiration would be negligible compared to the huge amount of irrig-ation water applied. This flux was compared to the input flux in the infiltrationwater.

Statistical differences of the means were determined using ANOVA and theTukey HSD test for post-hoc pairwise testing.

3. Results

3.1. SOIL

The pH of the organic layer increased rapidly after the start (25th June 1996) ofirrigation on the infiltration plot. The mean pH values on the infiltration plot andcontrol plot were approximately the same prior to irrigation, but increased from4.7 to >6.5 on the infiltration plot already during the first growing season (1996).The pH of the organic layer remained high during the 1997 and 1998 growingseasons (mean pH 6.4–7.0). The pH of the organic layer was at its highest on thelast sampling date in 1997 (Figure 1a). The pH of the 0–10 cm mineral soil layeralso increased from 4.9 to 6.4 as a result of infiltration (Figure 1b).

158 A.-J. LINDROOS ET AL.

Figure 2. The exchangeable Ca concentration in the organic (a) and 0–10 cm mineral soil (b) layerson the control and infiltration plots in 1996 and 1998. Infiltration was started on 25th June 1996.Mean and standard error of the mean are indicated (n = 3). o.m. = organic matter, d.m. = dry matter.

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Figure 3. The exchangeable Mg concentration in the organic (a) and 0–10 cm mineral soil (b) layerson the control and infiltration plots in 1996 and 1998. Infiltration was started on 25th June 1996.Mean and standard error of the mean are indicated (n = 3). o.m. = organic matter, d.m. = dry matter.

160 A.-J. LINDROOS ET AL.

Figure 4. The exchangeable K concentration in the organic (a) and 0–10 cm mineral soil (b) layers onthe control and infiltration plots in 1996 and 1998. Infiltration was started on 25th June 1996. Meanand standard error of the mean are indicated (n = 3). o.m. = organic matter, d.m. = dry matter.

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TABLE III

The pH of the infiltration and percolation water on the infiltration and controlplots during the growing seasons 1996–1998. Statistical differences of the meanswere tested between infiltration and percolation water collected during the samesampling period (small letters) and between the different years on the infiltrationplot and a three year period on the control plot (capitals). The means marked withthe same letter do not differ significantly (p > 0.05)

Infiltration plot Control plot

1996 1997 1998 1996–1998

Infiltration x 7.7Aa 7.1Ba 7.2Ba

water S.D. 0.4 0.2 0.2

n 21 19 14

Percolation x 7.1ABb 7.0Ab 7.1Ba 4.9Ca

water S.D. 0.2 0.2 0.2 0.2

– below organic layer n 57 21 20 15

– depth 40 cm x 7.1ABb 6.9Ab 7.1Ba 6.1Cb

S.D. 0.3 0.2 0.2 0.3

n 16 18 17 12

– depth 100 cm x 7.1Ab 6.9Ab 7.1Aa 5.9Bb

S.D. 0.4 0.2 0.2 0.2

n 16 20 17 5

The exchangeable Ca and Mg concentrations increased strongly as a result ofinfiltration in both the organic layer and the 0–10 cm mineral soil layer. Prior toinfiltration the mean values on the infiltration plot were similar to those on thecontrol plot (Figures 2 and 3). In contrast, there was no apparent change in the ex-changeable K concentration in the organic layer as a result of sprinkling infiltration,despite the apparent increase in the mineral soil (Figure 4).

3.2. PERCOLATION WATER

The percolation water pH was much higher on the infiltration plot than on thecontrol plot. The mean pH of the infiltration water was 7.7 during the 1996 growingseason, and decreased as the water percolated down through the organic layer. ThepH values in the infiltration and percolation water were very similar during the1997 and 1998 growing seasons (Table III).

162 A.-J. LINDROOS ET AL.

TABLE IV

The Ca concentration (mg L−1) of the infiltration and percolation water on the infiltra-tion and control plots during the growing seasons 1996–1998. Statistical differences ofthe means were tested between infiltration and percolation water collected during thesame sampling period (small letters) and between the different years on the infiltrationplot and a three year period on the control plot (capitals). The means marked with thesame letter do not differ significantly (p > 0.05)

Infiltration plot Control plot

1996 1997 1998 1996–1998

Infiltration x 9.17ABa 9.61Aa 8.54Ba

water S.D. 0.39 1.39 0.25

n 22 21 14

Percolation x 8.17Ab 9.27Ba 9.03Bb 4.56Ca

water S.D. 1.14 1.03 0.40 2.80

– below organic layer n 54 23 20 15

– depth 40 cm x 7.61Ab 9.4Ba 9.01Bab 3.62Ca

S.D. 1.15 0.74 0.71 0.55

n 14 18 17 17

– depth 100 cm x 7.51Ab 9.42Ba 8.81Bab 4.53Ca

S.D. 1.64 1.14 0.46 0.49

n 15 21 17 10

The Ca and Mg concentrations in the percolation water on the infiltration plotwere higher than those on the control plot. The mean Ca and Mg concentrationsin the percolation water on the infiltration plot were lower than the levels in theinfiltration water during the 1996 growing season, but at a similar level or slightlyhigher during 1997 and 1998 (Tables IV and V). In contrast, the K concentrationbelow the organic layer and at a depth of 40 cm on the control plot was higher thanthe concentrations during irrigation on the infiltration plot. The K concentrationsin the infiltration and percolation water remained at the same level throughout theinfiltration treatment during 1996–1998 (Table VI).

In general, the pH and base cation concentrations in the percolation water wererelatively similar at all sampling depths during infiltration (Tables III–VI).

On the infiltration plot, the Ca input in infiltration water was about 10 kg m−2

and the output 8 kg m−2 at a depth of 100 cm during 1996. The correspondingvalues for Mg were 1.7 and 1.5 kg m−2, respectively.

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TABLE V

The Mg concentration (mg L−1) of the infiltration and percolation water on the infiltra-tion and control plots during the growing seasons 1996–1998. Statistical differences ofthe means were tested between infiltration and percolation water collected during thesame sampling period (small letters) and between the different years on the infiltrationplot and a three year period on the control plot (capitals). The means marked with thesame letter do not differ significantly (p > 0.05)

Infiltration plot Control plot

1996 1997 1998 1996–1998

Infiltration x 1.48Aa 1.67BCa 1.52ACa

water S.D. 0.06 0.40 0.02

n 22 21 14

Percolation x 1.38Ab 1.58Ba 1.60Bb 1.06Cab

water S.D. 0.10 0.16 0.09 0.53

- below organic layer n 54 23 20 15

– depth 40 cm x 1.34Ab 1.59Ba 1.60Bb 0.92Ca

S.D. 0.09 0.09 0.11 0.30

n 14 1 8 1 7 17

– depth 100 cm x 1.34Ab 1.58Ba 1.57Bab 1.4Ab

S.D. 0.15 0.17 0.06 0.13

n 15 21 17 10

4. Discussion

The amount of water applied in the sprinkling infiltration treatment during thegrowing season was very high, about 2000× the mean annual precipitation(ca. 600 mm) in the study area.

As a result, the moisture content of the uppermost soil layers was continuouslyabove field capacity. The effects of this change in the water conditions on nutri-ent mineralisation in the soil and nutrient uptake by the vegetation has not beenstudied, but we do know that the treatment has not had any adverse effects onthe growth and overall condition of the tree stand (Helmisaari et al., 1998). Sucha high moisture content in the soil (e.g. waterlogging or paludification) usuallyresults in anaerobic conditions. In this experiment, however, we can assume thatthe infiltration water had a relatively high oxygen content (not measured) becauselake water was sprayed onto the forest floor.

164 A.-J. LINDROOS ET AL.

TABLE VI

The K concentration (mg L−1) of the infiltration and percolation water on the infiltra-tion and control plots during the growing seasons 1996–1998. Statistical differences ofthe means were tested between infiltration and percolation water collected during thesame sampling period (small letters) and between the different years on the infiltrationplot and a three year period on the control plot (capitals). The means marked with thesame letter do not differ significantly (p > 0.05)

Infiltration plot Control plot

1996 1997 1998 1996–1998

Infiltration x 1.43Aab 1.2BCa 1.26ACa

water S.D. 0.08 0.37 0.05

n 22 21 14

Percolation x 1.5Ab 1.19Aa 1.33Aa 5.76Ba

water S.D. 0.25 0.33 0.15 3.24

– below organic layer n 54 23 20 15

– depth 40 cm x 1.27Aa 1.13Aa 1.35Aa 2.24Bb

S.D. 0.12 0.08 0.09 1.54

n 14 18 17 17

– depth 100 cm x 1.28Aa 1.10Aa 1.31Aa 1.09Ab

S.D. 0.17 0.19 0.07 0.51

n 15 21 17 10

Sprinkling infiltration significantly increased the exchangeable Ca and Mg con-centrations in the organic and uppermost mineral soil layers. The fact that theoutput of Ca and Mg from the 0–100 cm thick layer was lower than the input to thesoil surface via irrigation further supports the above observation. The saturation ofcation exchange sites by Ca and Mg took place within a relatively short periodof time, since retention was observed in 1996 but no longer in 1997 or 1998.Although base saturation (BS) was not determined in this study, it is generallyaccepted (Hallbäcken and Popovic, 1985) that the pH and effective base saturationare strongly correlated. We can therefore assume that the BS in the surface soillayers on the infiltrated plot was close to 100%, i.e. that Ca2+ and Mg2+ haddisplaced most of the other cations (primarily H+ and Al3+) from the exchangesites. The increase in the pH of the soil to >6.5 will result in the release of aconsiderable amount of H+ ions into the soil solution owing to the almost completedissociation of the pH-dependent organic functional groups (e.g carboxyls) in thesolid matrix. This will have strongly increased the cation exchange capacity of the

LAKE-WATER INFILTRATION 165

soil, and further increased the pool of Ca and Mg bound in the soil. However, therelease of large amounts of H+ ions will not have affected strongly the soil waterpH owing to the diluting effect of the large amounts of water added. The pool ofH+ ions in this normally undissociated fraction of total soil acidity is in fact veryhigh; in the organic layer of upland soils in Finland it accounts for about 90% oftotal acidity (Starr and Tamminen, 1992). Conversely, the free and exchangeablefraction of total soil acidity, which is equivalent to the protons measured as pH,is only about 10%. The increase in the pH as a result of sprinkling infiltration, aswell as increased ammonium availability, also initiated nitrification in the forestsoil (Lindroos et al., 1998).

Some of the H+ ions displaced from cation exchange sites by the input of Ca2+and Mg2+, as well as those released into the soil solution as a result of dissociation,appear to have been carried deeper into the mineral soil. The pH of the infiltrationwater decreased slightly as it percolated down through the organic layer in 1996.However the major part of these H+ ions will have been neutralised by the HCO−

3ions in the irrigation water. The alkalinity of the infiltration and percolation wa-ter at various depths down the soil profile remained relatively constant at around0.17 mmol L−1 during infiltration (data not presented).

According to soil analysis, infiltration had no effect on the K concentrations inthe organic layer of the forest soil. In contrast, the K concentrations increased in thetopmost part of the mineral soil. The increase was, however, very small comparedto that of Ca and Mg, and the relative importance of K on the cation exchangesites remained low also after the infiltration treatment. In addition, the measuredK concentrations in percolation water did not indicate any retention in either theorganic layer or mineral soil. The fact that the K concentration did not increase inthe organic layer as a result of infiltration is not surprising, because K+ has a verylow affinity for cation exchange sites on organic matter (Bohn et al., 1985), and thepresence of elevated levels of divalent Ca2+ and Mg2+ ions will further reduce itsability to bind to the exchange sites. More than 80% of the (free + exchangeable)K+ in the organic layer and mineral soil of coarse-textured forest soils in Finlandis free in the soil solution, and only a relatively small proportion bound on cationexchange sites on the organic matter (Derome, 1991).

Because significant changes were no longer observed in the pH or concentra-tions and fluxes of Ca and Mg in the water percolating down to a depth of 100 cm inthe soil in 1997 and 1998, it would appear that a state of chemical equilibrium hasbeen reached between the soil and infiltrating water in this soil layer. Lake waterinfiltration not only leads to neutralisation of the forest soil, but also increases thecapacity of the soil to withstand acidic inputs owing to the increased concentrationsof exchangeable base cations. In this respect, the effect of sprinkling infiltration onforest soil is very similar to that of liming (Derome et al., 1986). We do not, at thepresent time, have any information about the long-term effects of the increase inpH and elevated Ca and Mg concentrations, caused by sprinkling infiltration, onnutrient availability, microbial activity and nutrient uptake in forest soil. However,

166 A.-J. LINDROOS ET AL.

it is clear that we cannot draw direct inferences from the results of long-term limingexperiments because, in contrast to the strong pH increase brought about by liming,the increase produced by sprinkling infiltration can never exceed the pH of thewater applied to the soil.

5. Conclusions

Sprinkling infiltration with lake water amounting to more than 2000 times theannual recharge resulted in a considerable reduction in soil and percolation-wateracidity. The pH of the organic and uppermost mineral soil layers increased to about7, which was approximately the same pH as that of the lake water. Saturation ofthe cation exchange sites in the surface soil by calcium and magnesium from theirrigation water occurred during the first growing season after the start of infiltra-tion. In contrast, potassium was not bound at all to the cation exchange sites in theorganic layer, presumably owing to its inability to compete for exchange sites withthe two other base cations.

Lake water infiltration leads to neutralisation of the forest soil, and increasesthe buffering capacity of the soil to withstand acidic inputs owing to the increasedconcentrations of exchangeable base cations.

Acknowledgements

The study was sponsored by the Waterworks of the cities of Hämeenlinna, Jyväs-kylä, Mikkeli, the Tampere-Valkeakoski area, the Turku area and the Tuusula area,as well as the Ministry of Agriculture and Forestry. We are grateful to the Water-works of Hämeenlinna for the infiltration arrangements and assistance in the fieldwork.

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