Nutrient cycling and soil leaching in eighteen pure and mixed stands of beech (Fagus sylvatica) and spruce (Picea abies)

Forest Ecology and Management 258 (2009) 2578–2592

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Nutrient cycling and soil leaching in eighteen pure and mixed stands of beech (Fagus sylvatica) and spruce (Picea abies) Torsten W. Berger a,*, Erich Inselsbacher a, Franz Mutsch b, Michael Pfeffer b a b

Department of Forest- and Soil Sciences, Institute of Forest Ecology, University of Natural Resources and Applied Live Sciences (BOKU), Peter Jordan-Straße 82, 1190 Vienna, Austria Federal Research and Training Centre for Forests, Natural Hazards and Landscape, Department of Forest Ecology and Soil, Seckendorff-Gudent-Weg 8, 1131 Vienna, Austria

A R T I C L E I N F O

A B S T R A C T

Article history: Received 19 May 2009 Received in revised form 4 September 2009 Accepted 7 September 2009

Studies on the combined effects of beech–spruce mixtures are very rare. Hence, forest nutrition (soil, foliage) and nutrient fluxes via throughfall and soil solution were measured in adjacent stands of pure spruce, mixed spruce–beech and pure beech on three nutrient rich sites (Flysch) and three nutrient poor sites (Molasse) over a 2-year period. At low deposition rates (highest throughfall fluxes: 17 kg N ha1 year1 and 5 kg S ha1 year1) there was hardly any linkage between nutrient inputs and outputs. Element outputs were rather driven by internal N (mineralization, nitrification) and S (net mineralization of organic S compounds, desorption of historically deposited S) sources. Nitrate and sulfate seepage losses of spruce–beech mixtures were higher than expected from the corresponding single-species stands due to an unfavorable combination of spruce-similar soil solution concentrations coupled with beech-similar water fluxes on Flysch, while most processes on Molasse showed linear responses. Our data show that nutrient leaching through the soil is not simply a ‘‘wash through’’ but is mediated by a complex set of reactions within the plant–soil system. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Fagus sylvatica Leaching Mixed species effects Nutrient cycling Picea abies

1. Introduction There is a large body of scientific work comparing the mineral nutrition and nutrient cycling in pure spruce and pure beech stands, but studies on the combined effects of beech–spruce mixtures are very rare, although mixed spruce–beech forests are a major forest type in Central Europe (e.g., review by De Schrijver et al., 2007 and references therein). Atmospheric pollution is scavenged more efficiently by coniferous canopies, enhancing deposition through higher dry deposition of SO42, NH4+ and NO3 (Augusto et al., 2002; De Schrijver et al., 2008). In general, inputs of nitrogen (N) and sulfur (S) increase from beech to spruce (Rothe et al., 2002a,b; Rothe and Mellert, 2004; Berger et al., 2008) as a result of the higher filtering surface of spruce and taking into account the leafless period of beech. According to De Schrijver et al. (2007) the significantly higher stand deposition flux of N and S in coniferous forests is reflected in a higher soil seepage flux of SO42, NO3, base cations and aluminum. De Vries et al. (2007) indicated that sulfate is still the dominant source of actual soil acidification despite the generally lower input of S than N, due to the different behavior of S (near tracer) and N (strong retention). Their studies are based upon huge data sets of European forests

* Corresponding author. Tel.: +43 1 47654 4107; fax: +43 1 47654 4129. E-mail address: [email protected] (T.W. Berger). 0378-1127/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2009.09.014

(between 57 and 181 sites), none of these studies was done in mixed conifer-broadleaf stands. Nitrogen throughfall fluxes, together with soil C/N ratio, soil pH or temperature explained a major part of nitrate variation in the soil solution under different atmospheric deposition regimes (MacDonald et al., 2002; Kristensen et al., 2004; Van der Salm et al., 2007). Gundersen et al. (2006) compiled regional and continental data on inorganic N in seepage and surface water from temperate forests. Elevated N deposition explained only approximately half of the variability in N leaching. Rothe et al. (2002a,b) found a significant non-linear pattern between N input by throughfall and nitrate concentrations of the soil solution, assuming that nitrate soil solution concentrations may be influenced by factors other than N input, e.g., differences in litter distribution, decomposition and mineralization. Berger et al. (2009) concluded that element outputs from forest stands on nutrient poor soils are driven by atmospheric inputs, but dominated by internal sources in nutrient rich soils. Both beech, as well as spruce stands, in the Black Forest, Germany, showed a net output of S (v. Wilpert et al., 2000). Despite markedly reduced S deposition, desorption of sulfate still influences soil solution chemistry (Martinson et al., 2005). Average annual streamwater outputs of S exceeded S bulk deposition inputs at Hubbard Brook, USA. This discrepancy may be explained by net desorption of S and net mineralization of organic S largely associated with the forest floor (Likens et al., 2002).

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Nowadays, it is considered prudent, close-to-nature forest practice, to convert secondary spruce stands into mixed spruce– beech stands. However, the assumption that mixed spruce–beech stands are a suitable replacement for secondary spruce stands on former mixed broadleaf sites needs critical review. E.g., Binkley and Giardina (1998) concede acidification by spruce but question its negative consequences on stand growth, since aboveground net primary production is higher in spruce forests than in beech. There is no evidence that deciduous admixture improves foliar nutrition of conifers (Rothe et al., 2003), since soil nutrient pools are not necessarily linked with nutrient levels. The rare studies on the effect of beech–spruce mixtures (Rothe, 1997; Rothe and Binkley, 2001; Rothe et al., 2002b; Berger et al., 2002, 2004, 2006) do not generally justify the long-held ‘‘beech-mother of forests’’ concept. Sterba et al. (2002) also found that Norway spruce grew better in the pure stand than in an otherwise comparable mixed species stand and linked this to water stress arising from competition with beech as a result of the very shallow rooting system of spruce trees in a mixed stand. Patterns of properties observed in mixed stands cannot be predicted from patterns observed in monocultures (Finzi and Canham, 1998). Berger et al. (2009) addressed this key issue for adjacent stands of pure spruce, mixed spruce–beech and pure beech on a nutrient rich site (Kreisbach) and for adjacent spruce and mixed stands on a nutrient poor site (Frauschereck). They raised important questions on mixed species effects for these two intensively studied and typical sites on the quite different bedrocks Flysch and Molasse. Since there was only one full set of spruce, mixed and beech for one site during a 1-year period, the authors proposed to raise similar questions again and to draw conclusions for a larger scale. Consequently, this complementary study is to our knowledge the only one where mixed species effects are tested in a fully replicated statistical design. We measured selected forest nutrition parameters (soil, foliage) and nutrient fluxes via throughfall and soil solution in adjacent stands of pure spruce, mixed spruce–beech and pure beech on three nutrient rich sites (including Kreisbach) and three nutrient poor sites (including Frauschereck; yielding a total of 18 stands) over a 2-year period. While Berger et al. (2009) put much emphasis on litterfall and forest floor dynamics this study focuses on a comparison of solute nutrient fluxes derived from suction lysimetry with accumulation of nutrients on buried resin bags. In additions, this large scale design enabled the performance of regression analyses to select the driving forces of nitrate and sulfate seepage fluxes. We addressed the impact of tree species composition by asking the following questions: (1) Does admixture of beech improve forest nutrition of spruce stands? (2) Does tree species composition (spruce vs. beech) affect atmospheric inputs and element outputs? (3) What are factors controlling leaching of nitrate and sulfate? (4) What are tree species related differences for acidification and loss of base cations deduced from element input–output balances in the aqueous phase? 2. Materials and methods 2.1. Study sites We selected six sites on the two different bedrocks Flysch and Molasse (3 comparable sites on each substrate). These sites are part of 18 sites documented in Berger et al. (2002) and originally represented two adjacent stands of pure spruce and mixed spruce– beech of at least 1 ha size. Beech and spruce were similarly mixed, before one stand at each site was converted into the current pure

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spruce stand. According to Rothe and Binkley (2001) research could take advantage of the spatial scale at which trees interact in the absences of replicated-plot experiments. Hence, for this study we selected mono specific beech stands (5–7 canopy dominant trees) within the mixed species stands. Individual trees influence soil properties primarily within the radius of the canopy (e.g., review by Rhoades (1997)). This is in accordance to Lovett et al. (2004), who measured N cycling characteristics in small singlespecies plots (2–3 canopy dominant trees) of five dominant tree species within mixed forests of the same species. The current design with 3 tree species compositions (spruce, mixed, beech) per site and 3 site replicates per bedrock (total of 18 stands) enabled testing mixed species effects. Within previous studies, Berger et al. (2009) already performed a similar monitoring program at the extensively studied sites Kreisbach and Frauschereck, which are very typical sites on Flysch and Molasse. Detailed site information for those sites is given by Berger et al. (2009) and forest stand characteristics of all 18 plots are listed in Table 1. Later on, throughout this paper, only means and statistics are given for the sites on Flysch (Kreisbach, Gru¨nburg, Schlierbach) and on Molasse (Frauschereck, Bradirn 1 and Bradirn 2). Hence, mean stand characteristics are given in Table 1 as well. Standing timber volume and dominant tree heights are higher on Flysch, despite a somewhat younger stand age. On average, the stands are located on N (Flysch) to W (Molasse) facing slopes. Precipitation is declining from the western (Molasse) to the eastern (Flysch) parts of Austria. 2.1.1. Study sites on Flysch The Flysch zone is a narrow strip in the foothills of the Northern Limestone Alps from west to east throughout the country. Hence, the study sites are spread throughout Lower and Upper Austria at elevations between 480 and 730 m (Table 1). Flysch consists mainly of old tertiary and mesozoic sandstones and clayey marls. Nutrient release from this bedrock is high and consequently the prevalent humus forms are mull (beech and mixed stands) to intermediate types between mull and moder (pure spruce stand), indicating quick turnover of the forest litter layer (usually less than 3 cm thickness). Soil parameters (Table 2) indicate nutrient rich soils. All soils of these study sites were classified as pseudogley (Scheffer and Schachtschabel, 1998; FAO classification: stagnic cambisol), since horizons with a high fraction of fine material (loam to clay) cause temporary waterlogging (stagnation zone at approximately 40–50 cm soil depth). The natural forest vegetation of the mixed stands on Flysch is Asperulo odoratae-Fagetum (Mucina et al., 1993). 2.1.2. Study sites on Molasse These study sites are located in Upper Austria, in a forested landscape, called Kobernausserwald, at elevations between 570 and 710 m (Table 1). Parent material for soil formation are tertiary sediments (so-called ‘‘Hausruck-Kobernausserwald’’ gravel), which consist mainly of quartz and other siliceous material (granite, gneiss, hornblende schist, pseudotachylite and colored sandstone). Because of this acidic bedrock with low rates of nutrient release, the dominant soil types are mainly semi-podzols (Scheffer and Schachtschabel, 1998; intermediate soil type between cambisol and podzol; FAO classification: dystric cambisol) and partly podzols. Humus form is acidic moder and the thickness of the forest litter layer varies between 5 and 10 cm, indicating slow turnover and accumulation of nutrients. In general, soils on Molasse contain more organic carbon and are more acidic, more sandy and less supplied with nutrients than soils on Flysch (Table 2). The natural forest vegetation of the mixed stands is Luzulo nemorosae-Fagetum (Mucina et al., 1993).

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Table 1 Forest stand characteristics of adjacent pure and mixed species stands at the experimental sites on Flysch and on Molasse according to a 1997 survey. Mono specific beech stands (5–7 canopy dominant trees) were selected within the mixed species stands. Hence, ha-related stand characteristics are the same for the mixed and the pure beech stand, except for Kreisbach, where the pure beech stand was large enough. Site

Age (years)

Stems (N ha1)

Timber volume (m3 ha1)

Basal area (m2 ha1)

Dominant tree height (m)

Elevation (m a.s.l.)

Slope (8)

Aspect (8, through E from N)

Mean (1971–200) precipitation (mm)

Coordinates N

E

850 850 850

48805’5000 48805’5000 48805’5000

15839’4600 15839’4900 15839’5400

337.5 337.5 337.5

1100 1100 1100

47856’3600 47856’2100 47856’2100

14813’2000 14812’4300 14812’4400

16 12 12

0.0 0.0 0.0

1180 1180 1180

47856’2500 47856’2500 47856’2500

14811’1200 14810’4600 14810’4800

590 587 587

14 13 13

352.5 352.5 352.5

1043 1043 1043

47859’3700 47859’3200 47859’3200

14841’2600 14841’0600 14841’0900

22.0 29.5 28.0

710 700 690

8 7 7

292.5 292.5 315.0

1180 1180 1180

48805’2700 48805’3300 48805’3500

13818’3600 13818’3900 13818’3600

Flysch Kreisbach Spruce Mixed Beech

53 65 65

1012 976 960

567 487 588

57 44 47

27.0 27.5 28.0

480 480 480

11 11 11

0.0 0.0 0.0

Gru¨nburg Spruce Mixed Beech

80 86 86

664 413 413

717 730 730

70 46 46

36.0 36.0 38.0

560 600 600

16 15 15

Schlierbach Spruce Mixed Beech

71 76 76

574 328 328

614 679 679

67 42 42

35.0 39.5 39.0

730 680 680

Mean Spruce Mixed Beech

68 76 76

750 572 567

633 632 666

65 44 45

32.7 34.3 35.0

Molasse Frauschereck Spruce 58 Mixed 89 Beech 89

1264 414 414

432 384 384

51 42 42

Bradirn 1 Spruce Mixed Beech

82 77 77

658 457 457

415 434 434

45 45 45

28.0 31.0 30.0

570 610 610

14 11 11

247.5 292.5 292.5

1180 1180 1180

48805’1400 48805’1700 48805’1800

13814’0800 13814’1400 13814’1400

Bradirn 2 Spruce Mixed Beech

96 108 108

486 301 301

442 245 245

52 25 25

33.0 28.5 29.0

640 640 640

5 18 15

247.5 180.0 247.5

1180 1180 1180

48805’1000 48805’1000 48805’1100

13816’2700 13816’3400 13816’4200

79 91 91

803 391 391

430 354 354

49 37 37

27.7 29.7 29.0

640 650 647

9 12 11

262.5 255.0 285.0

1180 1180 1180

48805’1700 48805’2000 48805’2100

13816’2400 13816’2900 13816’5100

Mean Spruce Mixed Beech

2.2. Sampling and analytical methods 2.2.1. Soils The forest floor (O-horizon) and mineral soil cores were taken with a core sampler of 70 mm diameter to a depth of 50 cm. There were three distributed replicate samples at each stand, which were pooled before analysis, divided into the following horizons: Ohorizon, 0–10, 10–20, 20–30, 30–40, 40–50 cm. The mineral soil within each volumetric horizon was separated into the fine soil (sieving, <2 mm) and the coarse fraction (>2 mm). Roots were sorted by hand. Soil chemical parameters were determined by routine procedures as suggested by Blum et al. (1989) for the standardization of Austrian soil surveys. Samples of forest floor and mineral soil were analyzed for total content of C (LECO SC 444, USA), N (Kjeldahl ¨ NORM L1082; 2300 Kjeltec Analyzer Unit, method according to O Tecator, Sweden), P and S (both after digestion with HNO3/HClO4 ¨ NORM L1085; ICPS, inductive coupled plasma according to O spectrometry, Optima 3000 XL, PerkinElmer, USA). Organic carbon (referred to as carbon (C) throughout the remainder of this paper) was calculated as total carbon minus CCaCO3 (Scheibler method: reaction of carbonates with HCl and volumetric determination of ¨ NORM L1084). Calcium, Mg, K, Na, emerging CO2 according to O Mn, Fe, Al were measured as total contents after digestion with HNO3/HClO4 in the forest floor and as exchangeable cations (0.1 M BaCl2 extract) in the mineral soil by ICPS. Soil acidity was measured

as pH with a glass Ag/AgCl combination electrode with KCl reference electrode (10 g of soil was mixed with 25 ml of 0.01 M CaCl2 or deionized H2O, stirred, and the pH was measured next morning 30 min after stirring again). Elemental stocks were then calculated as the product of dry (105 8C) fine soil masses (related to area and soil depth) and corresponding element contents. 2.2.2. Foliage Foliage of beech and spruce was collected at the six mixed spruce–beech stands. In early September, leaf samples of beech were collected with a shot gun from the upper crown of three trees per site (3 subsamples per beech tree were pooled for chemical analyses). In late October, one big branch of the seventh whorl was cut off from the top of each of three spruce trees per site. Chemical analyses of N, P, S, Ca, Mg, K, Na, Fe and Mn were performed as described for forest floor samples above, separately for fresh (1year-old) and old needles (older than 1 year). Measured dry masses of fresh and old needles per branch enabled calculations of weighed mean foliar concentrations per tree. 2.2.3. Solute samples Soil solution, bulk precipitation and throughfall were monitored for this study over a 2-year period from 1 November 2005 to 31 October 2007. Samples were collected every 3 weeks. Solute samples were filtered (0.2 mm) and stored in clean, high density polyethylene bottles at 4 8C until analysis. pH was determined with

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Table 2 Mean soil properties of the forest floor, 0–10 cm mineral soil, top soil (forest floor + 0–10 cm mineral soil) and the total soil profile (down to 50 cm) under the pure and mixed stands of spruce and beech on the bedrocks Flysch and Molasse: total stores of Corg (kg m2 per horizon) and N, P and S (g m2 per horizon); stores of Ca, Mg, K, Na, Al, Fe and Mn, given as total content in the forest floor and exchangeable content in the mineral soil (g m2 per horizon); cation exchange capacity (CEC), sum of base cations (Ca, Mg, K, Na) and sum of acid cations (Al, Fe, Mn, H+) in molc m2 per horizon; base saturation (%); C/N ratio. Site Flysch Forest floor

Species

C

Spruce Mixed Beech

1.0 0.7 0.4

0–10 cm

Spruce Mixed Beech

Top soil

Total soil

N

P

S

Ca

Mg

39 29 12

2.8 1.4 0.7

5.1 3.1 1.3

17.0 17.6 16.8

23.0b 10.5ab 4.3a

2.7b 2.3ab 2.0a

170 120 123

24.2 22.2 24.8

26.0 22.1 22.3

29.7 41.8 55.5

2.8 3.4 4.9

Spruce Mixed Beech

3.6 2.9 2.4

208 148 135

27.0 23.6 25.5

31.0 25.2 23.6

46.7 59.4 72.3

25.8b 14.0ab 9.2a

Spruce Mixed Beech

7.7 6.8 6.1

535 525 469

61.6 59.7 57.1

81.0 74.9 73.7

208.3 434.9 607.6

Spruce Mixed Beech

5.2b 4.7b 3.7a

206 187 172

9.2 8.8 7.8

26.7 23.4 20.5

0–10 cm

Spruce Mixed Beech

3.7 3.2 4.6

161 148 184

21.9 22.5 32.7

Top soil

Spruce Mixed Beech

8.9 7.9 8.4

366 335 356

Total soil

Spruce Mixed Beech

15.6 13.9 14.2

Molasse Forest floor

Factor bedrock (total soil) Flysch All 6.9*** Molasse All 14.5

K

Na

Fe

Mn

CEC

Base cat.

Acid cat.

Base sat.

C/N ratio

16.3a 25.6ab 32.7b

25.4 30.7 35.5

2.0 1.3 0.4

118.5b 57.2ab 18.3a

98.9b 43.4ab 13.3a

5.7 2.4 1.6

22.3b 10.9ab 4.2a

3.6 2.2 1.4

18.7b 8.8ab 2.8a

12.9 11.8 18.9

7.2 10.3 9.6

58.5 51.0 33.3

0.0 0.0 0.0

2.2 1.3 1.0

9.0 8.9 7.8

2.4 3.1 4.1

6.6b 5.7ab 3.7a

23.7 34.1 50.2

17.0 19.2 19.9

43.4 26.0 24.6

9.3 11.6 10.0

177.1b 108.2ab 51.6a

98.9b 43.4ab 13.3a

7.9 3.7 2.6

31.3b 19.8ab 12.0a

6.0 5.3 5.4

25.3b 14.5ab 6.6a

19.3 32.1 44.7

18.4 20.3 20.6

52.4 43.3 67.9

104.6 110.8 110.5

26.0 24.8 35.3

339.4 233.8 124.0

99.6 43.7 13.8

24.8 23.7 19.1

62.6 58.5 55.5

18.5 29.2 40.3

44.1 29.3 15.3

30.0 51.8 69.7

14.7 13.2 13.1

35.7 36.0 33.7

26.2 32.8 29.4

34.0 47.4 42.4

6.5 6.4 5.0

245.7 300.0 235.1

239.4 245.4 187.7

3.2 3.6 3.3

45.4 52.7 41.8

5.1 6.0 5.4

40.3 46.7 36.4

11.3 11.7 13.2

25.3b 25.2b 21.6a

28.6 26.6 41.1

1.4 0.7 1.7

0.5 0.6 1.0

4.9 5.7 9.8

4.6 4.2 6.5

40.5 43.7 58.8

0.0 0.2 0.1

0.0 0.1 0.0

5.0 5.3 7.3

0.4 0.4 0.7

4.5 4.9 6.6

9.1 8.1 9.6

23.3 23.2 24.4

31.1 31.3 40.5

55.3 50.0 61.5

37.1 36.7 35.4

26.7 33.4 30.4

38.8 53.1 52.2

11.1 10.6 11.6

286.2 343.7 293.9

239.4 245.6 187.8

3.2 3.6 3.4

50.4 58.0 49.0

5.5 6.4 6.1

44.9 51.6 42.9

11.1 11.2 12.6

24.4 23.8 23.2

672 627 686

93.3 83.4 92.6

139.7 116.7 118.2

40.9 39.3 38.3

28.5 34.7 31.7

48.4 61.5 62.0

18.6 15.7 16.1

378.8 416.5 360.7

251.3 249.0 189.9

4.8 5.3 5.1

62.3 67.0 57.4

6.4 7.1 6.8

55.9 60.0 50.6

10.3 10.7 12.0

23.3 22.3 20.7

510** 662

59.5** 89.8

76.5** 124.8

28.7** 16.8

232.4** 385.3

52.4*** 230.1

29.5** 55.5

50.5*** 11.0

13.7*** 22.1

416.9** 39.5

54.5(*) 31.6

30.5b 14.2ab 5.7a

Al

108.6*** 57.3

22.5** 5.1

58.9 ns 62.2

29.3*** 6.8

A one-way ANOVA (factor species composition) was performed for each bedrock and horizon separately and results of a Duncan multiple range test are given only, if differences were significant (different letters indicate significant differences, p < 0.05; a represents the lowest mean). Another one-way ANOVA (factor bedrock) was done to test mean differences between Flysch and Molasse for the total soil profile; level of significance is shown as: ns: not significant, p > 0.10; (*)p < 0.10; *p < 0.05; **p < 0.01; *** p < 0.001.

a glass Ag/AgCl combination electrode with KCl reference electrode, ammonium by FIA (flow injection analysis, Tecator, Sweden), metal cations by ICPS (ICP-OES Optima 3000, PerkinElmer, USA) and anions by ion chromatography (Dionex DX 500, USA). Total dissolved organic carbon (DOC) was analyzed in the soil solution with a Shimadzu TOC-5050 Total Carbon Analyzer, Japan. 2.2.3.1. Soil solution. Tension lysimeters (ceramic cups) were installed at 10 and 50 cm depth in the mineral soil (3 replications per depth). The applied suction was 50 kPa and regular sampling was occasionally interrupted during dry periods or when the soil water was frozen. 2.2.3.2. Bulk precipitation and throughfall. Bulk precipitation (PD) was collected in open fields or clearings (open) adjacent (within 100 m) to the stands of each site, using one polyethylene funnel with a 200 mm upper diameter, placed 170 cm above ground on a wooden stick. Throughfall (TF) was collected with 3 similar funnel collectors per stand (100 cm above ground). The funnels were connected to polyethylene reservoirs by black norprene tubings, which were half buried in the soil to keep the solution at low temperature. The volume of throughfall and precipitation was measured in the field with a graduated cylinder. During winter (snow, frozen rain samples) one (open) and three (stand) polyethylene pails (245 mm upper diameter) were used. Funnels and pails were rinsed with deionized water after each sampling

event. Cleaned (rinsed with deionized water) polyester plugs were used in the funnels to minimize particulate inputs and replaced each time of collection with plastic gloves. All samples of the individual collectors at each stand were pooled before chemical analysis to give one sample per stand. Element fluxes were calculated according to measured solution volumes per area of the collector. 2.2.4. Resin bags Resin bags were used as indicators of nitrate and cation leaching. At each sampling event, three small pits per stand were dug with a shovel, each one for two adjacent bags below the forest floor and at 10 cm mineral soil depth. Thereafter, the blade of the shovel was inserted horizontally into the cleared, vertical upslope face. Each bag was squeezed between a firm double layered metallic mesh and could be easily pushed below the blade of the slightly lifted shovel. The metallic mesh, with the bag in between, was tied up with a string to a plastic stick in the ground. Then, the pits were backfilled. By using two different sets of bags (18 stands  3 replicates  2 depth = 108 bags per set) retraction of exposed bags and insertion of recharged bags was done within one working step. Exposure periods varied between 2.5 (winter), 2.0 (spring, autumn) and 1.5 (summer) months from 1 November 2005 to 31 October 2007. Bags were labeled by stand and depth to make sure that each bag was always buried at the same location.

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Each resin bag (10 cm diameter; 120 mm mesh) was filled with 6 g Dowex 1x8 (counter ion: Na+) and 6 g Amberlite IR 120 (counter ion: Cl). The retrieved bags were placed in plastic bags, stored in cooling boxes until arrival at the laboratory, cleaned with deionized water and extracted three-fold with 100 ml of 1 M NaCl solution (shaking for 30 min) and filtered. Before chemical analysis all three extracted solutions per stand and horizon were pooled. Accumulation of nitrate and cations on the resin bags was measured within a week in the laboratory. After neutralisation with 1.6N NaOH both NO3-N (as NO2-N after reduction with copper-sheathed granulated zinc) and NH4+-N (modified indophenol reaction) were determined colorimetrically according to Schinner et al. (1996). The metal cations were measured in the extracted solutions by ICPS. Thereafter, the resin bags were charged with 200 ml of 1.6N NaCl for 30 min while being shaken, floated 4 times in deionized water and air-dried. Right before the next use, the bags were rinsed with deionized water, placed in plastic bags until arrival and exposed in wet condition in the field. Doubtless, resin bags can be used as relative indicators of element leaching, expressed as g per bag (e.g., Lovett et al., 2004), however, we present the results as area related elemental resin bag fluxes (RB), since the bags of known size were buried horizontally. 2.3. Separation of throughfall into its contributions It is easy to measure element fluxes in throughfall but difficult to separate them according to their source. Nutrients in throughfall may result from (1) incident precipitation, passing through the canopy; (2) material deposited as particles, gases, or cloud droplets prior to the precipitation event, being washed off during the event, and; (3) exchange processes within the canopy (including foliage, woody parts, epiphytes, and microorganisms). Hence, according to Lovett and Lindberg (1984), net throughfall flux (NTF) can be defined as: NTF ¼ TF  PD ¼ DD þ CE

(1)

indicating that the difference between throughfall and precipitation deposition (PD) is equal to the sum of dry deposition (DD) and canopy exchange (CE). According to Eq. (1) we ignored stemflow flux which is often a minor percentage of throughfall flux. In fact, at Kreisbach, stemflow is negligible for spruce (Jost et al., 2004) and amounted 5% and 8% of throughfall (mm year1) for the mixed and beech stand, respectively (Berger et al., 2008). The same percentage (5%) was measured for the mixed stand at Frauschereck (Berger et al., 2009). Canopy exchange includes both leaching (efflux from the canopy) and uptake or retention (influx to the canopy). Because neither dry deposition nor canopy exchange is easily quantified, it is difficult to separate their respective contributions to total chemical deposition (TD) in throughfall. Distinguishing between the two is important, as dry deposition represents an input to the ecosystem, while canopy exchange is an intrasystem transfer. The estimate of TD is based on the filtering approach model of Ulrich (1983), with some modifications: DD was calculated as NTF of sulfate instead of sodium assuming that CEsulfate equals zero. Although this method is rather old, it is still a common way to solve this notoriously difficult problem of separating throughfall fluxes into its contributions as documented in the recent literature (e.g., Rothe et al., 2002a; De Schrijver et al., 2004, 2007; Zeng et al., 2005; Berger et al., 2008, 2009). Ratios between deposition rates of sodium in the open field and in the stand are not useful to estimate filtering by the canopy in a situation where this element is scarce (far away from the sea), since DD is underestimated, partly indicating absorption or retention processes in the canopy (CE not equal 0; for details see Berger et al., 2008).

2.4. Seepage fluxes through the soil profile Solute nutrient fluxes through the soil profile were calculated for each period between two sampling events as product of measured soil solution concentrations with corresponding modeled water fluxes for a given soil depth. These event-to-event seepage fluxes (SP) were summed up to yield annual fluxes. When tension lysimeters did not collect any sample for a specific event (during very dry periods or when the soil was frozen), a concentration was assumed by using the mean soil solution concentration of the previous and the following event. This estimate seems justified, since in these cases modeled water fluxes were very low, hardly affecting annual nutrient fluxes through the soil profile. Water fluxes through the soil were calculated on a daily basis with the hydrological model BROOK90 (Federer et al., 2003; Federer, 2004) using daily meteorological input parameters (precipitation, radiation, minimum temperature, maximum temperature, relative humidity and wind speed) from the following nearest weather stations, operated by ZAMG (Zentralanstalt fu¨r Meteorologie und Geodynamik, Vienna, Austria): LilienfeldTarschberg for study site Kreisbach (488010 4200 N, 158350 1600 E, 681 m a.s.l., tmin and tmax were corrected via monthly height gradients deduced from long term records of Lilienfeld-Tarschberg and weather station Rax at 1547 m a.s.l.), Kremsmu¨nster for study sites Gru¨nburg and Schlierbach (488030 1900 N, 148070 5600 E, 383 m a.s.l.; tmin and tmax were corrected for the corresponding elevations using long term records of Kremsmu¨nster and weather station Feuerkogel at 1618 m a.s.l.) and Wolfsegg for study sites Frauschereck, Bradirn 1 and Bradirn 2 (488060 1900 N, 138400 2000 E, 660 m a.s.l.). For all sites on Flysch, we used the same parameter files for the pure spruce, mixed and pure beech stands as applied by Berger et al. (2009) for Kreisbach. These authors used the comparison between modeled and measured soil water storage in several soil depths for calibration of the hydrologic model. Solely the location parameters were adjusted according to Table 1. The same authors calibrated the model for the pure spruce and mixed stand at Frauschereck as well. We linearly extrapolated the individual parameters for the pure beech stand from the spruce and the mixed stand, if no other data were available. Again, the same parameter files were used for all sites on Molasse according to the three species compositions and location parameters. Details are given in Berger et al. (2009), but it is worthy to point out that on Flysch, water flow is the sum of macropore infiltration, vertical matrix drainage and bypass flow. Bypass flow simulates pipe flow of new water, an important flow path within the loamy to clayey soil, which is characterized by frequent swelling (wet periods) and shrinking (dry periods). The BROOK90 model enables the exact characterization of each soil horizon, e.g., of the stagnation zone at 40–50 cm soil depth by specific setting of a number of different soil variables within the soil parameter file. On Molasse, soil texture (0–50 cm) is sandy to sandy loam, hence, bypass flow was turned off and the water flow used for estimation of nutrient fluxes, is the sum of macropore infiltration and vertical matrix drainage. 2.5. Acidification Proton load resulting from proton deposition, N and S transformations was calculated according to the following equation (Rothe et al., 2002a): Hþ ¼ Hþ in TD þ ðNH4 þ in  NH4 þ out Þ þ ðNO3  out  NO3  in Þ þ ðSO4 2 out  SO4 2 in Þ in mmolc Hþ m2 year1 :

(2)

T.W. Berger et al. / Forest Ecology and Management 258 (2009) 2578–2592

2.6. Statistics Statistical differences of soil solution concentrations, TF-, TD-, SP- and RB fluxes, input–output balances and proton loads were tested by analyses of variances (ANOVA), factor species composition nested within site effect, for each bedrock (and soil depth) separately (N = 3 species compositions  3 sites per bedrock  2 years = 18) and results of multiple pairwise comparisons (Bonferroni adjustment) are given. Nested terms (species composition within site) are useful for modeling the effect of a factor (species composition) whose values do not interact with the levels of another factor (site). These analyses were performed for annual values and not for, e.g., sampling periods, to be consistent with meta-analyses and reviews of the current literature, working with annual fluxes as well (see recommendations of De Schrijver et al., 2007). In most cases, an additional one-way ANOVA (factor bedrock) was performed to test differences between the two bedrocks Flysch and Molasse. Mean soil properties were tested by a simple one-way ANOVA (factor species composition) and results of a Duncan multiple range test are given due to a shortage of degrees of freedom for a nested design (N = 3 species compositions  3 sites per bedrock = 9). Stepwise regressions were performed to find the driving forces of nitrate and sulfate fluxes through the soil profile after pre-selecting possible independent variables via bivariate correlations (at each step, the independent variable not in the equation that has the smallest probability of F is entered, if that probability is sufficiently small; the method terminates when no more variables are eligible for inclusion or removal). All statistics were performed with the package SPSS 15.0 for Windows (Release 6 September 2006). 3. Results and discussion 3.1. Forest nutrition 3.1.1. Soils Nutrient stores and soil properties of the total soil profile (down to 50 cm depth, where deep lysimeters were installed) indicated significant differences between the soils on Flysch and Molasse for all listed parameters (Table 2) except Mg stores and cation exchange capacity. Hence, we called soils on Flysch nutrient rich (e.g., higher Ca- and K stores, sum of base cations and base saturation) and soils on Molasse nutrient poor (e.g., higher stores of C, P, S, Al, Fe and sum of acid cations). The mean C/N ratio on Flysch (14) was significantly lower (p < 0.001) than on Molasse (22), indicating a much higher nutrient turnover on Flysch. The fact that N stores were higher on Molasse (660 g m2) than on Flysch (510 g m2) can be attributed to a disproportional high accumulation of N in the forest floor. These signs of retarded decomposition were also expressed by significantly higher (p < 0.001) C stores on Molasse (15 kg m2) than on Flysch (7 kg m2). Table 2 gives soil parameters for the forest floor (corresponding depth of first layer of exposed resin bags), the mineral soil horizon between 0 and 10 cm, and for the top soil (forest floor + 0–10 cm horizon) which has to be considered for interpretations of the second layer of resin bags and the soil solution collection at 10 cm depth. On Flysch, feedback effects from stand composition were significant in these top layers for C, Al, Fe, sum of acid cations and base saturation as indicated by the trend for the total soil profile. Mean pH (H2O) at 0–10 cm increased from 4.3 (spruce) over 4.7 (mixed) to 5.2 (beech; beech > spruce; mixed = spruce, beech; not shown in Table 2). As a consequence of retarded decomposition spruce sequestered more C, Mg and K (compare higher CEC) in the forest floor on Flysch. On Molasse, however, the C/N ratio in the forest floor declined from spruce to beech (beech < spruce, mixed) since C stores showed a similar pattern (beech < spruce, mixed).

2583

Table 3 Mean element content (mg g1) of spruce and beech foliage at 6 mixed spruce– beech stands on the two bedrocks Flysch and Molasse. Spruce needles were calculated as dry mass weighed means of fresh and old needles. Site

N

P

S

Ca

Mg

K

Na

Fe

Mn

Flysch Spruce Beech Difference

13.57 23.82

1.21 1.28 ns

1.08 2.03

6.75 12.54

0.86 1.47

4.70 6.71

0.09 0.15

0.07 0.11

***

***

**

*

1.42 0.99 ns

0.82 1.06

4.34 6.61

0.09 0.27

0.06 0.12

*

***

*

***

ns ns

ns (*)

ns ns

Molasse Spruce Beech Difference

***

***

***

13.82 23.89

1.15 1.43

0.98 2.04

***

***

***

ns

*

***

ns

**

ns

***

*

Factor bedrock Spruce ns Beech ns

2.43 3.48 (*)

0.81 0.85 ns

ns ns

A one-way ANOVA (factor species composition) was performed to test differences between spruce and beech foliage for each bedrock separately (N = 3 stands  3 trees  2 species = 18; ns: not significant, p > 0.10; (*)p < 0.10; *p < 0.05; **p < 0.01; *** p < 0.001). The factor bedrock was tested by another one-way ANOVA within the spruce and beech trees, respectively.

Except these two parameters no significant feedback effects (and hardly any trends) from tree composition were visible. Mean pH (H2O) at 0–10 cm was the same for all species compositions (4.1). 3.1.2. Foliage Comparisons between beech and spruce foliage at the 6 mixed spruce–beech stands (Table 3) indicated significantly higher nutrient concentrations of beech foliage for all elements, except for Mn (both substrates) and P (Flysch), justifying the assumption that the quality of beech litter is better than that of spruce litter (see Section 1). Surprisingly, the lower nutrient content of soils on Molasses than on Flysch was reflected significantly only for Ca (both species) and Mg (beech). Although S storage was higher for soils on Molasse (see Table 2), uptake of S into spruce foliage was lower (p < 0.05), probably, because the major part of S is bound as organic S in the top soil in a non available form. Higher (p < 0.01) P concentrations of beech foliage on Molasse reflected higher P soil contents coupled with higher P availability at lower soil pH. 3.1.3. Soil solution Volume-weighted mean annual soil solution concentrations were calculated for each of the 2 study years. The volume weight was derived from modeled water fluxes and 2-year means are given in Table 4. Soil solution concentrations of H+ declined significantly and sharply at 10 cm from the spruce to the mixed stand (corresponding pHs for Flysch and Molasse: spruce: 4.9 and 4.1; mixed: 5.2 and 4.6), however the soil solution under the mixed stand did not always react as expected (non-linear effects) from the single-species stands. There was a significant decline of dissolved organic carbon in accordance with measured C stores of the soil (compare Table 4) from spruce to beech dominated stands at both depths on both substrates. In all these four cases, nitrate concentrations decreased from spruce – over mixed – to beech stands as well. Nitrate concentration levels were higher on Flysch (for levels of significance see Table 4: factor bedrock), caused by higher nitrification rates at higher soil pH. On Flysch, nitrate concentrations under spruce did not decline from 10 to 50 cm depth, maybe due to reduced plant uptake (unlike beech, spruce may exclude nitrate uptake; Rothe et al., 2002a,b). In contrast to nitrate, sulfate concentrations within the same species composition increased significantly in all cases from 10 to 50 cm soil depth (Table 4). Possible reasons are concentration effects (transpiration), relatively low uptake rates (Marschner, 1995), release of adsorbed SO4 from the soil exchanger (Reuss and

T.W. Berger et al. / Forest Ecology and Management 258 (2009) 2578–2592

2584

Table 4 Volume-weighted mean annual soil solution concentrations (calculated from modeled water fluxes) of total dissolved organic carbon (DOC; mg l1), of nitrate, sulfate, chloride, phosphate, calcium, magnesium, potassium, sodium, ammonium, aluminum, iron, manganese and H+ (mmolc l1) at 10 and 50 cm soil depth at the pure and mixed stands of spruce and beech on the bedrocks Flysch and Molasse from November 2005 to October 2007. Mn

H+

2.6c 1.2b 0.4a

6.4c 4.1b 0.8a

13.3b 6.7a 4.2a

10a 5a 6a

0.3a 0.2a 0.2a

0.8a 1.2a 0.4a

2.4b 0.4a 1.0ab

3.5a 3.4a 3.4a

101a 90a 72a

10.1a 5.8a 6.9a

1.0a 0.4a 0.2a

74.7b 26.0a 45.6ab

0.3a 0.3a 0.3a

88b 71ab 41a

1.0a 0.7a 0.6a

6.5b 1.6a 1.0a

17.3a 17.3a 10.9a

ns ns ns

ns

(*)

(*)

*

**

**

*

***

(*)

*

ns

ns

ns ns ns

**

ns ns ns

ns ns ns

**

Site

DOC

NO3

SO4

Cl

PO4

Ca

Flysch 10 cm Spruce Mixed Beech

12.9b 11.0b 4.0a

330b 225a 151a

98b 102b 56a

58a 47a 38a

0.3a 1.9a 2.9a

290b 249b 158a

95b 55a 54a

22c 13b 8a

54b 37a 29a

3.0a 1.8a 2.3a

79b 48a 28a

50 cm Spruce Mixed Beech

3.3b 1.9a 1.6a

335c 160b 68a

195b 172b 107a

99b 43a 41a

0.4a 1.2a 1.7a

598a 945a 753a

127b 91ab 65a

18c 10b 6a

130c 61b 36a

0.3a 0.3a 0.3a

Molasse 10 cm Spruce Mixed Beech

20.4b 12.6a 12.1a

94b 27a 8a

39b 39b 26a

53a 45a 41a

0.6a 0.7a 0.6a

8b 3a 4ab

31b 23b 18a

8b 9b 3a

26b 25b 16a

54b 8a 6a

123a 157b 103a

40a 33a 26a

0.5a 2.4a 0.2a

14b 7ab 5a

26a 22a 19a

10b 5a 3a

30a 30a 24a

(*)

*

*

ns ns

*

**

*

**

ns ns ns

*

***

ns ns ns

*

*

**

(*)

*

(*)

*

ns ns ns

(*)

ns ns ns

***

***

ns

(*)

*

*

(*)

(*)

ns

**

ns ns

50 cm Spruce Mixed Beech

3.3b 2.7ab 1.6a

Factor bedrock 10 cm Spruce ns Mixed ns ** Beech 50 cm Spruce Mixed Beech

ns ns ns

(*)

ns

ns ns

Mg

K

Na

(*)

NH4

Al

* *

Fe

** **

An ANOVA (factor species composition nested within site effect) was performed for each bedrock and soil depth separately (N = 3 species compositions  3 sites per bedrock  2 years = 18) and results of multiple pairwise comparisons (Bonferroni adjustment) are given: different letters indicate significant differences, p < 0.05; a represents the lowest mean. Another one-way ANOVA (factor bedrock) was done for each soil depth to test mean differences between Flysch and Molasse within the same species composition; level of significance is shown as: ns, not significant, p > 0.10; (*)p < 0.10; *p < 0.05; **p < 0.01; ***p < 0.001; N = 3 species compositions  2 bedrocks  2 years = 12.

Johnson, 1986) and mineralization of organic S (Likens et al., 2002), which have to be discussed later. Sulfate concentrations were significantly higher on Flysch than on Molasse at 10 cm depth for each tree composition, but at 50 cm depth the factor bedrock was not significant any more. It is very striking that sulfate concentrations under the mixed stand were never lower than under the spruce stand, at 50 cm depth on Molasse even significantly higher. As expected, the concentrations of the divalent base cations Ca and Mg were enriched on the nutrient rich soil on Flysch and declined from spruce – over mixed – to beech stands in most cases (Table 4). Monovalent K in the soil solution on Flysch seemed to be an excellent marker for the species composition (10 and 50 cm: spruce > mixed > beech). Aluminum was quantitatively the most important acid cation, mirroring the acidifying effect of spruce admixture and dominated the soil solution chemistry on Molasse. At relatively low soil solution concentrations in the top soil on Flysch, Fe and Mn pointed exactly to the studied species composition (spruce > mixed > beech). 3.2. Element fluxes 3.2.1. Atmospheric deposition Annual inorganic N TF fluxes (Table 5) were lowest under beech and not statistically different between the spruce and the mixed stands, except for NO3-N TF fluxes on Molasse, which were even highest for the mixed stands. For several elemental TF fluxes, the mixed stand fluxes were not in-between the pure species stand fluxes but yield the highest records (not always statistically proven). This non-additive response of the mixture, in this case,

enhanced (synergistic response) TF fluxes, would not be expected based on the single-species patterns. Although SO4-S TF fluxes under the mixed stands were statistical not different from those under pure spruce, the data were again higher. Maybe, the vertically structured mixed stands with slightly higher tree heights than in the adjacent pure spruce stands (see Table 1) were able to scavenge higher amounts of pollutants (higher DD). The highest 2-year means of inorganic N TF fluxes were 16.5 kg N ha1 (mixed stand, Molasse). These N fluxes were in the low to moderate range of values reported for Austrian spruce and beech forests (Glatzel et al., 1988) and at the lower end of N throughfall fluxes (18–33 kg ha1 year1) in adult stands of Norway spruce across Switzerland below 1500 m a.s.l. (Flu¨ckinger and Braun, 1998). The highest 2-year mean of SO4-S TF fluxes was 5.1 kg S ha1 (mixed stand, Flysch) and much lower than reported for other Austrian spruce forests in the 1980s (15–54 kg S ha1 year1) by Glatzel et al. (1988) and was undoubtedly due to lower inputs of SO4-S in recent years in atmospheric deposition. SO4-S bulk deposition in the open (PD) amounted between 4.3 (Molasse) and 4.6 kg ha1 year1 (Flysch), indicating that the study area is relatively remote from emission sources. Despite the assumptions, limitations and modifications discussed in Section 2, estimated total deposition (TD) fluxes of elements (Table 5) permit the following conclusions. Total deposition fluxes are much higher than TF fluxes for H+ (negative CE) but lower than TF fluxes for the base cations K, Ca and Mg (positive CE). In this study N TF fluxes were higher than N TD fluxes since both NH4 and NO3 were leached (positive CE), except under beech, where CE flux of NH4 was negative on both substrates. Due to the assumption that CE of S equals zero, SO4-S TD and SO4-S TF

T.W. Berger et al. / Forest Ecology and Management 258 (2009) 2578–2592

2585

Table 5 Two-year means (11/05–10/07) of measured element fluxes in precipitation (PD) and throughfall (TF) in g m2 year1 (except H+ in mg m2 year1) and precipitation amount (H2O, mm). Total deposition (TD) was estimated after Ulrich (1983) with some modification according to the given equations. Flux

H2O

H+

K

Na

Ca

Mg

Mn

NH4-N

NO3-N

SO4-S

Cl

PO4-P

1159** 1376

4.6 8.2

0.44 0.41

0.31 0.37

0.37* 0.27

0.12 0.09

0.01 0.01

0.40(*) 0.54

0.50 0.58

0.46 0.43

0.96 1.03

0.02 0.02

Flysch Spruce Mixed Beech

769a 832b 851b

3.4 3.2 3.1

1.33b 1.23b 1.05a

0.22 0.29 0.20

0.64b 0.58b 0.43a

0.23 0.23 0.18

0.05c 0.03b 0.01a

0.62b 0.58b 0.34a

0.84b 0.86b 0.55a

0.49ab 0.51b 0.41a

1.04 1.00 0.86

0.01 0.01 0.02

Molasse Spruce Mixed Beech

938a 1026b 1102c

6.8 6.9 6.6

1.31b 1.47c 1.08a

0.25 0.32 0.25

0.37 0.41 0.38

0.18 0.22 0.17

0.02 0.03 0.02

0.61b 0.71b 0.43a

0.80a 0.94b 0.68a

0.44 0.49 0.44

1.21 1.29 1.12

0.02 0.02 0.03

4.8 4.9(*) 3.9(*)

0.46 0.49 0.39

0.32 0.34 0.27

0.40* 0.41* 0.33*

0.13 0.13 0.10

0.01 0.01 0.01

0.44 0.46 0.36

0.54 0.57 0.45

0.49ab 0.51b 0.41a

0.97 1.01 0.80

0.02 0.03 0.02

8.1 9.1 8.0

0.41 0.47 0.42

0.37 0.41 0.37

0.27 0.31 0.28

0.09 0.10 0.09

0.01 0.01 0.01

0.54 0.61 0.56

0.58 0.66 0.60

0.44 0.49 0.44

1.02 1.14 0.99

0.02 0.02 0.02

Site

Measured fluxes PD Open Flysch Molasse TF

Estimated fluxes TD Flysch Spruce Mixed Beech Molasse Spruce Mixed Beech

A one-way ANOVA (factor bedrock) was done to test differences of PD fluxes (open fields) and TD fluxes (within the same species composition) between Flysch and Molasse ((*)p < 0.10; *p < 0.05; **p < 0.01; not significant cases are not labeled). Different TF and TD fluxes were tested by another ANOVA (factor species composition nested within site effect) for each bedrock separately (N = 3 species compositions  3 sites per bedrock  2 years = 18) and only significant results of multiple pairwise comparisons (Bonferroni adjustment) are given (different letters indicate significant differences, p < 0.05; a represents the lowest mean). PD: precipitation deposition, measured in the open. TF: throughfall, measured: TF = PD + DD + CE; DD: dry deposition: DDsulfur = TF  PD; DDx = (DD/PD)sulfur  PDx; x = each element except sulfur. CE: canopy exchange: CE = TF  PD  DD; foliar leaching (+) or foliar uptake (buffering) (); TD: total deposition: TD = PD + DD.

Table 6 Two-year means (11/05–10/07) of annual fluxes (SP) of water (H2O, mm), total dissolved organic carbon (DOC), nitrate, sulfate, chloride, phosphate, calcium, magnesium, potassium, sodium, ammonium, aluminum and manganese in g m2 year1 and H+ in mg m2 year1 through the soil profile at 10 and 50 cm mineral soil depth (suction lysimeters) at the adjacent stands of spruce, mixed spruce–beech and beech on the bedrocks Flysch and Molasse. H+

Site

H2O

DOC

NO3-N

SO4-S

Cl

PO4-P

Ca

Mg

K

Na

NH4-N

Al

Fe

Mn

Flysch 10 cm Spruce Mixed Beech

541a 719b 723b

6.95b 7.93b 2.92a

2.59a 2.23a 1.56a

0.87b 1.18c 0.65a

1.11a 1.22a 0.98a

0.002a 0.014a 0.022a

3.29ab 3.55b 2.31a

0.64b 0.48a 0.49a

0.48b 0.37b 0.23a

0.68a 0.62a 0.48a

0.0220a 0.0184a 0.0236a

0.373b 0.312ab 0.187a

0.0373c 0.0247b 0.0077a

0.097b 0.082b 0.016a

7.0b 4.8ab 3.1a

50 cm Spruce Mixed Beech

103a 316b 357c

0.34a 0.58b 0.58b

0.46ab 0.69b 0.32a

0.30a 0.86c 0.61b

0.38a 0.49a 0.52a

0.000a 0.004a 0.007a

1.22a 6.27b 5.32b

0.16a 0.37b 0.27ab

0.07a 0.13b 0.08a

0.32a 0.46b 0.30a

0.0004a 0.0012b 0.0014c

0.009a 0.014ab 0.019b

0.0008a 0.0017b 0.0025c

0.002a 0.012a 0.004a

0.2a 0.1a 0.4a

Molasse 10 cm Spruce Mixed Beech

308a 390b 409b

6.11b 4.81a 4.86a

0.41b 0.16a 0.05a

0.20a 0.26a 0.17a

0.53a 0.59a 0.54a

0.002a 0.003a 0.003a

0.04a 0.02a 0.03a

0.12a 0.12a 0.09a

0.10ab 0.14b 0.05a

0.17a 0.22b 0.15a

0.0127a 0.0157a 0.0166a

0.267a 0.311a 0.257a

0.0792a 0.0617a 0.0784a

0.010a 0.005a 0.002a

22.8a 9.9a 20.1a

50 cm Spruce Mixed Beech

284a 342a 360a

0.92a 0.91a 0.57a

0.21b 0.04a 0.03a

0.57a 0.84a 0.61a

0.35a 0.35a 0.30a

0.002a 0.011a 0.001a

0.08a 0.04a 0.03a

0.09a 0.09a 0.09a

0.11b 0.08ab 0.03a

0.18a 0.23a 0.20a

0.0011a 0.0013a 0.0014a

0.208b 0.185ab 0.128a

0.0076a 0.0058a 0.0052a

0.049b 0.013a 0.009a

ns ns

(*)

**

*

ns

*

*

(*)

**

*

***

*

(*)

**

**

(*)

**

(*)

*

***

ns

ns

**

*

*

*

ns ns ns

ns ns ns

***

**

ns

ns ns ns

*

(*)

ns ns

(*)

ns

ns ns ns

***

(*)

ns ns ns

(*)

ns ns

ns ns ns

***

ns ns

ns ns

Factor bedrock 10 cm *** Spruce *** Mixed *** Beech 50 cm Spruce Mixed Beech

ns

**

4.7ab 5.5b 3.6a

ns

(*)

*

*

*

ns

ns

***

*

(*)

***

**

(*)

***

*

ns

ns ns

ns **

**

An ANOVA (factor species composition nested within site effect) was performed for each bedrock and soil depth separately (N = 3 species compositions  3 sites per bedrock  2 years = 18) and results of multiple pairwise comparisons (Bonferroni adjustment) are given: different letters indicate significant differences, p < 0.05; a represents the lowest mean. Another one-way ANOVA (factor bedrock) was done for each soil depth to test mean differences between Flysch and Molasse within the same species composition; level of significance is shown as: ns, not significant, p > 0.10; (*)p < 0.10; *p < 0.05; **p < 0.01; ***p < 0.001; N = 3 species compositions  2 bedrocks  2 years = 12.

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fluxes are the same. It is important to note that TF fluxes are not a reliable measure of atmospheric deposition to forest ecosystems. The main result is that TD fluxes were statistically not different between species composition for both substrates (except: S TD flux under beech < S TD flux under spruce). A one-way ANOVA (factor bedrock) did not reveal any significant differences of TD fluxes between Flysch and Molasse (except for Ca). This is in accordance to the fact that elemental PD fluxes (open field) were not significantly (p > 0.05) different either (except of Ca). 3.2.2. Seepage fluxes Two-year means of annual SP fluxes of water and elements through the soil profile at 10 and 50 cm soil depth are given in Table 6. Since nutrient fluxes are the product of soil solution concentration and water fluxes, much emphasis has to be put on the modeled water fluxes. Due to the stagnation zone below 40 cm soil depth on Flysch, there was an abrupt decrease of water and consequently nutrient flow between the two planes of the installed lysimeters. Hence, focusing on output fluxes below 50 cm, assuming that there is no plant uptake below, is correct but biases comparisons between the two bedrocks in regard of nutrient turnover within the whole soil profile. Water fluxes at 10 cm mineral soil depth were smaller on Molasse, since a major part had already been transpired into the plants from the thick organic layer on top of the mineral soil, but further loss of water flow through the sandy soil down to 50 cm was small (spruce: 8%; mixed: 12%; beech 12%), yielding no significant differences of water seepage between the tree species. However, on Flysch, water flow below the thin organic layer started with higher amounts than on Molasse, but loss down to 50 cm was severe and dependent on tree

species (spruce: 81%; mixed: 56%; beech 51%) resulting in significantly different water seepage fluxes between species composition (spruce < mixed < beech). As a consequence, striking differences among tree species observed in soil solution concentrations (Table 4) became smaller (nitrate) or were turning round (sulfate, Ca, Mg) for the corresponding SP flux rates (Table 6), since water fluxes in the mixed and beech stands were higher than in spruce. Hence, soil solution concentrations are important for evaluating water quality but do not necessarily justify conclusions about nutrient losses from ecosystems. 3.2.3. Solute flux profiles Nutrient fluxes and their changes from input via precipitation deposition (PD) over throughfall and seepage (SP) through the soil at 10 cm depth and loss below 50 cm are plotted in Fig. 1 as 2-year means of three stands per species composition and bedrock. Increases of anion fluxes from PD to TF were small under the spruce and the mixed stands and negligible under beech due to very low dry deposition rates, indicating that all study sites are remote from emission sources. Nitrate was the dominant anion in PD and TF (all sites) and in the top soil on Flysch (all species) and under spruce on Molasse. On Flysch, nitrate fluxes under spruce (mixed, beech) increased from 60 (61, 39) in TF to 185 (159, 112) at 10 cm and declined sharply to 33 (49, 23) at 50 cm soil depth (data in mmolc m2 year1). Hence, the top soil above 10 cm (forest floor + 0–10 cm mineral soil) represented a huge nitrate source (SP 10 cm minus TF), which declined from spruce over the mixed to the beech stand. However, absolute nitrate losses below 50 cm were highest under the mixed stands. On Molasse, the top soil represented a nitrate sink for all species compositions (TF minus SP

Fig. 1. Mean solute flux profiles from precipitation deposition (PD) over throughfall (TF) through the soil at 10 and 50 cm mineral soil depth (suction lysimeters; SP) in mmolc m2 year1 at the pure and mixed stands of spruce and beech on the bedrocks Flysch and Molasse (2 years: November 2005 to October 2007; 3 stands per species composition and bedrock). The difference of the flux balance is plotted either as anion deficit (cations minus anions; Cat–An) or as cation deficit (anions minus cations; An–Cat).

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10 cm in mmolc m2 year1: spruce: 28, mixed: 56, beech: 45). Absolute nitrate losses below 50 cm were lower than on Flysch, declining from spruce (15) over the mixed (3) to the beech stand (2; data in mmolc m2 year1). On Flysch, the top soil was a source of sulfate (SP 10 cm minus TF: 15–41 mmolc m2 year1) and the corresponding fluxes decreased from 10 to 50 cm soil depth. Hence, in accordance with Likens et al. (2002) and Berger et al. (2009) we suggest that net mineralization of organic S is the major S source on Flysch. This seems reasonable, since at the high pH soils on Flysch sulfate adsorption (and consequently desorption) can be assumed to be negligible. Contrary to most biogeochemical models which consider only inorganic sulfate sorption and desorption, a considerable proportion of the atmospherically deposited sulfate is cycled through the organic S pool before being released to the soil solution (Alewell, 2001). On Molasse, the top soil was a sink of sulfate (TF minus SP 10 cm: 15–17 mmolc m2 year1) and sulfate fluxes increased from 10 to 50 cm soil depth. Hence, in accordance with Rothe et al. (2002a) and Berger et al. (2009) we postulate that desorption of historically deposited S is the major S source on Molasse, supported by the fact that the acidic B-horizon has the highest adsorption capacity. Due to declining nitrate fluxes (as discussed above) with increasing depth, fluxes of sulfate dominated over nitrate in all stands (except for spruce on Flysch) at 50 cm soil depth. In TF, the dominant anions nitrate, sulfate and chloride were balanced by ammonium and base cations. Ammonium was quickly nitrified, taken up by the plants or adsorbed on exchange sites in the top soil and did not play an important role in the ion balance of the soil solution. Calcium and, to a lesser extent, Mg were the main cations in seepage on Flysch, while Al and H+ were the dominant cations on Molasse. At 50 cm soil depth on Flysch, admixture of

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beech (mixed and beech) caused a mismatch of analyzed anions and cations, characterized by a huge anion deficit (mixed: 251; beech: 229; data in mmolc m2 year1). Organic anions and HCO3 are frequently assessed by the anion deficit of a solution. Since under the (mixed) beech stands on Molasse DOC SP fluxes were higher and did not cause an anion deficit (but a small analyzed cation deficit due to chelating Fe, Al and Mn), this anion deficit can be attributed to bicarbonate only. Mean annual H+ soil solution concentrations of 0.4 (mixed) and 1.0 (beech) mmolc l1 (see Table 4) correspond to pHs of 6.4 and 6.0, levels at which bicarbonate is the dominant mobile anion (Reuss and Johnson, 1986). Hence basification, coupled with higher soil and root respiration of deep rooting beech stands (formation of bicarbonate is a function of soil pH and CO2 partial pressure) increased natural leaching (i.e., without anthropogenic acid input) of base cations under the (mixed) beech stands on Flysch. This is in accordance with the fact that in the soil solution samples, e.g., Ca concentrations under beech on Flysch at 50 cm soil depth were highly correlated with the not analyzed anion deficit (presumably bicarbonate; r2 = 1.00, p < 0.001) but not with nitrate concentrations (r2: ns), while at 10 cm depth the corresponding relations were opposite (r2 between Ca and nitrate = 0.57, p < 0.001; r2 between Ca and anion deficit: ns). In all other cases the analyzed anions and cations were well balanced and small anion deficits in TF and SP 10 cm can be attributed to both organic anions and bicarbonate. 3.2.4. Resin bag fluxes Accumulation of elements on the ion exchange resin of approximately 2-monthly exposed bags is summed up and expressed as 2-year mean annual nutrient RB fluxes below the forest floor and at 10 cm mineral soil depth in Table 7. First, we

Table 7 Two-year means (11/05–10/07) of element fluxes (RB) in g m2 year1, estimated from 2-monthly exposed resin bags below the forest floor (0 cm mineral soil depth) and at 10 cm mineral soil depth at the pure and mixed stands of spruce and beech on the bedrocks Flysch and Molasse. Site

NO3-N

NH4-N

K

Flysch Forest floor Spruce Mixed Beech

1.09a 0.70a 0.51a

0.76b 0.66ab 0.38a

3.23a 3.00a 2.92a

10 cm Spruce Mixed Beech

1.17a 0.87a 0.75a

0.44b 0.24a 0.13a

MOLASSE Forest floor Spruce Mixed Beech

0.45b 0.30ab 0.16a

10 cm Spruce Mixed Beech Factor bedrock Forest floor Spruce Mixed Beech 10 cm Spruce Mixed Beech

Mg

Al

Fe

Mn

Si

9.46a 8.66a 9.51a

1.55a 1.37a 1.32a

0.46c 0.23b 0.11a

0.06c 0.03b 0.02a

0.66c 0.19b 0.09a

2.98a 3.03a 3.05a

2.47a 1.97a 2.28a

12.67a 12.61a 16.74b

2.00a 1.69a 2.05a

0.73a 0.44b 0.37b

0.04a 0.03a 0.03a

0.64c 0.34b 0.12a

3.04a 3.04a 3.05a

0.96b 0.64ab 0.35a

1.92a 1.86a 1.64a

1.54a 1.70a 2.04a

0.42a 0.53ab 0.62b

0.12a 0.14b 0.20c

0.14a 0.12a 0.17a

0.07a 0.09a 0.14a

3.68a 3.72a 3.71a

0.38b 0.24ab 0.15a

0.79b 0.32a 0.20a

1.39a 1.00a 1.49a

1.51a 1.36a 1.75a

0.46ab 0.45a 0.57b

0.32a 0.40a 0.33a

0.17a 0.17a 0.18a

0.07a 0.08a 0.12a

3.70a 3.80a 3.77a

ns

ns ns ns

**

(*)

*

***

ns

*

(*)

**

*

**

**

***

(*)

**

(*)

ns

*

ns

ns ns ns

ns ns ns

*

(*)

(*)

**

**

*

*

**

**

**

*

(*)

**

*

ns ns

*

ns

* **

ns * ***

Ca

ns ns ns

An ANOVA (factor species composition nested within site effect) was performed for each bedrock and soil horizon separately (N = 3 species compositions  3 sites per bedrock  2 years = 18) and results of multiple pairwise comparisons (Bonferroni adjustment) are given: different letters indicate significant differences, p < 0.05; a represents the lowest mean. Another one-way ANOVA (factor bedrock) was done for each soil horizon to test mean differences between Flysch and Molasse within the same species composition; level of significance is shown as: ns: not significant, p > 0.10; (*)p < 0.10; *p < 0.05; **p < 0.01; ***p < 0.001; N = 3 species compositions  2 bedrocks  2 years = 12.

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simply focus on RB flux rates as relative indicators, since multiplying the amount of accumulated ion per bag (g bag1) with the fixed factor 127.3 (10 cm diameter) yields g m2 period1. Resin bag fluxes of the base cations Ca, Mg and K reflected the different soil properties between Flysch and Molasse (factor bedrock) for all tree species. Nitrate fluxes below each species composition were higher on Flysch than on Molasse for both horizons (differences for spruce were not significant). Iron RB fluxes at 10 cm were elevated on Molasse and, once more (see above), pointed at relatively low flux rates in the top soil on Flysch exactly to the studied species composition. Again, Mn seemed to be an excellent marker of tree species composition (compare soil solution concentrations; Table 4) and bedrock. Ammonium and Si showed no significant bedrock impact at all. However, the clear trend of raised Si fluxes on Molasse, may be interpreted as increased, acid induced, weathering. There were clear effects of species composition within each horizon and bedrock for NO3-N and NH4-N fluxes, which declined from the spruce over the mixed to the beech stands, although not always significantly (results of multiple comparisons are given in Table 7). There was a trend that Ca and Mg RB fluxes increased under beech (significant increase: Ca: Flysch, 10 cm; Mg: Molasse, forest floor and 10 cm). Comparisons between the forest floor- and 10 cm RB fluxes indicated a sink of the top mineral soil for NH4 and K (not significant for NH4 on Molasse), since these cations were strongly retained on the soil exchanger or taken up by the plants. The 0– 10 cm horizon was a source for Ca and Mg on the nutrient rich soils on Flysch, but a sink on the nutrient poor soils on Molasse (significant only for Ca on Flysch). In accordance with the above interpretations of the solute flux profiles, NO3-N RB fluxes indicated that the 0–10 cm mineral soil (top soil without forest floor) represented a nitrate source (RB 10 cm minus RB Forest floor is positive) on Flysch but a nitrate sink (RB 10 cm minus RB Forest floor is negative) on Molasse for all species combinations. 3.2.5. Differences between estimated seepage and resin bag fluxes through the top soil Comparisons between seepage fluxes at 10 cm (SP 10 cm, Table 6) and resin bags fluxes at 10 cm (RB 10 cm, Table 7) revealed significantly higher RB fluxes for all cations (paired samples t-tests for each substrate separately: N = 3 species compositions 3 sites per bedrock  2 years = 18). While these enrichment factors were relatively small for Al and Fe (1.3–2.4), they were much higher for the base cations Ca, Mg and K (Flysch: 3.6–6.2; Molasse: 4.5–49.5). Ammonium RB fluxes were 12.6- (Flysch) and 28.8-fold (Molasse) higher. Nitrate RB 10 cm fluxes were roughly within the same range but lower than the corresponding SP fluxes (Flysch: p < 0.01; Molasse: ns). Hence, we assume that these discrepancies between estimated SP and RB fluxes are not caused by different water flux calculations, but by element specific retention mechanisms of the resin, additionally influenced by ambient soil (solution) concentrations. The mobile anion nitrate tends to go with the water and is underrepresented on the resin. For the cations, however, the resin is an almost endless sink, fixing them via unsaturated water flow which is not collected via suction lysimetry at much higher soil moisture tensions (<50 kPa). The use of the resin bags as relative indicator was proven by significant positive correlations (p < 0.001; N = 3 species compositions  3 sites  2 substrates  2 years = 36) between RB 10 cm and SP 10 cm fluxes for all elements except NH4-N and Al (r2: Ca: 0.78; Mg: 0.69; Mn: 0.65; K: 0.45; Fe: 0.37; NO3-N: 0.34). In addition, the fact that resin bag fluxes below the forest floor (RB Forest floor) and the corresponding RB 10 cm fluxes were highly (p < 0.001) correlated for each element support the applicability of this method (r2: NO3-N: 0.90; NH4-N: 0.59; K:

0.36; Ca: 0.88; Mg: 0.89; Al: 0.37; Fe: 0.45; Mn: 0.84; Si: 1.00). However, correlations between throughfall and RB Forest floor fluxes for the studied elements were not significant, except for Mn (r2: 0.42; p < 0.001). Paired samples t-tests yielded significantly higher RB Forest floor- than TF fluxes for all elements, except for NH4-N (both substrates) and NO3-N (Flysch). Hence, in accordance to the discussion above for the whole top soil (compare Fig. 1; above 10 cm mineral soil depth), solely the forest floor (above 0 cm mineral soil depth) released huge amounts of nutrients. We conclude that seepage flux calculations (lysimetry) are the most appropriate way to calculate solute nutrient fluxes and input–output budgets, and the applied resin bag method is doubtless very useful as relative indicator of nutrient leaching. Nevertheless, we hypothesize that the much higher absolute values for the cations are not necessarily wrong but simply indicate that a major nutrient flux fraction is not caught by the lysimeters but retained on the soil exchanger. Similar to the soil exchanger the ion exchange resin functions as a huge sink. E.g., mean base cations stores under beech on Flysch amounted 168 (Ca), 43 (Mg) and 57 (K) kg ha1 (Table 2) and the corresponding annual RB Forest floor fluxes were 95, 13 and 29 kg ha1 year1 (Table 7). Dividing the forest floor content by these fluxes yields average residence times in years (turnover; Tn) of 1.8, 3.3 and 2.0 and is slightly higher than Tn estimated for Kreisbach (1.7, 2.2 and 0.7) by Berger et al. (2009) for Ca, Mg and K, respectively. Higher turnover rates at Kreisbach are expected due to the lowest elevation within the Flysch sites (Table 1). Hence, we suggest that our RB fluxes rather represent decomposition and mineralization fluxes than seepage fluxes. 3.3. Input–output budgets Mean annual input–output element budgets for the pure and mixed stands of spruce and beech on Flysch and Molasse are plotted for each year of the study in Fig. 2, characterized by similar precipitation amounts (Flysch: 1148 vs. 1170 mm; Molasse: 1345 vs. 1407 mm). Total deposition is our estimate of the true input to the system (according to the equations given in Table 5), while calculated elemental seepage fluxes passing the plane of the 50 cm soil depth (SP 50 cm, compare Table 6) represent the output of the system. 3.3.1. Nitrogen balance As discussed above, nitrate output (SP 50 cm) decreased from spruce – over mixed – to beech stands on Molasse, which is in accordance with the literature (e.g., De Schrijver et al., 2007). However, nitrate losses at 50 cm on Flysch indicated the highest loss under the mixed stands. Our simplified explanation for this not expected synergistic pattern in the mixed spruce–beech stands is a very unfavorable combination of high nitrate solution concentrations (similar to pure spruce) coupled with high water fluxes (similar to pure beech). As a consequence, the mean nitrate balance was negative for the mixed stands on Flysch (1.2 kg N ha1 year1) but positive in all other cases. On average, stands on Molasse retained significantly (p < 0.01) more nitrate (net balance: spruce, 3.7 < beech, 5.7; mixed, 6.2) than stands on Flysch (spruce, 0.8 = mixed: 1.2 = beech, 1.3; all data in kg N ha1 year1). This finding agrees with numerous studies (e.g., Kristensen et al., 2004; Van der Salm et al., 2007), stating that N leaching is negatively correlated with the C/N ratio of the soil (compare Table 2; factor bedrock). Since ammonium losses were negligible, the 2-year mean total inorganic N balances (NH4-N + NO3-N) were always positive and higher than NO3-N balances (Flysch: 3.5–5.2 kg N ha1 year1; Molasse 9.1– 12.3 kg N ha1 year1). As discussed earlier, input–output budgets on Flysch obscure the nutrient turnover within the soil profile. The fact, that the top

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Fig. 2. Mean annual input–output element budgets for the pure and mixed stands of spruce and beech on the bedrocks Flysch and Molasse for the years 11/05–10/06 and 11/ 06–10/07. Standard errors were calculated for 3 stands per species composition. Total deposition (TD) is the estimate of the true input to the system (according to the equations given in Table 5), while calculated element fluxes passing the plane of the 50 cm soil depth (suction lysimeters; compare Table 6) represent the output of the system. Element inputs and outputs are given in g m2 year1, except for H+ which are expressed in mg m2 year1.

soil above 10 cm (forest floor + 0–10 cm mineral soil) represents a huge nitrate source (SP 10 cm minus TF), which is declining from spruce over the mixed to the beech stand (see Fig. 1), is not visible from these balances at all. 3.3.2. Sulfur balance Biogeochemistry of S seems to be a major controlling factor of the element input–output budgets, plotted in Fig. 2. In each year, at both substrates, small amounts of S were retained at the spruce stands (except in the first year on Molasse), while the S balance was always negative for the (mixed) beech stands. Mean 2-year net SO4-S balances decreased from spruce over beech to the mixed stands (Flysch: spruce, 1.9 > beech, 2.0 > mixed, 3.5; Molasse: spruce, 1.4 = beech, 1.7 = mixed, 3.4; all data in kg S ha1 year1).

Again, we do not get any information about these S sources within the soil profile from input–output balances, which were discussed earlier (see Section 3.2.3): (i) mobilization of pedogenic S (net mineralization of organic S compounds) in the top soil on Flysch, and (ii) desorption of historically deposited S below 10 cm soil depth on Molasse. Net outputs of S are documented in many other studies (see citations in Section 1), but we do not know of any study (except for Kreisbach and Frauschereck by Berger et al., 2009) reporting that S losses under mixed deciduous stands are higher than under coniferous stands. 3.4. Factors controlling N and S seepage Bivariate correlations between NO3-N and SO4-S seepage fluxes (g m2 year1), respectively, and elemental seepage (Table 6) and

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Table 8 Bivariate correlations between NO3-N and SO4-S seepage fluxes (g m2 year1), respectively, and elemental seepage (Table 6) and throughfall fluxes (Table 5) top soil and total soil parameters (Table 2) and elemental foliage contents (Table 3: data of spruce and beech were used for the pure stands and intermediate values for the mixed stands); only significant correlations are given in bold (p < 0.001), italic (p < 0.01) and normal (p < 0.05) letters. In a second step, these selected parameters were used to run stepwise regressions to select the driving forces (independent variables) of NO3-N and SO4-S seepage fluxes passing the plane of 10 and 50 cm soil depth; units are given in the captions of the cited tables; significance of adjusted coefficients of determination (R2) and partial regression coefficients: ns, not significant, p > 0.05; *p < 0.05; **p < 0.01; ***p < 0.001. Bivariate correlations

NO3-N seepage 10 cm Seepage 10 cm Throughfall Top soil Foliage NO3-N seepage 50 cm Seepage 10 cm Seepage 50 cm Throughfall Top soil Total soil Foliage SO4-S seepage 10 cm Seepage 10 cm Throughfall Top soil Foliage SO4-S seepage 50 cm Seepage 10 cm Seepage 50 cm Throughfall Top soil Total soil Foliage

Stepwise regressions R2

Independent variables

Coefficients

Ca, Mg, K, SO4-S, Mn, H2O –H, Mg –C, Al, Mn, –Acid cat., –CEC, –Fe, Ca, –N, –S Mn, Ca

0.49**

(Constant) Foliage Mn -Top soil C

1.255411 ns 1.252559* 0.000239*

NO3, Ca, Mg, K, SO4-S – – –N, –Al, –Acid cat., –K, –CEC, –C –C/N ratio, –Base sat., –C, –Acid cat., Base cat., –Al, Ca –

0.24*

(Constant) -Total soil C/N ratio

1.134881** 0.047176*

Mn, Ca, K, Mg, H2O, NO3-N, Cl, Na, –Fe –H, –Cl, Mg, Ca –C, –N, –Al, –Acid cat., –Fe, –CEC, –S, Ca, Base sat., Mn Mn, Ca

0.83***

(Constant) -Top soil C Foliage Mn -Throughfall H

0.824756*** 0.000058* 0.367434*** 0.062204*

– – – – – –

ns

(No model)

throughfall fluxes (Table 5), top soil and total soil parameters (Table 2), and elemental foliage contents (Table 3: data of spruce and beech were used for the pure stands and intermediate values for the mixed stands) are given in Table 8. Correlations were performed for all 18 sites (N = 18), since possible species combination are already expressed in these parameters. It is not surprising that the number of significant correlations is generally higher in the top than in the deep soil, since numerous processes which are not caught by these parameters occur during the passage from 10 to 50 cm soil depth. It is striking that virtually not a single element in TF was correlated with nitrate or sulfate seepage at 50 cm soil depth, which is contradictory to the literature (see citations in Section 1). However, at generally low N and S deposition rates at the study sites (Table 5), tree related differences of TF were minimal and can not explain variations of N and S in SP 50 cm fluxes. In a second step, these selected parameters were used to run stepwise regressions to select the driving forces (independent variables) of NO3-N and SO4-S seepage fluxes passing the plane of 10 and 50 cm soil depth (Table 8). Foliage Mn concentrations (positive coefficient) and C stores of the top soil (negative coefficient) explained nitrate (r2 = 0.49; p < 0.01) and sulfate (r2 = 0.83; p < 0.001, including H+ in TF, negative coefficient) seepage at 10 cm. Manganese was mentioned before as excellent marker of species combination (soil solution concentration, TF-, SP- and RB fluxes). A negative correlation with top soil C strengthens the role of mineralization of organic compounds as source of N (after nitrification) and S (as expressed by a negative correlation with H+ TF as well). All variables, correlating with nitrate seepage at 50 cm could be reduced by stepwise regression, taking inter-correlations into account, to one single parameter: C/ N ratio of the total soil profile (negative coefficient; p < 0.05). This is in accordance with many studies cited in the literature (see Section 1). However, no similar model exists to explain factors controlling sulfate seepage below 50 cm soil depth within this research.

3.5. Acidification and loss of base cations According to Eq. (2) the 2-year mean proton load was significantly (p = 0.054) higher on Flysch (mean of all species: 36 mmolc H+ m2 year1) than on Molasse (14 mmolc H+ m2 year1). This difference is mainly attributed to the fact that retention of nitrate is negligible on Flysch (2 mmolc H+ m2 year1; retention means consumption of H+) but high on Molasse (37 mmolc H+ m2 year1). On Flysch, each component of Eq. (2) was highest for the mixed stands, yielding significantly higher soil acidification under the mixed stands than under the corresponding pure stands: mixed (57) > beech (26), spruce (24); all data in mmolc H+ m2 year1. On Molasse, acidification declined from spruce – over mixed – to beech stands, mainly due to higher nitrate retention of the (mixed) beech stands: spruce (20) > mixed (14), beech (9); all data in mmolc H+ m2 year1. In general, these proton loads are within very low ranges. Rothe et al. (2002a) reported proton loads, estimated according to Eq. (2), from 80 to 800 mmolc H+ m2 year1 under spruce and from 10 to 300 mmolc H+ m2 year1 under beech. As a consequence, input– output balances (Fig. 2) indicated a net gain of Ca and Mg on Molasse (not affected by species composition). On Flysch, however, net losses of Ca and Mg were less a consequence of proton deposition, N and S transformation but mainly caused by accompanying leaching with naturally formed bicarbonate. Again, the highest losses were estimated for the (mixed) beech stands: net balance for Ca: spruce (8.2) > beech (49.9), mixed (58.5); for Mg: spruce (0.3) > beech (1.6), mixed (2.4); all data in kg ha1 year1. As discussed earlier, K was strongly retained within all studied forest ecosystems and net balances were not affected by species composition. 4. Conclusions Our data show that admixture of beech may improve forest nutrition of spruce in pure spruce stands depending on site

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conditions (question 1; see Section 1). It is concluded that tree species composition may affect atmospheric input, however effects are negligible in unpolluted areas. At generally low atmospheric deposition rates there was almost no linkage between nutrient inputs and outputs (question 2). The highest nitrate losses on Flysch and sulfate losses on both substrates below 50 cm were recorded under the mixed stands. An explanation for this not expected synergistic pattern in the mixed spruce–beech stands is a very unfavorable combination of high solution concentrations (similar to pure spruce) coupled with high water fluxes. Decomposition, mineralization and nitrification are the controlling factors of nitrate leaching on Flysch, while these processes are of minor importance on Molasse. Net mineralization of organic S compounds on Flysch and desorption of historically deposited S on Molasse are the main S sources, controlling sulfate leaching. Our data show that leaching of nitrate and sulfate through the soil is not simply a ‘‘wash through’’ but is mediated by a complex set of reactions within the plant–soil system (question 3). Acidification under the mixed stands was higher than under the corresponding pure stands on Flysch. On Molasse, acidification declined from spruce – over mixed – to beech stands, mainly due to higher nitrate retention of the (mixed) beech stands (question 4). In general, these conclusions agree with those drawn by Berger et al. (2009) from only two intensively studied sites. We conclude, that combining large scale studies (including replicated stands; e.g., this study) with detailed studies of processes and patterns on a plot scale (e.g., Berger et al., 2009) is a powerful tool for evaluating mixed species effects, since current knowledge is still limited. Acknowledgements This research was supported by the Austrian Science Fund (FWF, project number P18208; granted to T.W. Berger). We thank Eugenie Fink, Anita Gruber, Gerlinde Mistlberger, Brigitte Schraufsta¨dter, Monika Sieghardt and Karin Wriessnig for performing the chemical analyses at the laboratories. Klaus Dolschak helped us sampling and analyzing the soil samples. Thanks to Sophie Zechmeister-Boltenstern for launching the usage of resin bags within this study. We thank the forest owners (Lilienfeld Abbey, Kremsmu¨nster Abbey, Austrian Federal Forests) and family Karl Haiderer (open field at Kreisbach) for the possibility to perform this research on their properties. Torsten W. Berger thanks his family, especially his wife Pe´tra and his sons Ralf and Joachim for their support in the field during many extended weekends. Finally, we thank Jean-Paul Laclau and two anonymous reviewers for their critical comments for the improvement of this paper. References Alewell, C., 2001. Predicting reversibility of acidification: the European sulfur story. Water Air Soil Pollut. 130, 1271–1276. Augusto, L., Ranger, J., Binkley, D., Rothe, A., 2002. Impact of several common tree species of European temperate forests on soil fertility. Ann. For. Sci. 59, 233– 253. Berger, T.W., Neubauer, C., Glatzel, G., 2002. Factors controlling soil carbon and nitrogen stores in pure stands of Norway spruce (Picea abies) and mixed species stands in Austria. For. Ecol. Manage. 159, 3–14. Berger, T.W., Ko¨llensperger, G., Wimmer, R., 2004. Plant–soil feedback in spruce (Picea abies) and mixed spruce–beech (Fagus sylvatica) stands as indicated by dendrochemistry. Plant Soil 264, 69–83. Berger, T.W., Swoboda, S., Prohaska, T., Glatzel, G., 2006. The role of calcium uptake from deep soils for spruce (Picea abies) and beech (Fagus sylvatica). For. Ecol. Manage. 229, 234–246. Berger, T.W., Untersteiner, H., Schume, H., Jost, G., 2008. Throughfall fluxes in a secondary spruce (Picea abies), a beech (Fagus sylvatica) and a mixed spruce– beech stand. For. Ecol. Manage. 255, 605–618. Berger, T.W., Untersteiner, H., Toplitzer, M., Neubauer, J., 2009. Nutrient fluxes in pure and mixed stands of spruce (Picea abies) and beech (Fagus sylvatica). Plant Soil 322, 317–342. Binkley, D., Giardina, C., 1998. Why do tree species affect soils? The warp and woof of tree-soil interactions. Biogeochemistry 42, 89–106.

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