Soil solution chemistry and impact of forest thinning in mountain forests in the Bavarian Alps

Forest Ecology and Management 108 (1998) 231±238

Soil solution chemistry and impact of forest thinning in mountain forests in the Bavarian Alps Rupert Baeumler1,*, Wolfgang Zech Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany Received 20 June 1997; accepted 15 December 1997

Abstract The soil solution of two forested catchments in the Bavarian Alps was studied over a period of not quite four years with regard to the dynamics of solutes and the impact of forest thinning on the solution chemistry. Soil solution was obtained by porcelain tension cups from three depths (10, 30 and 50 cm). It is characterized by high concentrations of alkali and alkaline earth cations, and of silicate and bicarbonate. Solution chemistry in topsoil is in¯uenced by atmospheric input, but the deposited acidity is almost completely and rapidly buffered through cation exchange and silicate weathering. The solution chemistry is further controlled by biogenetic processes including the release of ions by mineralization, and the temporary ®xation of ions by biological immobilization and root uptake. Gaseous losses of N may occur due to a dominance of hydromorphic soils. Thinning (removal of 40% of the trees by thinning) caused strong changes of the solution chemistry. Except for silicate and sulfate the concentrations of all cations and anions increased mainly due to increased mineralization and the reduction of nutrient uptake by roots. Ammonium and nitrate showed the strongest effects. Solution chemistry returned to pre-event conditions one year after thinning due to the high buffer capacity of the soils. # 1998 Elsevier Science B.V. Keywords: Soil solution; Mountain forests; Forest thinning; Buffering processes; Mineralization

1. Introduction The chemical composition of the soil solution changes with time and space (Tokuchi et al., 1993) re¯ecting biological and chemical processes during transport and/or storage of the soil water. Soil solution chemistry is strongly in¯uenced by soil characteris*Corresponding author. Fax: ++49-8161-714466; e-mail: [email protected] 1 Present address: Department of Soil Science, Technical University of Munich, D-85350 Freising-Weihenstephan, Germany 0378-1127/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0378-1127(98)00238-2

tics, and by the total load of atmospheric input. The soil solution is intermediary between input and output of forest ecosystems. The aim of the present study is to characterize the element dynamics in the soil solution of mountain forest ecosystems of the Bavarian Alps near the lake Tegernsee, and the impact of disturbances simulated by thinning. Alpine and subalpine ecosystems are very sensitive to such in¯uences, and we must understand the multiple interactions within those mountain ecosystems after any kind of interference for future risk assessment or minimization.

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2. Materials and methods 2.1. Site description The two small S- to SSW-exposed catchments (WS) are located in extended mountain forests south of Marienstein in the Tegernsee Alps (about 50 km SW of Munich). The area of the catchments is 4.2 ha (WS1) and 6.3 ha (WS2). The average slope is 218 and 198, and the distance between them averages 350 m. The difference in altitude between the weirs and the ridges of both catchments is about 180 m. The ridge is at 1210 m a.s.l. (WS1) and 1220 m a.s.l. (WS2). A full description of the areas is given by Breitsameter (1996). During the monitoring period (1990±1993) the annual precipitation varied between 1700 mm (1992) and 2500 mm (1993). The long-time average is about 1800±2000 mm with a maximum from June to August and minimum in the autumn and winter. Mean annual temperatures are ‡58C and ÿ78C; mean wind directions are NW±W and SE (BaÈumler and Zech, 1997). Parent materials are Cretaceous sandstones and clayey sediments of different color and thickness interstrati®ed with solid to smooth carbonatic sediments. The bedrock strongly in¯uences topography and soils. The landscape is characterized by rounded mountain tops of 1200±1400 m a.s.l. The soils of the catchments are mainly strati®ed with redoximorphic and aquic conditions due to the clayey parent material, which make the subsoil almost completely watertight. Small areas near the ridge are characterized by acid brown-colored Inceptisols and Spodosols developed from mica-rich sandstone (BaÈumler et al., 1995). The two watersheds are covered by mixed mountain forests of 74% and 71% Picea abies, 15% and 14% Abies alba, and 11% and 14% Fagus sylvatica. 3. Experimental design Measurements were made over four years (1990± 1993) at the two watersheds WS1 and WS2. After the ®rst two years, 40% of the stem volume in WS1 was removed by thinning. Under undisturbed conditions

the degree of canopy protection was 0.9±1.0 over 94% of the total area. It was reduced to 0.5±0.7 at 79% of the area (Breitsameter, 1996). The stems were removed with a mobile crane to reduce soil damage and erosion. Residues remained at the soil surface. 4. Methods Porous porcelain tension cups were installed at three depths (10, 30, 50 cm below soil surface, nˆ12). Before installation the tension cups were conditioned with 10% HCl-solution for one week, and afterwards with soil solution from the aquic sites of the study areas for three months (Hughes and Reynolds, 1990). The ®eld samples of the ®rst month were rejected to allow equilibration. The tension was adjusted weekly according to the actual soil water tension. Soil solution samples were taken weekly. pH and conductivity were measured immediately after sampling. The solutions were stored at ÿ158C and analyzed every three months. Cations were measured by atomic absorption (Varian SpectrAA 400), anions and total organic carbon (TOC; UV-oxidation) were determined colorimetrically (Alpkem Rapid Flow Analyzer RFA-300). Organic anions were calculated according to Oliver et al. (1983). 5. Results and discussion 5.1. Soil solution chemistry pH increased with soil depth from 4.6 at 10 cm to 6.5 at 50 cm (Table 1). There were no seasonal variations in mean monthly pH, as it was found in highly acidi®ed ecosystems (Schaaf, 1992). This indicates a rapid buffering of deposited and internally produced acidity. pH decreased brie¯y in early spring (March and April) after the beginning of snowmelt, showing increased acidity accumulated in the snow cover during winter (Rascher et al., 1987; Arthur and Fahey, 1993). Except for Ca2‡; mean concentrations of alkali and alkaline earth cations decreased with increasing depth (Table 1). However, at 10 cm the mean concentration of Ca2‡ of 7.2 mg lÿ1 in WS2 was six times greater than the Ca2‡ concentration in throughfall

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Table 1 Volume weighted means (mg lÿ1) of pH, conductivity, ion concentrations, TOC, and acid neutralization capacity (ANC) in the soil solution of three depth of WS1 and WS2 before thinning (08/90±10/92) (standard errors are given in parentheses) WS1

pH [H‡] (meq lÿ1) Ca2‡ Mg2‡ K‡ Na‡ NH‡ 4 NOÿ 3 SO2ÿ 4 ÿ1 PO3ÿ 4 (mg l ) H4SiO4 Clÿ HCOÿ 3 TOC Conduct.(mS cmÿ1) ANC (meq lÿ1)  cations (meq lÿ1)  anions (meq lÿ1) Difference

WS2

10 cm (mg lÿ1)

30 cm

50 cm

10 cm (mg lÿ1)

30 cm

50 cm

4.6 24.5 5.4(4.2)a 0.8(0.5) 0.7(0.9)a 0.8(0.6)a 0.1(0.1) 7.1(8.2)a 8.6(3.8)a 16.5(40.9) 0.4(0.3) 1.6(1.0) 1.2(1.1) 21.0(12.7)a 56(53)a 67a 419 503 ÿ84

5.0 8.6a 5.6(3.0)a 0.7(0.3) 0.5(0.4)a 0.9(0.5)a 0.1(0.1) 4.6(5.6)a 7.5(3.9)a 14.7(60.6)a 2.7(1.9)a 1.6(1.5)a 2.9(3.2) 11.4(16.0)a 42(19)a 207a 360 413 ÿ53

6.5 0.3 13.4(11.6)a 0.9(0.6)a 0.4(0.4)a 1.0(0.7)a 0.05(0.2) 1.1(2.7)a 9.7(2.7)a 13.6(55.2) 9.7(4.3) 1.6(1.5)a 14.5(12.4)a 5.9(5.8)a 73(81)a 1018a 798 560 ‡238

4.6 24.1 7.2(7.1) 0.9(0.9) 2.9(3.7)b 0.6(0.6)b 0.1(0.1) 19.0(18.9)b 6.0(3.9)b 15.5(46.6) 0.4(0.5) 1.7(3.0) 1.1(2.0) 10.5(13.5)b 65(78)b 303b 562 563 ÿ1

5.7 1.9 7.9(10.9)b 0.7(1.0) 0.3(0.6)b 0.6(0.7)b 0.1(0.1) 15.5(27.1)b 5.2(5.2)b 25.3(146.7)b 4.5(2.9)b 1.1(2.8)b 2.4(3.3) 2.7(4.4)b 50(76)b 404b 481 452 ‡29

6.5 0.3 7.3(4.3) 0.4(0.4)b 0.2(0.2)b 0.5(0.6)b 0.05(0.1) 5.0(9.1)b 5.3(3.6)b 15.1(57.7) 8.4(1.3) 1.1(1.2)b 6.3(7.2)b 2.1(1.3)b 45(108)b 462b 423 346 ‡77

Values within the rows followed by different letters are significant differently at p<0.01 between WS1 and WS2 (Wilcoxon matched pairs test).

(1.2 mg lÿ1; BaÈumler and Zech, 1997). Ca2‡ was the dominant cation and together with exchange processes due to buffering of acidity and subsequent input by dry or wet deposition, Ca2‡ will be released by mineralization of organic matter and weathering of soil minerals. Mineral weathering is indicated by increasing concentrations of silicic acid with increasing soil depth (Table 1). Besides leaching the concentrations of K‡, Mg2‡ and Na‡ decreased with increasing depth presumably due to uptake by roots already from top soil. The concentrations of alkali and alkaline earth cations did not vary seasonally. Different weather conditions from year to year may minimize the effects of seasonality (Litaor, 1988; Kazda and Katzensteiner, 1993). This is indicated by maxima of Ca2‡, Mg2‡, and K‡ in summer 1992, the driest season of the monitoring period. The concentration of NH‡ 4 in the soil solution was negligible and just above the detection limit. Deposited NH‡ 4 will be taken up by the roots or oxidized to by microorganisms (Stams et al., 1991; Knoepp NOÿ 3 et al., 1993). This is indicated by increased concen-

trations of NOÿ 3 in the soil solution (Table 1). Mean concentrations of 19 mg lÿ1 in 10 cm depth, and of 15.5 mg lÿ1 in 30 cm depth of WS2 were three to four times greater than in throughfall of 4.6 mg lÿ1. Mineralization of organic N-compounds and the oxidation ÿ ÿ of NH‡ 4 to NO3 are the main reasons. NO3 varied seasonally with a minimum at the beginning of winter. The concentrations increased during spring, decreased in summer, and increased again in autumn. This is clearly different from results of highly acidi®ed ecosystems with maximum in summer (Rascher et al., 1987; Nodvin et al., 1988; Schaaf, 1992; Tokuchi et al., 1993). Lower concentrations in the summer are probably caused by a high N supply of trees (BaÈumler et al., 1995) and microorganisms. This may compensate an increased release of NOÿ 3 by mineralization of organic matter in the warmer summer months, as it was shown by Laudelot et al. (1984), Driscoll et al. (1987), Fahey and Yavitt (1988) and Cook et al. (1994). Another reason may be increased denitri®cation rates during summer (Bowden, 1986; Wiklander et al., 1991). Important controlling factors for denitri®-

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cation are given by aquic conditions in the soils at more than 80% of the total study area (Virzo de Santo, 1992; Hanson et al., 1994; Lockaby et al., 1994) together with a maximum of precipitation from June to August which was 40% of the total annual amount. The concentrations of chloride were low (1.12± 1.67 mg lÿ1; Table 1) representing the mean concentrations of the input by throughfall (0.65± 0.98 mg lÿ1). Sulfate increased by 2±6 mg lÿ1 compared to the throughfall chemistry (3.5 mg lÿ1), but in general the concentrations were low characterizing the low deposition rates of about 12 kg haÿ1 aÿ1 SO4-S (Table 1). There were no signi®cant differences between the three sampling depths. The concentrations decreased slightly during the monitoring period (1990±1993; slope between ÿ0.05 and ÿ0.08) probably due to a continuously decreasing atmospheric input during the last decade (Krebs and Moritz, 1995). Silicic acid and bicarbonate increased with increasing depth (Table 1) by buffering of acidity (Probst et al., 1992). Both species were not found in precipitation and throughfall (BaÈumler and Zech, 1997). The concentration of bicarbonate may additionally be controlled by microbial activity and to a limited extent by root exudates (Nonnen, 1980). Silicic acid varied seasonally in comparison to bicarbonate. During summer the mean concentrations were 2.6±2.7 mg lÿ1 higher than in winter (P<0.05). Mineral weathering is forced by increased mineralization rates and soil respiration due to higher temperatures during summer (Cleaves et al., 1970). However, the concentration of ions is additionally in¯uenced by the soil moisture content and the contact time between solution and soil matrix. Swistock et al. (1990) and Drever and Zobrist (1992) showed increased ion concentrations in soil solutions with increasing period of storage. In our study sites the soil moisture content was lowered by evapotranspiration during the growing season (Breitsameter, 1996), even though June, July and August are the most rainy months. This may additionally concentrate the soil solution. Total organic carbon (TOC) was low (Table 1). The mean concentrations decreased from 10±20 mg lÿ1 at 10 cm to about 2±6 mg lÿ1 in the subsoil due to microbial decay and sorption or precipitation reactions (Guggenberger, 1992; Nelson et al., 1993; Kaiser et al., 1997). There were no seasonal trends except for

dilution during the snowmelt in the spring. This may indicate problems with the used suction cups. Filtration of the soil solution and sorption of organic compounds at the surface of the porcelain cups can not be excluded. However, mean concentrations of TOC were 6.5 mg lÿ1 in the throughfall and 2.0 mg lÿ1 in the groundwater of headwater areas and receiving streams. The concentrations in the soil solution ®t well in between considering leaching and organic matter decay in the topsoil. We found a surplus of anions at 10 cm, and a surplus of cations at greater depth (Table 1). Similar results were shown by van Wesemael and Verstraten (1993). The differences represent the sum of errors by the calculation of means. In addition the concentration of organic anions may be overestimated (van Wesemael and Verstraten, 1993). In the subsoil, pH was above 5 and the ANC strongly above zero. Gaseous losses of CO2 can occur during sampling with tension cups causing an underestimation of HCOÿ 3 and an increase of pH (Reuss et al., 1987). This is con®rmed by an increase of ANC, pH and anion de®cit with increasing depth. Ca2‡ was 60%±85% of the total equivalent sum of 2ÿ cations. Corresponding anions were NOÿ 3 and SO4 . ÿ With increasing depth NO3 was replaced more and more by bicarbonate. All other cations and anions are of minor signi®cance. 6. Impact of thinning Despite of similar soils and parent material at the sampling sites of both watersheds differences occurred in the solution chemistry already before thinning (Table 1). The in¯uence of the atmospheric input caused an adaption of the solution chemistry in the topsoil between WS1 and WS2. However, even there we found signi®cant differences in the concen2ÿ trations of Ca2‡, K‡, Na‡, NOÿ 3 , SO4 , TOC, conductivity, and ANC (p<0.01). The differences increased with increasing depth except for NH‡ 4, which was close to detection limit anyhow. They may be caused by different mineralization rates at both sites in addition to differences in the period of water storage, inter¯ow, and exchangeable cations due to the natural heterogeneity of different sites (Sollins and McCorison, 1981; Jardine et al., 1989; Swistock et al., 1990).

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Nevertheless thinning caused distinct changes in the soil solution chemistry of WS1. Except for Si, Clÿ, the concentrations of all ions increased and SO2ÿ 4 directly after thinning in comparison with the preevent period and WS2 (Table 2). By contrast, high precipitation in the year after the thinning diluted the concentrations of most solutes in the control watershed WS2 by 30%±50% compared to the mean equivalent concentration during the pre-event period. Highest dilution rates occurred in the topsoil due to the direct in¯uence of throughfall (Litaor, 1988). However, we found no changes in the solution chemistry of WS2 (Fig. 1). In the disturbed watershed WS1 the equivalent concentration of ions decreased by only 7%±10% after thinning, despite the fact that the throughfall was 300 mm (17%) higher than in the control area due to the thinning of the canopy. Similar results were found by LundstroÈm (1993) and Dahlgren and Driscoll

(1994) after cutting of trees. Ceased or reduced nutrient uptake by the roots and increased mineralization compensated dilution in the disturbed area. The conductivity decreased by about 10±15 mS cmÿ1 in the control WS2 after thinning, whereas no changes or even an increase were found in WS1 (Tables 1 and 2). ÿ NH‡ 4 and NO3 increased strongest after thinning (Fig. 2). The concentrations of NH‡ 4 ®rst strongly increased in the topsoil directly after cutting and removal of the trees due to a reduction or cease of root uptake, and decreased again by nitri®cation and microbial immobilization of N indicating a rapid change or adaption of the microorganisms to the new conditions (Fig. 2). Similar pattern occurred after clear felling in the Hubbard Brook Experimental Forest (Dahlgren and Driscoll, 1994). Maximum concentration of NOÿ 3 , however, occurred in 10 and 30 cm, six months after the disturbance, but decreased to pre-event concentrations after one year similar to ÿ NH‡ 4 (Fig. 2). At 50 cm the concentrations of NO3 slightly but continuously increased indicating in®ltration of NOÿ 3 released by microbial decay and a gradual release from necrotizing roots of the cut trees. While the relative portion of NOÿ 3 increased from 20% of the equivalent sum of anions to 50%, all other

Fig. 1. Volume weighted equivalent means of cations and anions in the soil solutions of WS1 and WS2 at 10, 30, and 50 cm depth before and after thinning.

Fig. 2. Time series of the mean monthly concentrations of NH‡ 4 and NOÿ 3 in the soil solutions of WS1 and WS2 before and after thinning.

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Table 2 Volume weighted means (mg lÿ1) of pH, conductivity, ion concentrations, TOC, and acid neutralization capacity (ANC) in the soil solution of three depth of WS1 and WS2 after thinning (11/92±12/93) (standard errors are given in parentheses) WS1 (thinned)

pH [H‡] (meq lÿ1) Ca2‡ Mg2‡ K‡ Na‡ NH‡ 4 NOÿ 3 SO2ÿ 4 ÿ1 PO3ÿ 4 (mg l ) H4SiO4 Clÿ HCOÿ 3 TOC Conduct.(mS cmÿ1) ANC (meq lÿ1) Cations (meq lÿ1) Anions (meq lÿ1) Difference

WS2 (control)

10 cm (mg lÿ1)

30 cm

50 cm

10 cm (mg lÿ1)

30 cm

50 cm

4.4 36.3a 4.5(4.7)a 0.8(0.8)a 0.7(1.3)a 0.7(1.1)a 0.3(2.8)a 15.2(22.5)a 4.5(4.1)a 23.4(55.7) 0.2(0.2) 0.7(1.0) 0.9(1.6) 15.4(12.8)a 56(52) 84a 389 474 ÿ85

4.6 23.8a 4.0(2.9)a 0.7(0.7)a 0.4(0.7)a 0.8(0.8)a 0.01(0.04) 11.2(16.2) 4.1(2.8)a 15.8(31.4) 1.0(2.1)a 0.6(0.7)a 1.0(3.7)a 11.1(17.1)a 43(31)a 111a 324 376 ÿ52

6.0 1.0a 12.4(14.5)a 0.9(0.8)a 0.3(0.2)a 1.0(0.6)a 0.01(0.02) 5.8(14.9)a 4.5(1.8)a 16.2(36.6) 4.9(3.7)a 1.4(1.0)a 15.5(27.8)a 4.8(3.7)a 80(80) 1080a 747 523 ‡224

4.5 29.2b 3.0(2.1) 0.5(0.3)b 1.2(1.1)b 0.5(0.5)b 0.01(0.03)b 9.3(10.5)b 3.7(1.5)b 23.2(45.7) 0.2(0.1) 0.6(0.9) 0.9(1.3) 9.3(4.1)b 40(34)b 48b 272 321 ÿ49

5.4 4.0b 4.8(3.6)b 0.5(0.4)b 0.2(0.3)b 0.5(1.0)b 0.01(0.02) 10.7(14.5) 3.5(1.2)b 30.3(54.7) 3.2(0.8)b 0.5(0.6)b 1.4(2.0)b 2.7(1.0)b 40(36) 235b 12 305 ‡7

6.1 0.7b 4.8(1.6)b 0.3(0.2)b 0.1(0.2)b 0.6(1.3)b 0.00(0.03) 2.0(4.6)b 4.0(1.5)b 22.9(45.5) 5.8(0.4)b 0.5(0.8)b 4.6(4.0)b 1.9(1.2)b 34(14)b 328b 294 223 ÿ71

Values within the rows followed by different letters are significant differently at p<0.01 between WS1 and WS2 (Wilcoxon matched pairs test).

anions decreased after the disturbance in WS1 (Fig. 1). SO2ÿ 4 was reduced by about 15%. A higher stress of acidity due to higher mineralization and nitri®cation might have induced increased sorption of SO2ÿ 4 (Mitchel et al., 1989; Rustad et al., 1993), or increased NOÿ 3 concentrations simply reduced desorption of SO2ÿ 4 . All other cations and anions were minor affected (Fig. 1). The changes in the soil solution chemistry of WS1 returned to pre-event conditions about one year after thinning. Plants with a high N-supply started to grow in the lower shrub and ground layer in the second year after the disturbance (Blessing, 1996). Their high demand of nutrients might have caused an overall reduction of increased ion concentrations in the soil solution. 7. Conclusions The soils of the Tegernsee Alps south of Munich compensate the atmospheric input by processes of

buffering, i.e. cation exchange and silicate weathering despite minor topsoil acidi®cation. Thinning of mountain forests by taking out 40% of the trees caused strong but short-term changes in the chemical composition of the soil solution, especially of ammonium and nitrate. The disturbed area compensated the acute effects of the thinning within about 12±14 months. Acknowledgements The authors gratefully acknowledge the ®nancial support by the German Ministry of Education, Science and Technology (BMBF). We are indebted to the forestry of®ce at Kreuth and Bad Wiessee (Mr. P. ZoÈlch, Mr. W. Kuhn), the water supply and distribution of®ce at Rosenheim (Mr. W. Kraus), and to the members of the institute of landuse and nature conservation at the Ludwig-Maximilian-University of Munich (Prof. Dr. U. Ammer) for their manifold support during the ®eld work. We are very grateful to two unknown reviewers for their comments.

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