Influence of dolomite lime on leaching and storage of C, N and S in a Spodosol under Norway spruce (Picea abies (L.) Karst.)

Forest Ecology and Management 146 (2001) 55±73

In¯uence of dolomite lime on leaching and storage of C, N and S in a Spodosol under Norway spruce (Picea abies (L.) Karst.) S. Ingvar Nilssona,*, Stefan Anderssona, Inger Valeurb, Tryggve Perssonb, Johan Bergholmb, Anders WireÂnc a

Department of Soil Sciences, Swedish University of Agricultural Sciences, P.O. Box 7014, SE-750 07 Uppsala, Sweden b Department of Ecology and Environmental Research, Swedish University of Agricultural Sciences, P.O. Box 7072, SE-750 07 Uppsala, Sweden c Department of Computer Services, Swedish University of Agricultural Sciences, P.O. Box 7079, SE-750 07 Uppsala, Sweden Received 24 September 1999; received in revised form 5 April 2000; accepted 7 April 2000

Abstract Dolomite lime (8750 kg ha 1) was applied in a replicated ®eld experiment (nˆ4) in a Norway spruce stand in southern Sweden (568240 N, 138000 E). The soil was a Typic Haplorthod. Soil pH and effective base saturation increased signi®cantly in the forest ¯oor and mineral soil 10 years after the lime treatment. The estimated C mineralisation rates (heterotrophic CO2 respiration) in the forest ¯oor were 1000 and 1800 kg C ha 1 per year in the control and lime treatment, respectively. No signi®cant treatment effects were found on soil storage of C, although a decline in C storage was strongly indicated by signi®cant declines in the C/N and C/S ratios in the Oe‡Oa layer of the lime treatment (20.2 and 203, respectively) compared to the control (24.5 and 222). The S/N ratio was the same in the two treatments (0.11), indicating that N and S were mineralised in the same proportions in both treatments. Increasing concentrations of dissolved organic and inorganic C, N and S forms associated with the lime treatment were found at several soil depths. The most consistent effect of treatment, comprising both C, N and S, was found in the soil solution from the O layer (Oi‡Oe‡Oa) and at 15 cm depth in the mineral soil. The chemical composition of the dissolved organic matter changed with increasing soil depth. On a percentage basis, operationally de®ned hydrophilic compounds tended to increase, while hydrophobic compounds decreased signi®cantly, indicating a selective adsorption of hydrophobic compounds, particularly in the B horizon. Hydrophilic compounds tended to have a lower C/N ratio than hydrophobic compounds. This difference was statistically signi®cant at the 30 and 50 cm soil depth in the control treatment. Changes in the chemical composition related to the lime treatment could not be shown. Estimates of the adsorption in the B horizon of dissolved components from the forest ¯oor (annual water percolation 400 mm) showed that the adsorption of DOC and DON was higher in the lime treatment (202 kg C ha 1 per year and 7.6 kg N ha 1 per year) than in the control treatment (128 kg C ha 1 per year and 3.4 kg N ha 1 per year) which accounted for 83±90% of the DOC and DON leaching from the forest ¯oor. The adsorption of DOS was only 31% (control) and 14% (lime), corresponding to 0.8 and 0.6 kg S ha 1 per year, respectively. The DOC leaching from the B horizon (at 50 cm depth) was equivalent to ca. 1.2% of the estimated annual heterotrophic CO2 respiration, while the DOC adsorption was equivalent to 6±7% of the respiration. # 2001 Elsevier Science B.V. All rights reserved. Keywords: CO2 respiration; DOC; DON; DOS; Forest liming; Hydrophilic compounds; Hydrophobic compounds; N and S mineralisation; NH4‡; NO3 ; Norway spruce; SO42 ; Spodosol

* Corresponding author. Tel.:‡46-18-671276; fax: ‡46-18-672795. E-mail address: [email protected] (S. Ingvar Nilsson).

0378-1127/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 4 5 2 - 7

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S. Ingvar Nilsson et al. / Forest Ecology and Management 146 (2001) 55±73

1. Introduction Carbon losses from soils occur mainly as CO2 evolution, while leaching of dissolved organic carbon (DOC) from the soil is less important from a quantitative viewpoint. Guggenberger (1992) estimated that the annual heterotrophic CO2 respiration accounted for 1200 kg C ha 1 in stands of Norway spruce on podsols in southern Germany, while DOC leaching from the lower part of the B horizon accounted for only 27.5 kg C ha 1. Estimates of the C storage in Finnish podsolised soils showed that 70% of the total storage was con®ned to the upper 1 m of the mineral soil, while the remaining amount was found in the forest ¯oor (Liski and Westman, 1997). The organic matter in the mineral soil and its contents of N, P and S is likely to be of limited availability for the microorganisms compared to the organic matter in the forest ¯oor, because of the close association of the former organic matter pool with mineral particle surfaces (Sollins et al., 1996). Part of the mineral soil C has been translocated by leaching from the forest ¯oor (O horizon), and part has been formed in situ from decomposing roots (Zech et al., 1996). The translocated organic compounds differ in their mobility. Hydrophobic compounds (Leenheer, 1981), i.e. mainly high-molecular dissolved fulvic acids, have a strong adsorption capacity and a large fraction of these compounds is selectively adsorbed at a rather shallow soil depth, while more low-molecular, hydrophilic compounds (mainly hydrophilic acids) move to greater soil depths. As a result, the dissolved organic matter (DOM) which is leached through the soil is successively enriched with respect to hydrophilic compounds (Cronan and Aiken, 1985; Qualls and Haines, 1991; Guggenberger and Zech, 1993). Hydrophilic compounds have lower C/N and C/P ratios than hydrophobic compounds (Qualls and Haines, 1991). The same tendency holds for the C/S ratio (David et al., 1995). In relative terms, the leaching losses of organic N, P and S may therefore be greater than the leaching loss of organic C. 1.1. Effects of forest liming Forest liming has been suggested as a practical means for mitigating soil acidi®cation induced by acid deposition, particularly in the Nordic countries

and in Germany. However, large-scale forest liming is a rather controversial issue. Although measurable changes in soil chemistry such as increases in pH and base saturation and concomitant declines in exchangeable and soluble Al are well documented (HallbaÈcken and Popovic, 1985; Kreutzer et al., 1991; Kreutzer, 1995; Blette and Newton, 1996), positive effects on tree growth, tree vitality or soil biota have not been conclusively shown (HuÈttl and Schneider, 1998). There may be substantial losses of soil organic matter and nitrogen after forest liming. A greater net N mineralisation and nitri®cation at N rich sites, implying a high potential for NO3 leaching and eutrophication of adjoining surface waters, was shown by Persson and WireÂn (1995) and Persson et al. (1995). Long-term declines in soil C and N storage were documented for 40-year-old liming experiments where CaCO3 had been applied in amounts corresponding to 9000±10000 kg ha 1, indicating an increasing net release of CO2 to the atmosphere (Persson et al., 1995). Declines in C and N storage were also documented in a ®eld-liming experiment in a stand of Norway spruce on a Hapludalf in HoÈglwald in southern Germany, 7 years after an application of dolomite lime corresponding to 4000 kg ha 1 (Kreutzer, 1995). Because of the increase in pH, liming may increase the SO42 desorption and leaching in the mineral soil (cf. Eriksson and Karltun, 1995). There may also be a SO42 desorption due to an increased competition with organic anions as discussed by Kreutzer et al. (1991) and others. There is con¯icting evidence concerning liming effects on sulphur mineralisation both in a short-term and a long-term perspective. Marschner (1993) conducted laboratory incubations and found that the net sulphur mineralisation (SO42 formation) increased after liming in the A horizon of a Cambic Arenosol in a stand of Scots pine (Pinus sylvestris L.). WoÈlfelschneider (1994) conducted ®eld incubations of limed and unlimed forest ¯oor in a podsolised soil under Norway spruce (Picea abies Karst. [L.]) and obtained similar results. However, WoÈlfelschneider (1994) claimed that there was a reversed trend in net sulphur mineralisation, i.e. a lower net mineralisation after liming, when forest ¯oor material was incubated in the laboratory at 248C. A lower net sulphur mineralisation after liming was conclusively shown in a laboratory incubation of forest ¯oor material at 158C (Valeur and Nilsson, 1993).

S. Ingvar Nilsson et al. / Forest Ecology and Management 146 (2001) 55±73

Laboratory experiments have shown that the leaching of DOC, dissolved organic nitrogen (DON) and dissolved organic sulphur (DOS) from the forest ¯oor or mineral soil increases after liming (Hildebrand, 1990; Valeur and Nilsson, 1993; Andersson et al., 1994, 1999, 2000; Erich and Trusty, 1997). Changes in the chemical composition of dissolved organic matter, mainly an increase in the share of hydrophobic compounds, have been shown in some studies (GoÈttlein et al., 1991; Andersson et al., 2000). The increased ¯ux of DOC is partly attributed to an increase in the hydration of the organic molecules due to proton dissociation from carboxylic endgroups, and partly to an increase in the organic C mineralisation rate (Stevenson, 1982; Zech et al., 1996; Andersson et al., 2000). To the best of our knowledge there are no statistically replicated ®eld studies showing the temporal development in the mobility of organic and inorganic forms of C, N and S after forest soil liming. GoÈttlein and Pruscha (1991) collected soil samples from one limed plot (dolomite lime corresponding to 4000 kg ha 1) and one unlimed plot in a stand of Norway spruce in southern Germany. The samples were extracted with distilled water and DOC was measured in the ®ltered extracts. A liming effect (increasing concentration of DOC) was found in the forest ¯oor and in the upper 0±5 cm layer of the mineral soil. In the same experiment, soil solution collected below the forest ¯oor and at 40 cm depth in the mineral soil had elevated concentrations of DOC, DON and NO3 in the limed plot (Kreutzer, 1995). Geary and Driscoll (1996) collected soil solutions below the O and Bs horizons in limed plots (CaCO3 corresponding to 6890 kg ha 1) and unlimed plots in a watershed in the Adirondack Mountains, USA. During the monitoring period (1989±1992), liming had no visible effect on DOC leaching (Geary and Driscoll, 1996). The vertical penetration rate of lime broadcast on the soil surface is usually rather slow (HallbaÈcken and Popovic, 1985; Hildebrand, 1990). Therefore, ®eld monitoring of processes such as C, N and S mineralisation and leaching should preferably be extended for as long a time-period as possible after the lime application. Here, we report changes in (i) the soil storage of total C and N, (ii) C/N and C/S ratios, and (iii) the leaching dynamics of C, N and S forms, in a statistically replicated forest liming experiment in south-western Sweden established in 1984. The

57

specific aims of the study were:  To determine changes in the heterotrophic soil respiration (CO2 respiration) and in the storage of total C and total N.  To determine changes in the C/N and C/S ratios in the soil.  To determine to what extent the lime treatment had affected the leaching and adsorption of DOC, DON and DOS, and whether there were changes in the chemical composition of dissolved organic matter with respect to the percentage distribution of hydrophobic and hydrophilic compounds.  To determine to what extent the lime treatment had affected the leaching of inorganic N (NH4‡ and NO3 ) and inorganic S (SO42 ). Documented changes in acid/base chemistry (pH, effective base saturation and exchangeable Al) are provided as background information. 2. Materials and methods 2.1. Site description The forest liming experiment HassloÈv is situated in south-western Sweden (568240 N, 138000 E) on a ridge, 190 m above mean seawater level, close to the Kattegatt Strait. The soil is a Typic Haplorthod and the tree stand consists of Norway spruce (Picea abies [L.] Karst.), which was planted on former heath land. At the time of the dolomite lime application (in autumn 1984) the trees were 35 years old. Due to the presence of humi®ed organic matter in the uppermost 10±15 cm of the mineral soil, this soil layer was categorized as an A horizon rather than an E horizon. CaMg(CO3)2 (dolomite lime) was applied at three different rates: 1550, 3450 and 8750 kg ha 1. Each of the treatments (including the control treatment) was replicated in four blocks. In the present investigation only the control and highest dolomite lime treatment (called lime treatment) were studied. 2.2. Field sampling Ð Soil Soil samples were collected on two different occasions in 1994. On the ®rst occasion a soil inventory comprising the Oi (L) and Oe‡Oa (F‡H) layers and

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®ve mineral soil layers (0±5, 5±10, 10±20, 20±30 and 30±40 cm) was carried out. In each plot, 40 samples were collected from each of the Oi and Oe‡Oa layers, and 20 samples were collected from each of the mineral soil layers. The samples were bulked to one composite sample per plot and soil layer. The composite samples were brought to the laboratory and sieved (Oi and Oe‡Oa layers, 5 mm mesh size; mineral soil 2 mm mesh size). On the second occasion, samples from the Oi and Oe‡Oa layers were collected to measure net S mineralisation in a laboratory incubation study (Valeur et al., unpublished manuscript). In each plot, 10 samples were collected from each soil layer and these samples were bulked to one composite sample per plot and soil layer. Before the incubation the samples were characterised with respect to their total content of C, N and S. 2.3. Field sampling Ð Soil solution In 1993 zero-tension lysimeters (ZTLs), were installed immediately below the O horizon. The ZTLs were made of Perspex and furnished with a nylon net to ®lter coarse particulate material. Each ZTL was connected to a 1 l Erlenmeyer ¯ask. Porcelain suction cups (P80, KPM, Berlin) were installed in 1988 at the 15 and 30 cm soil depths, counted from the mineral soil surface. In 1994 suction cups were also installed at the 50 cm depth. Each suction cup was connected to a plastic container equipped with a silicon stopper, which protruded above the ground. Two plastic tubes penetrated the silicon stopper. One tube was connected to a vacuum pump and the other was used for collecting water samples. In each plot, there were three lysimeters (ZTLs or suction cups) per soil depth. All four replicate plots were used for the ZTLs and the suction cups at 50 cm, while three of the replicate plots were used for the suction cups at 15 and 30 cm. Soil solution samples were collected at irregular time intervals. One week before each sampling, an approximate suction of 0.7 bar was applied to the P80 cups and the Erlenmayer ¯asks connected to the ZTLs were carefully rinsed with distilled water. The collected samples were immediately transported to the laboratory (transport time <24 h). All samples were immediately ®ltered through a 0.45 mm Millipore ®lter (HA 0.45 HAWP04700). Each ®lter was rinsed beforehand with 50 ml deionised water to minimise the effect of

any DOM bleeding from the ®lters. The ®ltered samples were kept in a refrigerator until analysis. Soil solution samples from each lysimeter were analysed separately. Plot means for each soil depth and sampling date were used as input data for the statistical tests. The soil water sampling in the present study at the 15 and 30 cm mineral soil depths commenced on 23 November 1992, i.e. 8 years after the lime application. The ZTLs were ®rst sampled on 25 October 1993 (DOC) or on 6 December 1993 (the remaining variables). The suction cups at the 50 cm mineral soil depth were ®rst sampled on 23 November 1994. The sampling was ®nished on 21 November 1997 (suction cups) and on 1 December 1997 (ZTLs). 2.4. Chemical analyses Ð Soil On the ®rst sampling occasion, total C and N were analysed on vacuum-dried (658C) samples, using a Carlo±Erba analyser. On the second sampling occasion, total C, N and S were analysed on oven-dried (1058C) samples using a LECO analyser. The analytical values obtained were used as estimates of total organic C, N and S. pHH2 O and exchangeable cations were analysed on fresh soil samples. pHH2 O was measured electrometrically after equilibrating soil Ð distilled water slurries (volume ratio 1:2) overnight. Exchangeable cations and BSeff, i.e. base saturation at ambient soil pH, were determined by extracting soil samples (soil/extractant mass ratio 1:10 for the Oi and Oe‡Oa layers; 1:4 for the mineral soil) with (i) 1 M CH3COONH4 at pH 7, and (ii) unbuffered 1 M KCl. The base cations were determined in the CH3COONH4 extract, while Al and H‡ were determined in the unbuffered KCl extract. After ®ltration (OOR ®lter), base cations were analysed by ICP (Jobin Yvon JY-24) and Al and H‡ were determined by a two-step titration of the KCl extract, ®rst to pH 4.3 and then to pH 7 with 0.05 M NaOH. The concentrations of H‡ ions and Al were estimated from reference samples with known additions of HCl. 2.5. Heterotrophic CO2 respiration In November 1990, the Oi and Oe‡Oa layers and the 0±5, 5±10, 10±20 and 20±30 cm mineral soil layers were collected from control and lime plots. The Oi, Oe‡Oa and 0±5 cm layers were sampled using

S. Ingvar Nilsson et al. / Forest Ecology and Management 146 (2001) 55±73

quadratic frames and the remaining mineral soil layers with a soil corer. In each plot, samples from ®ve different spots were bulked to one composite sample per soil layer. The samples were immediately brought to the laboratory and put in a refrigerator. Green plant material and coarse twigs were removed from the Oi samples. Oe‡Oa and mineral soil samples were sieved as described above. Water content (drying at 1058C) and loss on ignition (5508C) were determined. Fresh soil material from each plot and soil layer was transferred to separate plastic containers in amounts corresponding to 6, 16 and 100 g dry mass for the Oi, Oe‡Oa and mineral soil layers, respectively. Each container had a lid with an aperture for gas exchange. The soil was incubated at 158C for 202 days. Distilled water was added once a month to keep the water content at 50±60% of WHC. The containers were periodically closed with airtight lids and gas samples were collected from the headspace with a syringe after 15 min and injected into a gas chromatograph (HP 5890, H.P. Company, Avondale, PA). The gas sampling was repeated after 2±24 h, depending on the respiration rate. The mass of C evolved per container and hour was calculated according to Persson and WireÂn (1993). CO2 measurements were conducted once a week during the ®rst month and thereafter each fourth week. The cumulative CO2 evolution was estimated by linear interpolation. CO2 evolution rates obtained in the laboratory were multiplied by the dry soil mass of the respective soil layers and by a correction factor, which was based on temperature and moisture response functions (Seyferth, 1998) and on simulated soil moisture and soil temperature values obtained at the nearby Skogaby Norway spruce site (Alavi, 1999), the local climate of which was assumed to be the same as that at HassloÈv. 2.6. Chemical analyses Ð Soil solution The filtered samples were analysed for pH (combination electrode); DOC (Shimatdzu TOC-500; TC±IC method); Total dissolved N (TDN) (as NO3 by flow injection after K2S2O8 digestion, or as NO on a Mitsubishi TN-05 analyser); NH4‡-N by flow injection; NO3 -N and SO42 -S by anion chromatography (Dionex Series 2000i/SP column);

59

Total dissolved S (TDS) by ICP (Jobin Yvon JY-24 or Perkin±Elmer Optima 3000 DV). Dissolved organic nitrogen (DON) was calculated as DONˆTDN (NH4‡‡NO3 ). Dissolved Organic sulphur (DOS) was calculated as DOSˆTDS SO42 . The dissolved organic matter (DOM) was fractionationated according to Leenheer (1981) for C, and according to the modi®ed method developed by Qualls and Haines (1991) for N. DOM was divided into hydrophobic and hydrophilic acids, bases and neutrals. An acidi®ed water sample (pH 2) was passed through a hydrophobic XAD-8 resin, followed by Amberlite cation and anion exchange resins. Appropriate sample fractions according to the fractionation scheme were analysed for DOC, TDN, NH4‡ and NO3 , and the DOC and DON concentrations in each of the DOM categories mentioned above were calculated. Hydrophobic bases were assumed to be negligible (cf. Vance and David, 1991; Easthouse et al., 1992). 2.7. Data treatment BSeff was obtained as Sbase cations/CEC100. CEC was obtained by summation of cations. pHH2 O , exchangeable cations, BSeff, CEC, storage of C and N, C/N and C/S ratios, as well as the fractions of DOM were statistically analysed by one-way ANOVA for signi®cant differences with respect to soil depth or treatment (p<0.05). The soil solution concentrations of C, N and S forms were analysed by PROC MIXED (SAS Inst.), which tested for treatment effects, temporal patterns and interactions of treatment and time (p<0.05) for the entire period of investigation. In the graphs, average values  2 S.D. are presented for each sampling event. The three replicates of the P80 suction cups installed at the 15 and 30 cm levels, were not consistently located within the same blocks. However, all four blocks were included in the calculations, and `empty' blocks were treated as missing values. 3. Results 3.1. Acid±base chemistry Ten years after the lime treatment, carried out in 1984, pHH2 O in the forest ¯oor (except for the Oi layer)

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S. Ingvar Nilsson et al. / Forest Ecology and Management 146 (2001) 55±73

Table 1 pHH2 O , effective base saturation (BS), exchangeable Al and cation exchange capacity (CEC) in the control and limed treatment in 1994 pHH2 O a

Oi Oe‡Oa 0±5 cm 5±10 cm 10±20 cm 20±30 cm 30±40 cm a b

BSeff (%)a

Al3‡ (mmolc kg 1)a

CEC (mmolc kg 1)a

Control

Limed

Control

Limed

Control

Limed

Control

Limed

4.9 4.3 4.1 4.2 4.3 4.4 4.4

4.5 5.9 5.2 4.8 4.8 5.0 4.9

90 a 54 a 17 a 8a 6a 9a 7a

97 b 100 b 96 b 62 b 36 b 45 b 30 a

±b 34.5 28.6 a 29.4 a 19.7 a 11.9 a 8.9 a

± ± 2.4 b 14.0 b 14.9 a 10.6 a 7.1 a

156 a 190 a 40 a 35 a 22 a 13 a 10 a

358 b 703 b 73 b 40 a 26 a 19 a 11 a

a a a a a a a

b b b b b b b

Values with different letters are significantly different with respect to treatment (p<0.05). Below the detection limit.

and in the mineral soil layers including the 30±40 cm layer had increased signi®cantly (Table 1). There was a signi®cant increase in the effective BSeff to a depth of 20±30 cm in the mineral soil. CEC increased signi®cantly in the forest ¯oor and the 0±5 cm layer in the mineral soil. Signi®cantly declining amounts of exchangeable Al could only be detected to a depth of 10 cm in the mineral soil, indicating that the increase in BSeff in the deeper mineral soil layers was chie¯y accounted for by increasing amounts of exchangeable Ca2‡ and Mg2‡, rather than by declines in exchangeable Al (Table 1).

the lime treatment (20.2) than in the control treatment (24.5). No signi®cant differences were found in the mineral soil layers, however (Table 2). The C/S ratio in the Oe‡Oa layer (second sampling occasion) was signi®cantly lower in the lime treatment compared to the control treatment (203 versus 222) and the C/N ratios were close to those mentioned above, i.e. 21.2 in the lime treatment and 25.1 in the control treatment. The S/N ratio was about the same (0.11) in both treatments. There were no signi®cant treatment differences in C and N storage, either in individual soil layers, or in the entire soil pro®le sampled (Table 2). However, the C storage tended to be smaller in the forest ¯oor and the upper 5 cm of the mineral soil in the lime treatment compared to the control treatment. A reversed trend, i.e. a higher C storage in the lime treatment, was indicated for the remaining part of the mineral soil (Table 2).

3.2. C and N stores, C/N and C/S ratios The lime treatment had changed the quality of the soil organic matter in the forest ¯oor. The C/N ratio in the Oe‡Oa layer was signi®cantly lower (p<0.05) in

Table 2 Carbon and nitrogen storage (kg ha 1) and C/N ratios (w/w) in the Oi and Oe‡Oa layers, and in the mineral soil in the control and limed treatment, 10 years after the application of dolomite lime in 1984a Carbon

Oi Oe‡Oa Forest floor 0±5 cm 5±10 cm 10±20 cm 20±30 cm 30±40 cm Mineral soil a

Nitrogen

C/N ratio

Control

Limed

Control

Limed

Control

Limed

6860 20820 27680 23160 20100 35920 23110 14600 116890

5540 18920 24460 18230 24810 42840 29550 17810 133240

250 850 1100 1180 1060 2280 1200 740 6460

200 930 1130 900 1300 1850 1600 960 6610

26.3 24.5

27.1 20.2

19.8 19.0 19.3 19.2 19.6

20.5 19.1 18.9 18.8 19.1

The C/N ratio in the Oe‡Oa layer differed significantly between lime and control treatments (p<0.05).

S. Ingvar Nilsson et al. / Forest Ecology and Management 146 (2001) 55±73

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Table 3 Cumulative C mineralisation in the laboratory incubations (202 days) and annual C mineralisation in the field for each soil layer estimated from the incubations, the soil mass in the field and corrections for soil moisture and soil temperaturea Cumulative C mineralisation (mg C g 1 per 202 day)

Oi Oe‡Oa 0±5 cm 5±10 cm 10±20 cm 20±30 cm Forest floor Total a

Annual C mineralisation (kg C ha 1 per year)

Control

Limed

41.9 29.8 14.2 12.8 6.0 3.7

53.9 68.2 11.9 12.5 5.5 3.1

Control

Limed

1000 2100

1800 2800

The standard error is 10±20% of the mean values.

3.3. Heterotrophic CO2 respiration The cumulative CO2 evolution (C mineralisation) obtained in the laboratory at ‡158C was higher in the limed than in the unlimed forest ¯oor (Oi‡Oe‡Oa) at HassloÈv (Table 3). In the mineral soil there was no difference in C mineralisation between unlimed and limed plots. When the laboratory estimates were extrapolated to the ®eld, accounting for soil mass, moisture and temperature, the calculated C mineralisation in the forest ¯oor was much higher in the lime treatment than in the control treatment (1800 and 1000 kg C ha 1 per year, respectively). The values calculated for the whole soil pro®le differed less in relative terms (2800 and 2100 kg C ha 1 per year, respectively).

the 15 cm depth in the mineral soil (Fig. 2a; Table 4). Some long-term ¯uctuations were also recorded in the mineral soil. In the control treatment DOC ranged from ca. 50 mg C/l in the O horizon to <10 mg C/l at the 50 cm depth in the mineral soil. The corresponding

3.4. Soil solution pH and dissolved C, N and S forms Statistically signi®cant differences with respect to treatment or time (including interactions) are shown in Table 4 for each chemical species and soil depth. In accordance with the pHH2 O values in Table 1, pH in the soil solution collected in the lysimeters was signi®cantly higher in the lime than in the control treatment at all sampling depths (Fig. 1). In the control treatment pH was close to 4.5 at all soil depths. In the lime treatment pH ranged from about 6.5 (O horizon) to slightly below 5.0 at the 50 cm depth in the mineral soil. DOC was signi®cantly higher in the lime treatment than in the control treatment in the O horizon and at

Fig. 1. pH in the soil solution in the control and limed treatment in the O horizon and at 15, 30 and 50 cm depth in the mineral soil. The error bars show 2 S.D.

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S. Ingvar Nilsson et al. / Forest Ecology and Management 146 (2001) 55±73

Table 4 Summary of statistically significant treatment and time effects on the soil solution concentrations Variable

Soil layer

Treatment

Time

Treatmenttime

pH

O 15 cm 30 cm 50 cm

p<0.001 p<0.001 p<0.05 p<0.001

n.s. n.s. p<0.01 n.s.

n.s n.s n.s n.s

DOC

O 15 cm 30 cm 50 cm

p<0.001 p<0.001 n.s. n.s.

p<0.01 p<0.001 p<0.001 p<0.001

n.s p<0.05 p<0.05 n.s

DON

O 15 cm 30 cm 50 cm

p<0.05 p<0.05 n.s. p<0.01

n.s. p<0.001 p<0.001 n.s.

n.s n.s. n.s. n.s

DOS

O 15 cm 30 cm 50 cm

p<0.05 p<0.001 n.s. p<0.001

p<0.001 p<0.01 p<0.001 p<0.001

n.s n.s. n.s. p<0.001

DOC/DON ratio

O 15 cm 30 cm 50 cm

p<0.05 n.s. n.s. n.s.

p<0.001 p<0.001 p<0.001 p<0.01

n.s p<0.05 n.s. n.s

NH4‡-N

O 15 cm 30 cm 50 cm

n.s. p<0.05 n.s. n.s.

p<0.05 n.s. n.s. n.s.

n.s p<0.01 n.s. n.s.

NO3 -N

O 15 cm 30 cm 50 cm

n.s. n.s. n.s. p<0.01

n.s. p<0.01 p<0.001 n.s.

n.s p<0.05 n.s. n.s.

SO42 -S

O 15 cm 30 cm 50 cm

n.s. p<0.01 n.s. n.s.

p<0.01 p<0.05 n.s. p<0.001

n.s n.s. n.s. p<0.05

®gures for the lime treatment were ca. 50±100 and 10 mg C/l, respectively. High TDN concentrations and low DON concentrations increased the uncertainty of the DON estimates. This was found in the deeper mineral soil layers which had the highest concentrations of NO3 (Fig. 2b and Fig. 5b). However, a signi®cant treatment effect on the DON concentration (lime >control) was found in the O horizon and at the 15 and 50 cm depths in the mineral soil (Fig. 2b; Table 4). There were temporal trends at the 15 and 30 cm depths, in that the DON concentration in the lime treatment decreased and

approached the concentration level of the control treatment towards the end of the monitoring period. In the lime treatment DON ranged from 1±2.5 mg N/l in the O horizon to 0.2±0.3 mg N/l at the 50 cm soil depth. In the control treatment DON ranged from 1 mg N/l in the O horizon to <0.2 mg N/l at the 50 cm soil depth. The C/N ratio of the dissolved organic matter in the O horizon was signi®cantly lower in the lime than in the control treatment (Fig. 2c; Table 4). Otherwise there were no treatment effects. There were signi®cant time effects at all soil levels, as the C/N ratio in the lime treatment tended to

S. Ingvar Nilsson et al. / Forest Ecology and Management 146 (2001) 55±73

63

Fig. 2. (a) Dissolved organic carbon (DOC) (mg C/l), (b) dissolved organic nitrogen (DON) (mg N/l), (c) C/N ratio (w/w) of dissolved organic matter and (d) dissolved organic sulphur (DOS) (mg S/l). Details as in Fig. 1.

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increase towards the end of the monitoring period, indicating that the dissolved organic matter became more nitrogen poor. Like DON, dissolved organic S (DOS) was signi®cantly higher in the lime treatment than in the control treatment in the O horizon and at the 15 and 50 cm depths in the mineral soil (Fig. 2d; Table 4). There were temporal trends at all sampling depths, largely indicating an increase in the DOS concentration in both treatments towards the end of the monitoring period. In the O horizon and at the 50 cm depth this increase seemed to be more pronounced in the lime treatment than in the control treatment. The DOS concentration in the O horizon varied from 1.0 to 2.0 mg S/l in the lime treatment and was slightly lower in the control treatment (0.5±1.0 mg S/l). The DOS

Fig. 3. Chemical components of the dissolved organic matter (% DOC) in the O horizon (Control humus or limed humus) and at 15, 30 and 50 cm soil depth in the mineral soil, in the control and limed treatment. The soil water was collected on 18 November 1996 (mineral soil) and 2 December 1997 (O horizon). The components are: HphobA, hydrophobic acids; HphobN, hydrophobic neutrals; HphilA, hydrophilic acids; HphilB, hydrophilic bases; HphilN, hydrophilic neutrals. Different letters show statistically significant differences between soil layers within each individual treatment (p<0.05).

concentrations at the 50 cm depth in the mineral soil were similar to those in the O horizon (0.5±2.5 mg S/l in the lime treatment, versus 0.5±1.0 mg S/l in the control treatment). 3.5. Chemical characterisation of dissolved organic matter The fractional composition of the dissolved organic matter (% of DOC concentration) is shown in Fig. 3. The fractionation dates were 18 November 1996 (the mineral soil) and 2 December 1997 (the O horizon). There were differences between soil depths, although no treatment differences could be found. The percentage of hydrophobic acids declined signi®cantly with soil depth, particularly in the control treatment, while the share of hydrophilic acids showed a weak tendency to increase with soil depth. These trends indicated that hydrophobic acids were selectively adsorbed compared to hydrophilic acids. The percentage share of hydrophobic neutrals increased signi®cantly with soil

Fig. 4. The C/N (w/w) ratio in the total hydrophobic fraction (Hphob) and the total hydrophilic fraction (Hphil), in the control and limed treatment on the same collection dates as in Fig. 3. The difference between Hphob and Hphil was statistically significant (p<0.05) at the 30 and 50 cm depth in the control treatment.

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Fig. 5. (a) NH4‡-N (mg N/l), (b) NO3 -N (mg N/l) and (c) SO42 -S (mg S/l). Details as in Fig. 1.

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depth in both treatments, indicating either an ineffective soil adsorption or a net addition of organic matter having the chemical properties of hydrophobic neutrals. The different adsorption patterns of hydrophobic and hydrophilic compounds might have affected the N content of the dissolved organic matter. This is illustrated in Fig. 4, which shows the C/N ratio of the total hydrophobic and total hydrophilic fractions related to soil depth. The total hydrophilic fraction tended to have a lower C/N ratio than the total hydrophobic fraction regardless of treatment and this difference between the fractions was statistically signi®cant in the control treatment at the 30 and 50 cm levels. Fractionations were also conducted on two previous occasions, when only the total hydrophobic and total hydrophilic fractions were analysed for their C and N content. The results were qualitatively similar to those reported above (data not shown). 3.6. Soil solution concentrations of inorganic N and S forms NH4‡ concentrations were generally low, extending from <3 mg N/l in the O horizon to <0.10 mg N/l at the 50 cm depth in the mineral soil (Fig. 5a). There was a temporal trend in the O horizon, showing up as a more clear differentiation between the control and lime treatments (control >lime) by the end of the monitoring period. There was a statistically signi®cant treatment effect (again control > lime) at the 15 cm depth in the mineral soil (Table 4). The NO3 concentration was signi®cantly higher in the lime than in the control treatment at the 50 cm depth in the mineral soil (Fig. 5b; Table 4). Temporal trends, shown as increasing NO3 concentrations in the lime treatment towards the end of the monitoring period, were found at the 15 and 30 cm depths in the mineral soil. The NO3 concentration in the control treatment varied from ca. 1 mg N/l in the O horizon to values close to the detection limit in the mineral soil. In the lime treatment NO3 varied between 2 and 4 mg N/l in the O horizon. In the mineral soil the variation was much greater (from values close to the detection limit up to 6 mg N/l). SO42 concentrations were ca. 1±4 mg S/l in the O horizon, 2±6 mg S/l at the 15, and ca. 4±7 mg S/l at the 30 and 50 cm depths in the mineral soil (Fig. 5c). The concentration was signi®cantly higher in the lime

treatment than in the control treatment at the 15 cm depth. There were temporal trends in the O horizon and at the 15 and 50 cm depths (Table 4). At the latter depth the difference between the lime and control treatments (lime >control) became more consistent towards the end of the monitoring period. 4. Discussion 4.1. Effects of liming on the acid±base chemistry of the soil The results were in accordance with previous soil inventories in forest liming experiments (HallbaÈcken and Popovic, 1985; Derome et al., 1986; Hultberg et al., 1995; Kreutzer, 1995; Staaf et al., 1996). Liming had pronounced effects on pHH2 O , BSeff and exchangeable Al (Table 1). The increase in exchangeable Ca2‡ and Mg2‡ in the lime treatment indicated by the increase in base saturation was only partly accounted for by a decline in exchangeable aluminium (Table 1). This may to some extent be an analytical artifact as we did not distinguish between water soluble and truly exchangeable cations. Furthermore, CEC did not increase in the lime treatment below the 0±5 cm layer in the mineral soil. However, in the pH interval 4±5 (Fig. 1), hydrolysis of surface-bound Al should be important, implying a decline in the average Al charge and a net increase in available cation adsorption sites (Hargrove and Thomas, 1981). 4.2. Soil storage of C and N; C/N and C to S ratios The pHH2 O value in the lime treatment, which was as high as 6.5 in the O horizon and 5.2 in the upper 5 m of the mineral soil (approximately corresponding to the A horizon), was far above both present and past values found in coniferous forests on podsolised soils under unmanipulated conditions, both in southern and northern Sweden (HallbaÈcken and Tamm, 1986; Tamm and HallbaÈcken, 1988; Eriksson et al., 1992). Profound effects of changes in pH on the activity of forest soil microbial communities are well documented (Persson et al., 1990/91; BaÊaÊth and Arnebrant, 1994; BaÊaÊth et al., 1995; Persson and WireÂn, 1995; Persson et al., 1995; Andersson et al., 2000). Therefore, one would expect signi®cant effects of liming on

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the production and turnover of organic matter components at the HassloÈv site. A net decline in the soil storage of C would have been expected in the light of the difference between the lime and control treatments in estimated annual heterotrophic CO2 respiration from the forest ¯oor (800 kg C ha 1 per year, see Table 3). On the other hand, the lime treatment increased the volumetric stem growth from 18 to 22 m3 ha 1 per year from 1984 to 1994 (Andersson et al., 1996). This indicates that the litter input to the soil (needles, twigs and roots) might also have increased, counteracting a decline in the C, N and S pools. In laboratory incubations, we showed that the heterotrophic CO2 respiration from the O horizon was signi®cantly higher in the lime treatment than in the control treatment (Persson et al., 1990/91; Valeur and Nilsson, 1993; Andersson et al., 1994). The difference in C storage in the forest ¯oor between the control and the lime treatment at the sampling event in 1994 was 3220 kg C ha 1 (ca. 12%), corresponding to an average net loss of 322 kg C ha 1 per year since the lime application in 1984. This difference was not statistically signi®cant, however. The difference in C storage in the mineral soil was not signi®cant either, and the trend was actually reversed (lime >control), possibly indicating an increased translocation and adsorption of dissolved organic matter leached from the O horizon (see Section 4.3). Previous investigations at HassloÈv showed an increase in both net nitrogen mineralisation and nitri®cation in the lime treatment (Persson et al., 1990/91), and in the present study we showed an increase in the leaching of both DON (Fig. 2b) and NO3 (Fig. 5b). The N storage was not measurably affected, however (Table 2). Still, a net loss of C from the Oe‡Oa layer was indicated by signi®cant declines in both the C/N and C/S ratios in the lime treatment (24.5 and 222 in the control treatment, against 20.2 and 203 in the lime treatment). A higher net N mineralisation in the lime treatment (Persson et al., 1990/91), combined with a similar S/N ratio (0.11) in the two treatments as was found in the present study, strongly indicated that there was also a higher net mineralisation of S in the lime treatment. Consistent declines in the C storage in the forest ¯oor were previously documented by Persson et al. (1995) for four other forest sites in southern Sweden, which had received CaCO3 in amounts corresponding

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to 9000±10 000 kg ha 1 40 years before the soil sampling. Similar declines in C storage were reported by Marschner and Wilczynski (1991) and Kreutzer (1995), although their results are rather uncertain as the control and lime treatments were not statistically replicated. A null-effect with respect to both C and N transformations was shown by Smolander et al. (1998) in a ®eld experiment performed in a stand of Picea abies (Oxalis±Myrtillus type) growing on podsolised soil in south-eastern Finland. Liming (6000 kg CaCO3 ha 1) just tended to decrease the net N mineralisation and did not change the soil respiration rate. pHH2 O in the forest ¯oor was ca. 4 in the control and ca. 5 in the lime treatment. In a previous study at the HassloÈv site (Andersson et al., 1994), it was shown that forest ¯oor limed to pH 5±5.5 (dolomite lime application 3450 kg ha 1) had a respiration rate that was not signi®cantly different from the control treatment (pH 4). Obviously, pH 5±5.5 was too low to give a signi®cant increase in the total biological activity. The investigation of Smolander et al. (1998) may be interpreted in the same way, although their ®eld experiment was unfortunately not replicated. To the best of our knowledge, no other reports have been published concerning the total soil storage of S, or the C/S ratio, after forest liming. 4.3. Effects of liming on dissolved organic matter In quite a few ®eld and laboratory studies researchers have reported increased levels of dissolved organic matter (DOC and/or DON) after liming. Increased levels of DOC have generally been explained as caused by (1) an increase in biological activity (Kreutzer, 1995; Zech et al., 1996), (2) an increase in proton dissociation of functional endgroups (Hildebrand, 1990), (3) a combination of (1) and (2) (GoÈttlein and Pruscha, 1991) or (4) an increased decomplexation of organic Al complexes (Erich and Trusty, 1997). Concerning the HassloÈv site, we have recently shown that the effect of liming on the production and leaching of DOC from the Oe and Oa layers could be more dependent on the microbial activity than on pH (Andersson and Nilsson, unpublished manuscript). Some authors have found that liming had no effect at all on the dissolved organic matter. Cronan et al. (1992) measured soluble organic matter in the forest ¯oor of a red pine (Pinus resinosa) stand. Although

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5000 kg of dolomite lime had been applied and pH increased from 4.4 to 6.9, there was no difference in DOC concentration between the control treatment and the lime treatment. The DOC analysis was carried out rather soon (11 weeks) after the lime application. However, the authors did not check how much of the lime had actually dissolved, and did not report the Ca2‡ and Mg2‡ concentrations in the soil solution. Kaiser et al. (1996) showed that DOC could be effectively adsorbed on the surfaces of carbonate minerals and RoÈmkens et al. (1996) have demonstrated that Ca2‡ ions may complex organic matter and thereby probably increase its stability (cf. Andersson et al., 2000). Geary and Driscoll (1996) measured DOC in Spodosols in the Adirondack Mountains in NE USA during a 2-year period, and could not demonstrate any clear treatment effects, not even in the O horizon. The lime application was 6890 kg CaCO3 ha 1. Ca2‡±organic matter complexes may also have been important in this case. On none of the fractionation events were there any treatment effects with respect to the fractional composition of the dissolved organic matter. The total hydrophilic fraction in both the control and lime treatment tended to have a higher nitrogen content than the total hydrophobic fraction. This difference was statistically signi®cant at the 30 and 50 cm soil depth in the control treatment (Fig. 4).The hydrophobic acids were adsorbed to a greater extent in the mineral soil than were individual hydrophilic fractions (Fig. 3). Similar differences in adsorption properties (Cronan and Aiken, 1985; Qualls and Haines, 1991; Kaiser et al., 1996; Andersson et al., 1999) and in N content (Qualls and Haines, 1991; Andersson et al., 1999, 2000) were shown in previous investigations. However, we also noticed a relative increase in hydrophobic neutrals with increasing soil depth (Fig. 3), indicating a low adsorption of these hydrophobic compounds. There is con¯icting evidence in the literature concerning the adsorption of hydrophobic neutrals. Qualls and Haines (1991) and David et al. (1995) found an adsorption pattern similar to ours in Dystrochrepts and Hapludults under oak-hickory forest, and in Spodosols under mixed northern hardwood respectively, while Guggenberger and Zech (1993) found an increasing adsorption with increasing depth in podsolised soils under Norway spruce. Hydrophobic neutrals consist of rather fresh organic matter, such

as fatty acids, waxes and less degraded lignin and lignocellulose products (Zech et al., 1996). The occasionally increasing share of hydrophobic neutrals with soil depth may therefore hypothetically be explained by a net input of fresh organic matter from roots and fungal hyphae in the mineral soil. Although we could not demonstrate any effects of liming on the quality of the dissolved organic matter, such effects were previously shown in a column experiment, where we used reconstructed soil pro®les made from samples collected from each of the HassloÈv control plots and lime plots (Andersson et al., 1999). The pro®les consisted of either O‡A horizons (approximately corresponding to the O horizon ‡0±15 cm mineral soil) or O‡A‡B horizons (where we extended the soil pro®le to approximately the 30 cm level in the mineral soil). The share of hydrophobic compounds in the water leaving the A horizon was signi®cantly higher in the lime treatment than in the control treatment. Assuming that liming as such stimulates an increasing microbial production of potentially leachable hydrophobic compounds, a high water content (as in the laboratory columns) may have increased the solubility and leaching of those compounds (cf. Christ and David, 1996). The DON concentration of the hydrophobic compounds in the laboratory columns was signi®cantly higher in the lime treatment than in the control treatment, although the concentration was still signi®cantly lower than the DON concentration of the hydrophilic compounds in either treatment. Liming effects on the quality of dissolved organic matter were shown by GoÈttlein et al. (1991) in the HoÈglwald experiment in southern Germany. These authors compared water extracts from limed and unlimed forest ¯oor and upper A horizon and found a higher share of `less polar' (hydrophobic) compounds in the lime treatment than in the control treatment. Also in their study, the amount of soluble carbohydrates in the LOf1 (Oi) horizon was higher in the lime treatment than in the control treatment. In our column experiment (Andersson et al., 1999), the share of hydrophobic compounds was still higher in the lime treatment than in the control treatment in the water leaving the B horizon, i.e. the hydrophobic compounds of the lime treatment seemed to be less selectively adsorbed than those of the control treatment. This pattern is in accordance with Zech et al.

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(1996), who showed for a number of unlimed soils, which covered a wide pH range, that the hydrophobic acid fraction increased (i.e. hydrophobic acids were adsorbed to a smaller extent), while the hydrophilic acid fraction decreased as pH increased from 3.5 to 6.0. An increasing pH in the soil solution will have a strong in¯uence on the dissociation of acid end-groups, and hence on the hydration and water solubility of the organic molecules. This will be particularly important for polyprotic organic acids like the hydrophobic acids, containing relatively weak endgroups.

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the lime treatment at the 50 cm depth is consistent with the increase in pH and also with the successively increasing DOC concentration at the same depth, implying a competition between SO42 and organic anions for available adsorption sites (Gobran and Nilsson, 1988; Guggenberger, 1994). There was a pronounced retention (which should mainly be an adsorption) of both DOC and DON, but a surprisingly small retention of DOS. The DOS concentrations (0.5±2.5 mg S/l) were considerably higher than the 0.3 mg S/l found by Mitchell et al. (1989) in a Spodosol in New Hampshire, and also higher than the maximum values (0.6±0.8 mg S/l) reported by Homann et al. (1990) from eight temperate forests in USA and Canada. Concentrations of 1 mg S/l have only occasionally been found in other podsolised soils in Scandinavia (A. Stuanes, unpublished data, 1999). However, we still think that our data are reasonable as the DOC/DOS ratios reported by Homann et al. (1990) were fairly similar to the ratios obtained in soil water from the O horizon in the control treatment at HassloÈv (ca. 100; compare Fig. 2a and d). In laboratory incubations of O layer material (Oe‡Oa) collected at HassloÈv, the leaching of total dissolved S (SO42 ‡DOS) was constant, i.e. independent of treatment. The DOS leaching was signi®cantly higher and the SO42 leaching was signi®cantly lower in the lime treatment compared to the control treatment (Valeur and Nilsson, 1993; Valeur et al., unpublished manuscript). Thus, the net mineralisation was lower in the lime treatment. Much of the leached organic sulphur should have consisted of ester sulphates (David et al., 1995) and theoretically, SO42 may have reacted with the dissolved organic compounds in the O horizon to form DOS (McGill and Cole, 1981; Schoenau and Germida, 1992). According to Table 5, the leaching of SO42 from the O horizon under ®eld conditions was similar in the two treatments (10.0 and 10.2 kg S ha 1

4.4. Leaching and retention of organic and inorganic C, N and S forms in the mineral soil We used the concentration gradients of the organic and inorganic C, N and S forms to make an approximate calculation of the amounts leached from the O horizon and the amounts retained or released in the mineral soil. To get the ¯ux and adsorption data in Table 5 we assumed that the average water percolation through the unsaturated zone was 400 mm per year, i.e. similar to modelled percolation in a nearby stand of Norway spruce (Skogaby) growing on a similar podsolised soil (Alavi, 1999). We then used average concentrations for each treatment, chemical variable and soil depth for the period when soil water was collected from all soil depths (November 1994± November/December 1997). By this simpli®ed approach we found that there was generally a net retention of all components except SO42 , which showed a net release in both treatments. This is in accordance with observations of a declining atmospheric deposition of SO42 resulting in a SO42 desorption in the mineral soil (cf. Eriksson and Karltun, 1995; Hallgren-Larsson et al., 1997). The tendency for a successively increasing SO42 ¯ux in

Table 5 Estimated leaching of C, N and S forms from the O horizon and adsorption in the mineral soil (0±50 cm) in the control and limed treatmenta DOC

Leaching Adsorption a

DON

SO42 -N

Control

Limed

Control

Limed

Control

Limed

Control

Limed

Control

155 128

234 202

4.0 3.4

8.4 7.6

2.6 0.8

4.2 0.6

11.2 9.8

15.4 6.2

10.0 4.0

The figures are given in kg ha 1 per year. Mineral N is NH4‡-N‡NO3 -N.

b

Mineral Nb

DOS

Limed 10.2 11.4

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per year in the control treatment and lime treatment), while the DOS leaching from the O horizon was higher in the lime treatment (2.6 and 4.2 kg S ha 1 per year in the control treatment and lime treatment, respectively). Besides the leaching of dissolved inorganic and organic S, root uptake (which did not occur in the laboratory incubations) should be another factor contributing to the higher net loss of both S and N in the O horizon of the lime treatment compared to that of the control treatment (Table 2 and Section 3). Tree growth was found to be signi®cantly higher in the lime treatment (Andersson et al., 1996), and it was shown by Maynard et al. (1985) that the net sulphur mineralisation was greater in a cropped soil than in an uncropped soil, indicating an active role of the plant rhizosphere in determining the turnover of organic sulphur. A contribution of organic sulphur to root uptake was shown by Zhao and McGrath (1994). The estimated adsorption of DOC in the B horizon was 128 and 202 kg C ha 1 per year in the control treatment and in the lime treatment, corresponding to 83±86% of the leaching from the forest ¯oor. In absolute terms the adsorbed amount in the control treatment was ca. 63% of the adsorbed amount reported from a Norway spruce stand in southern Germany by Zech et al. (1996). The adsorption obtained in our study was equivalent to 12% of the heterotrophic CO2 respiration from the forest ¯oor of the control treatment and lime treatment, and to 6±7% of the total heterotrophic CO2 respiration from the forest ¯oor‡the 0±30 cm layer of the mineral soil. The leaching of DOC from the lower part of the B horizon, i.e. the 50 cm level, (32 kg C ha 1 per year in the lime treatment, and 27 kg C ha 1 per year in the control treatment; see Table 5) was equivalent to 1.2% of the total heterotrophic CO2 respiration as de®ned above, which was about one half of the relative ®gure of 2.2% reported by Zech et al. (1996). Thus, the adsorption of DOC in the mineral soil was quantitatively much more important than the DOC leaching. The adsorption of DON in the B horizon (Table 5), was 3.4±7.6 kg N ha 1 per year, i.e. 85±90% of the DON leached from the O horizon. In absolute terms this was considerably less than the 12±25 kg N ha 1 per year reported by Kreutzer (1995). The DOC/DON ratio in the latter study was ca. 20±23, while the ratio in our study was 30±40 in the O horizon and 40±60 in the B horizon (at the 50 cm level) (Fig. 2c).

The retention of mineral N (Table 5) should be due mainly to uptake by the trees and microbial immobilisation. The retention in the lime treatment (40%) was considerably less than in the control treatment (88%) which should have been a result of an increased nitri®cation and nitrate leaching in the mineral soil as indicated in Fig. 5b. Many studies have shown that the adsorbed organic matter in the B horizon becomes biologically stabilized because of the refractory character of the dominating hydrophobic and hydrophilic acids (Qualls and Haines, 1992) and the formation of sparingly soluble Al- or Fe-organic matter precipitates or surface complexes on Al and Fe oxides (Sollins et al., 1996). The stabilisation caused by Al and Fe was demonstrated as inverse relationships between the degree of metal complexation and carbon mineralisation (heterotrophic CO2 respiration) from low-molecular organic acids as well as humic compounds and litter material (Boudot et al., 1989; Boudot, 1992; Miltner and Zech, 1998). Organic matter stabilization was also indicated by Persson and WireÂn (1995), who investigated podsolised soils in southern Sweden and showed that net N mineralisation per unit total N (predominantly organic N) decreased signi®cantly with increasing soil depth. According to Table 2, the amount of organic C and N in the mineral soil in HassloÈv was 80±85% of the total amount, i.e. slightly above the average value of 70% reported for the upper 1 m of mineral soils in a Finnish study (Liski and Westman, 1997). A very large part of the soil organic matter could thus be assumed to have a limited microbial availability. Our ®eld data indicated that liming caused a net decline in the C storage in the soil, shown both by signi®cant declines in the C/N and C/S ratios in the forest ¯oor (O horizon) and by the increase in the heterotrophic CO2 respiration rate (which also implied an increased ¯ux of `greenhouse gas' to the atmosphere). However, there was also an increase in the organic matter translocation from the O horizon to the mineral soil, implying a further chemical stabilisation and decline in the microbial availability of the remaining soil carbon. We could assume, however, that some of the apparent increase in DON retention in the lime treatment may, to some extent, be accounted for by an increase in the net N mineralisation followed by an increase in the net nitri®cation (cf. Kreutzer, 1995). An increased translocation of organic matter also

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points to the possibility of a transient increase in the podsolisation rate (cf. Courchesne and Hendershot, 1997) after liming. Such an increase should remain as long as there exists the combination of an acid coarsetextured mineral soil and a limed forest ¯oor which has an enhanced production and leaching of DOC which could complex Al and Fe and become adsorbed in the B horizon. The eventual reacidi®cation of the forest ¯oor will set the time frame for this `pedological paradox'. 5. Conclusions Liming changed the quality of the soil organic matter in that the C/N ratio and the C/S ratio decreased signi®cantly in the Oe‡Oa horizon. Higher heterotrophic CO2 respiration and signi®cant declines in the C/N and C/S ratios in the Oe‡Oa horizon of the lime treatment strongly indicated that the C storage in the soil had declined, although this could not be con®rmed statistically. Leaching of dissolved organic matter from the O and A horizons signi®cantly increased in the lime treatment, shown as an increased leaching of DOC, DON and DOS. The DOC leaching from the lower part of the B horizon was equivalent to 1.2% of the annual C loss from the soil (forest ¯oor‡the 0±30 cm layer of the mineral soil), as heterotrophic CO2 respiration, and was thus of less quantitative importance than the DOC adsorption in the mineral soil, which was equivalent to 6±7% of the respiration. About 80±90% of the DOC and DON that leached from the O horizon was adsorbed in the B horizon regardless of treatment, while the adsorption of DOS was only 14±31%. Adsorption of organically bound nutrients may imply a decreased microbial availability and plant availability. The increase in DOC adsorption in the B horizon of the lime treatment implied a transient lime-induced increase in the podsolisation rate. Leaching of SO42 from the B horizon was higher than that from the O horizon in both treatments. Thus, there seemed to be a net desorption of SO42 from the B horizon which should be due mainly to declining amounts of sulphur deposition. The net desorption tended to be higher in the lime than in the control treatment, probably due to a higher pH and a higher level of DOC in the lime treatment.

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Liming tended to increase NO3 leaching from the B horizon, implying an increase in the net nitri®cation. Acknowledgements This study was funded by the Swedish Environmental Protection Agency, the Swedish Council for Agricultural and Forest Research, the Oscar and Lili Lamm Foundation and the Swedish University of Agricultural Sciences. We thank Mr. Kjell Bengtsson and MSc Ulf Johansson, for their invaluable help with soil and water sampling. We also thank Ms Marianne Cervenka, Ms Lise Gustafsson and Ms Gunilla Hallberg for their skilful analytical work in the laboratory. We thank Dr. Mary McAfee for correcting our `Swenglish'. Finally, our special thanks go to `the grand old man of forest liming' Dr. Budimir Popovic who established the HassloÈv experiment and several other Swedish liming experiments during the 1980s.

References Alavi, G., 1999. Climate, Leaf Area, Soil Moisture and Tree Growth in Spruce Stands In SW Sweden. Field Experiments and Modelling, Agraria 175, Swedish University of Agricultural Sciences, Uppsala, 26 pp. Andersson, F., HallbaÈcken, L., Popovic, B., 1996. Kalkning och traÈdtillvaÈxt. In: Staaf, H., Persson, T., Bertills, U. (Eds.), Skogsmarkskalkning, Resultat och slutsatser fraÊn NaturvaÊrdsverkets foÈrsoÈksverksamhet, Report 4559. Swedish Environmental Protection Agency, pp. 122±133 (in Swedish with a summary in English). Andersson, S., Valeur, I., Nilsson, I., 1994. Influence of lime on soil respiration leaching of DOC, and C/S relationships in the mor humus of a Haplic Podsol. Environ. Int. 20, 81±88. Andersson, S., Nilsson, S.I., Valeur, I., 1999. Influence of dolomitic lime on C and N leaching in a forest soil. Biogeochemistry 47, 297±317. Andersson, S., Nilsson, S.I., Saetre, P., 2000. Influence of temperature and pH on carbon respiration and the leaching of dissolved organic carbon and dissolved organic nitrogen in a mor humus layer. Soil Biol. Biochem. 32, 1±10. BaÊaÊth, E., Arnebrant, K., 1994. Growth rate and response of bacterial communities to pH in limed and ash treated forest soils. Soil Biol. Biochem. 26, 995±1001. Ê ., Pennanen, T., Fritze, H., 1995. Microbial BaÊaÊth, E., FrostegaÊrd, A community structure and pH response in relation to soil organic matter quality in wood-ash fertilized, clear-cut or burned coniferous forest soils. Soil Biol. Biochem. 27, 229±240.

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