Factors controlling soil carbon and nitrogen stores in pure stands of Norway spruce (Picea abies) and mixed species stands in Austria

Forest Ecology and Management 159 (2002) 3±14

Factors controlling soil carbon and nitrogen stores in pure stands of Norway spruce (Picea abies) and mixed species stands in Austria Torsten W. Berger*, Christian Neubauer, Gerhard Glatzel Institute of Forest Ecology, Univ. f. Bodenkultur, Peter Jordan-Strasse 82, 1190 Vienna, Austria

Abstract Soil data of 18 pairs of secondary pure spruce stands (Picea abies) and adjacent mixed stands of spruce and broadleaf trees (at least 40% broadleaf trees, mainly beech, Fagus sylvatica) on comparable sites were used to study effects of species composition and other driving forces on carbon and nitrogen stores. It was hypothesized that carbon input by litter and roots to different soil horizons has a marked effect on carbon and nitrogen stores and, therefore, silvicultural methods, e.g., admixture of beech versus spruce, can be used to manage nitrogen retention and release. The study sites of different stand age (pole to mature stage) were chosen on two different bedrock materials (Flysch and Molasse). In general, soils on Flysch are less acidic, better supplied with nutrients, and less sandy than soils developed on Molasse. Species composition did not affect total stores (forest ¯oor and 0±50 cm mineral soil) of carbon or nitrogen on Molasse, however, paired samples tests indicated that mean total stores were signi®cantly higher both for carbon and nitrogen for pure spruce stands than for mixed species stands on Flysch. Carbon stores of the whole soil pro®le were best predicted by admixture of spruce, stores of phosphorus, sodium and calcium as well as stand age on Flysch; on Molasse, the only given predictor was aluminum storage, indicating that acidic soil conditions favor accumulation of organic carbon. On Flysch, total nitrogen stores were best explained by stores of sulfur and of the sum of base cations. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Soil carbon; Soil nitrogen; Picea abies; Forest restoration

1. Introduction During the past decade both carbon and nitrogen stores have been in the focus of scienti®c interest. Global climate models require information on carbon dynamics of ecosystems because the atmospheric CO2 levels not only depend on CO2 emissions but also on CO2 conversion by plant and by storage of organic carbon in ecosystems (Emanuel et al., 1984; Anderson, 1991; Rastetter et al., 1991; Schroeder, *

Corresponding author. Tel.: ‡43-1-47654-4107; fax: ‡43-1-4797896. E-mail address: [email protected] (T.W. Berger).

1991; Vitousek, 1991; Freedman et al., 1992; King and Neilson, 1992; Post et al., 1992; Smith and Shugart, 1993). Nitrogen dynamics of ecosystems are of particular interest because human activities have led to a dramatic increase of available plant nitrogen compounds in the biosphere (e.g., Glatzel, 1990; FluÈckiger and Braun, 1998; Gunderson, 1998; Katzensteiner, 1999). Nitrogen is a plant nutrient which in¯uences plant growth and thus carbon ®xation and turnover in ecosystems (Bonan, 1990; Luxmoore, 1991; Rastetter et al., 1992; Nadelhoffer et al., 1999). Retention of organic matter in soils determines the storage capacity for nitrogen, since the major part of nitrogen in terrestrial ecosystems is found in organic

0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 1 ) 0 0 7 0 5 - 8

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T.W. Berger et al. / Forest Ecology and Management 159 (2002) 3±14

compounds (e.g., Schachtschabel et al., 1997). Carbon decomposition in mineral soils narrows the C/N ratio and ultimately limits nitrogen retention (e.g., Swift et al., 1979). The turnover of carbon in soils is mainly controlled by water regimes and temperature (e.g., Sing and Gupta, 1977). Hence, forest species composition will in¯uence this turnover due to its different microclimates at the forest ¯oor. Modifying factors are size and physiochemical properties of carbon additions in litter from the above-ground and below-ground ¯ora and fauna, distribution of the root systems of plants in the soil pro®le, distribution of carbon within the soil matrix and its interaction with clay surfaces (Oades, 1988). Different rooting patterns have not only direct effects on the carbon ¯ux to the soil pro®le, but through root movement due to wind action on the canopy, have strong effects on soil porosity and thus soil aeration (Berger and Hager, 2000). Several studies have shown that forest management and the history of land use may in¯uence carbon and nitrogen dynamics (e.g., Brown and Lugo, 1990; Adger et al., 1992; Johnson, 1992; McPherson et al., 1993). Glatzel (1990) has shown that a large variation of carbon and nitrogen storage cannot be explained by primary soil variables. Hence, it is hypothesized that carbon input by the roots to different soil horizons as well as the chemical quality of this input has a marked effect on carbon and nitrogen storage and turnover. If this holds, silvicultural methods can be used to manage nitrogen retention and release. This issue is important because nitrogen deposition from air pollution in many places by far exceeds the amount removed by forest harvesting (e.g., Glatzel, 1990; Katzensteiner, 1999). Excess nitrogen is oxidized in aerated soils and leached to the ground water, adding to nitrate levels already critical from the viewpoint of human health (e.g., Emmett et al., 1998; Gunderson, 1998). This study was performed to test whether silvicultural methods, i.e., admixture of beech versus spruce, can be used to manage nitrogen retention and release. Soil data of 18 pairs of secondary pure spruce stands (Picea abies) and adjacent mixed stands of spruce and broadleaf trees (at least 40% broadleaf trees, mainly beech, Fagus sylvatica) on comparable sites were used to study effects of species composition, stand age, soil type and primary soil variables on total carbon and

nitrogen stores of the upper 50 cm soil pro®le (a more general description of the study sites is given elsewhere; Berger et al., 1998; Glatzel et al., 2000; Neubauer, 2000). 2. Study sites and methods 2.1. Study sites Within the special research program (SRP) ``Restoration of Forest Ecosystems'', 30 pairs of secondary pure spruce stands (P. abies) and adjacent mixed species stands (at least 40% broadleaf trees, mainly beech, F. sylvatica) on comparable sites were established for studying differences between these forests (Berger et al., 1998). While pure spruce stands are the natural vegetation in the sub-alpine region, the studied monospeci®c stands of Norway spruce at elevations between 470 and 790 m must be considered manmade (Mayer, 1974). Thirty six study sites (18 pairs) of different stand age (mature to pole stage; approximately 14±118 years) were selected for this study on two different bedrock materials (Molasse, Flysch). Since forest site classi®cations, soil pro®le descriptions and selected chemical soil analyses did not differ between the pure spruce stand and the corresponding mixed species stand for these 18 pairs (Neubauer, 2000), it is justi®ed to assume that existing differences of soil physical and chemical properties within these pairs are caused by species composition only. We found out that the total amount of Ca (HNO3/ HClO4; see Neubauer, 2000), pH and cation exchange capacity (compare Table 1) of the lower mineral soil (40±50 cm) were the best indicators for this selection procedure, because these parameters are preferentially in¯uenced by geochemical cycles (soil formation) and not like C, N or phosphorus by biochemical cycles. Site history data were available for 60% of the study sites, indicating that species composition before the conversion into pure secondary spruce stands was very similar to the recent composition of the studied mixed species stands (Neubauer, 2000). We assume that the species composition within each of all 18 pairs was the same before the mixed species stands were converted into pure spruce stands, based on comparisons of site classi®cation data (elevation, aspect, slope, topography,

T.W. Berger et al. / Forest Ecology and Management 159 (2002) 3±14

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Table 1 Mean mineral ®ne soil properties of 18 pure spruce stands and 18 mixed species stands on the bedrock materials Flysch and Molasse: carbon and nitrogen (mg g 1), effective cation exchange capacity (CEC, mmolc g 1), base saturation (%) and pH (CaCl2) Soil depth (cm)

C Pure

N

CEC

Base saturation

pH

Mixed

Pure

Mixed

Pure

Mixed

Pure

Mixed

Pure

Mixed

Flysch 0±5 5±10 10±20 20±30 30±40 40±50

56.2 30.3 19.2 11.0 8.3 6.1

34.4 19.0 12.5 8.7 6.1 5.0

3.5 2.2 1.4 0.9 0.7 0.6

2.4 1.4 1.0 0.8 0.6 0.5

121.2 105.7 91.6 82.0 87.5 91.1

95.8 79.3 72.4 72.8 81.6 89.2

47.3 42.9 45.7 49.1 62.3 69.1

69.2 51.4 47.8 57.0 66.6 74.0

3.7 3.8 4.0 4.1 4.2 4.3

4.3 4.1 4.2 4.3 4.3 4.4

Molasse 0±5 5±10 10±20 20±30 30±40 40±50

177.9 92.4 55.4 33.7 20.9 13.5

159.1 75.1 45.6 27.6 17.7 11.9

8.0 4.5 2.6 1.7 1.2 0.9

8.3 3.8 2.3 1.5 1.0 0.8

123.8 129.7 92.4 44.0 25.4 20.0

115.9 111.8 76.0 33.1 24.4 19.5

19.1 9.1 7.6 9.4 13.1 16.2

19.8 8.6 8.7 10.6 12.9 15.9

2.9 3.1 3.6 4.0 4.2 4.3

3.0 3.2 3.7 4.1 4.2 4.3

etc.) between the sites of known history with those of unknown history. 2.1.1. Study sites on Molasse These study sites are located near Mattighofen (138080 E, 488070 N), Upper Austria, in a forested landscape, called Kobernausserwald, at elevations between 510 and 720 m. Parent material for soil formation are tertiary sediments (so-called ``HausruckKobernausserwald'' gravel), which consist mainly of quartz and other siliceous material. Because of this acidic bedrock material with low rates of nutrient release, the dominant soil types are semi-podzols (intermediate soil type between cambisol and podzol) and podzols. Humus forms are moder and mor, and the thickness of the forest litter layer is between 5 and 10 cm, indicating slow turnover and accumulation of nutrients. pH values (CaCl2) of the upper mineral soil (0±5 cm soil depth) are between 2.6 and 3.4. The natural forest vegetation of mixed species stands is Luzulo nemorosae-Fagetum. 2.1.2. 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 Austria and Upper Austria from approximately 138220 E, 478500 N (Mondsee) to

158370 E, 488110 N (St. PoÈlten) at elevations between 470 and 790 m. Flysch consists mainly of old tertiary and mesozoic sandstones and clayey marls. Nutrient release from this bedrock material is higher in comparison to Molasse and consequently the prevalent humus forms are moder and mull, indicating quick turnover of the forest litter layer (usually less than 5 cm thickness). pH values (CaCl2) of the upper mineral soil (0±5 cm soil depth) are between 3.1 and 5.5. All soils of these study sites were classi®ed as pseudogley (Schachtschabel et al., 1997; FAO classi®cation: stagnic Gleysol), since horizons with a high fraction of ®ne material (loam, clay) cause temporary waterlogging. In general, soils on Flysch are less acidic, better supplied with nutrients (compare Table 1), and less sandy than soils developed on Molasse. The natural forest vegetation of mixed species stands on Flysch is Asperulo odoratae-Fagetum. 2.2. Soil sampling and analysis The litter layer was sampled with a sampling frame of 900 cm2 area. Soil cores were taken with a core sampler of 70 mm diameter to a depth of 50 cm. There were ®ve randomly distributed replications at each site, which were pooled before analysis, divided into six geometric horizons (0±5, 5±10, 10±20, 20±30, 30±40, 40±50 cm). Soil chemical parameters were

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T.W. Berger et al. / Forest Ecology and Management 159 (2002) 3±14

determined by standard procedure: samples of forest ¯oor and mineral soil were analyzed for total content of C (LECO SC 444), N (Kjeldahl), P and S (both after digestion with HNO3/HClO4; ICPS, inductive coupled plasma spectrometry). Organic carbon (carbon is used for organic carbon throughout this paper) was calculated as total carbon minus CCaCO3 (Scheibler). Calcium, Mg, K, Na, Mn, Fe and Al were measured as total contents after digestion with HNO3/HClO4 in the forest ¯oor and as exchangeable cations (BaCl2 extract) in the mineral soil by ICPS.

in the soil is dependent on the local conditions (soil texture, climate, etc.), while the mineralizable C fraction can be affected by activities of man. Because the mineralizable part of carbon is probably relatively small on Flysch, pure spruce caused signi®cantly higher total C stores (mean difference 2050 g m 2). However, on Molasse, species composition did not affect total soil C stores. Amounts of total (organic) soil carbon on Flysch are in accordance to Chen and Glatzel (1988), who measured 9.1 kg m 2 C under pure spruce and 6.7 kg m 2 C under pure beech within the upper 50 cm of a comparable soil pro®le in the Viennian Woods. Total C stores for spruce on Molasse are in the same range as reported by Bauer (1989) (12.6 kg m 2, upper 50 cm soil depth) for the same bedrock material. Total nitrogen stores (50 cm soil pro®le) of all sites were in the range 390±910 g m 2 (Table 2), indicating no signi®cant differences between the two bedrock materials. As stated for carbon above, paired samples tests indicated that mean total N stores were signi®cantly higher for pure spruce stands than for mixed species stands on Flysch (difference 120 g m 2); no differences between species composition were found on Molasse. Mean soil N stores (forest ¯oor and mineral soil down to 50 cm soil depth), analyzed within the framework of the Austrian forest soil inventory (Englisch, 1992), are comparable to our study: 696 g m 2 on soils with waterlogged horizons (pseudogley, gley; similar to soils of this study on

3. Results and discussion 3.1. Carbon and nitrogen stores of the whole soil pro®le Carbon and nitrogen stores of the forest ¯oor and the mineral soil down to 50 cm soil depth are called total C or N stores throughout this paper, given in Table 2. Total (organic) carbon stores on Flysch (between 5780 and 10 740 g m 2) were signi®cantly lower than C stores on Molasse (8110±14 260 g m 2). According to KoÈrschens (1998) a positive correlation between the inert part of carbon and clay content would cause higher C stores on soils developed on Flysch, but obviously the mineralizable part of carbon is markedly accumulated in acidic soils on Molasse, causing the observed difference. The inert C fraction

Table 2 Total carbon and nitrogen stores (descriptive statistics) within the forest ¯oor and mineral soil (0±50 cm) of 18 pure spruce stands and 18 mixed species stands on the bedrock materials Flysch and Molasse (paired samples tests were performed to test differences between species composition) Element

Flysch Pure

Molasse Mixed

Difference

Pure

Mixed

Difference

8.1 11.0 14.1

1.2 N.S.

2

Carbon (kg m ) Minimum Mean Maximum Nitrogen (g m 2) Minimum Mean Maximum *

6.7 8.6 10.7 450 630 910

p < 0:05. p < 0:01. N.S.: not signi®cant, p > 0:05. **

5.8 6.5 7.3 390 510 630

2.1**

120*

9.9 12.2 14.3 490 590 700

390 570 720

20 N.S.

T.W. Berger et al. / Forest Ecology and Management 159 (2002) 3±14

Flysch) and 728 g m 2 (podzol) to 802 g m 2 (semipodzol) on podzolic soils, comparable to soils of this study on Molasse. Results of Bauer (1989) (728 g m 2 under pure spruce on Molasse), Chen and Glatzel (1988) (521 g m 2 under pure spruce and 473 g m 2 under pure beech, both on Flysch) and Blay (1989) (604 g m 2 under pure beech on Flysch) are more relevant to our study, indicating a nice match with data of Table 2. These results support the stated hypothesis that carbon input by litter and roots to different soil horizons has a marked effect on C and N stores. However, these results are surprising because P. abies is commonly described as a tree with a ¯at root system with a much lower penetration energy than other coniferous trees (in decreasing order: Abies alba, Pinus sylvestris, Pseudotsuga menziesii, Larix decidua, P. abies, Pinus strobus) and most deciduous trees (Rottmann, 1989; Korotaev, 1992). Especially waterlogged horizons, which are present in all soil pro®les on Flysch, prevent deeper rooting of spruce and contribute to a plate-shaped root system (Zoth and Block, 1992; Xu et al., 1997). Since natural mixed forests of F. sylvatica and P. abies root to deep mineral soil horizons, affecting the nutrient storage and turnover of the whole soil pro®le and not only of the upper mineral soil horizons as pure spruce stands do, we expected increasing and not decreasing C and N stores due to admixture of beech. For example, Kreutzer et al. (1986) found that replacement of beech by spruce led to a long lasting release of nitrate to the ground water.

Fig. 1. Organic carbon stores of individual soil horizons (kg m stands on the bedrock materials Flysch and Molasse.

2

7

Since managing nitrogen, in particular N retention and release, by silvicultural methods takes place within the rooted zone, much emphasis was put on evaluating controlling factors for the whole soil pro®le in this paper (i.e., total C and N stores of the upper 50 cm). However, this quite surprising ®nding makes a closer look on individual soil horizons necessary. 3.2. Carbon and nitrogen stores of individual soil horizons Carbon stores of individual soil horizons are given in Fig. 1. Total lengths of the columns correspond to total carbon contents of Table 2. Although total carbon stores are higher for soils developed on Molasse, differences between pure spruce stands and mixed species stand are higher and consequently signi®cant for all geometric mineral soil horizons on Flysch, except for 20±30 cm soil depth (Table 3). Differences between pure spruce and mixed species stands on Molasse are positive as well, however, paired samples tests indicate signi®cantly higher carbon stores only for the forest ¯oor. Nitrogen stores of individual soil horizons are plotted in Fig. 2 and results of paired sample tests between species composition are given in Table 3. In general, pure spruce stands accumulate more nitrogen throughout the whole soil pro®le (except at 0±5 cm depth on Molasse), however, differences were only signi®cant for the forest ¯oor and at 10±20 cm depth on Flysch. A general trend of higher C/N ratios in the

per horizon) down to 50 cm soil depth for pure spruce and mixed species

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T.W. Berger et al. / Forest Ecology and Management 159 (2002) 3±14

Table 3 Mean differences between pure spruce stand and mixed species stands (pure minus mixed) for C, N, Ca and Al stores (g m C/N ratio and pH (CaCl2). If no level of signi®cance is given, differences are not signi®cant ( p > 0:10) Soil horizon (cm)

C

N

C/N

pH

Flysch Forest floor 0±5 5±10 10±20 20±30 30±40 40±50

166.1(*) 404.3** 252.2* 626.9** 227.0 242.9* 135.3*

6.8* 14.9 17.0 35.6* 15.1 16.3 13.4

7.9* 2.0 0.6 1.1 0.8 0.6 0.0

± 0.54** 0.32** 0.16(*) 0.13(*) 0.07 0.06

Molasse Forest floor 0±5 5±10 10±20 20±30 30±40 40±50

357.3(*) 20.0 208.9 233.3 305.8 57.3 78.1

10.9(*) 16.7 5.9 3.8 6.9 4.6 7.4

0.8 3.7* 0.9 2.0 1.4 0.5 0.7

± 0.11(*) 0.12 0.07 0.08 0.01 0.03

Ca

2

per soil horizon), Al

1.3 11.1 3.2 11.1 3.7 10.5 15.6 1.4* 0.3 0.2 0.7 0.1 0.0 0.1

± 7.3* 4.7 7.4 7.3 6.8 6.1 ± 0.2 1.2 2.6 4.3 1.0 0.4

(*)

p < 0:10. p < 0:05. ** p < 0:01. *

upper mineral soil under pure spruce indicates higher accumulation rates for C than for N, because, e.g., sulfur and phosphorus are necessary for C sequestration as well. To sequester 10 000 kg of carbon in humus (slowly decomposed organic matter), 833 kg of nitrogen, 200 kg of phosphorus, and 143 kg of sulfur are needed (Himes, 1998). Faster decomposition of N-rich components (leaves) of the forest ¯oor on Flysch

Fig. 2. Nitrogen stores of individual soil horizons (kg m the bedrock materials Flysch and Molasse.

2

than on Molasse due to more favorable soil conditions (e.g., higher pH) leaves heavier components with a wider C/N ratio (branches, fruit capsules), resulting in signi®cantly lower C/N ratios of the forest ¯oor under pure spruce than under mixture of spruce and beech. As expected, top soil layers showed most clearly feedback effects from stand composition (Tables 1 and 3). Spruce, having a highly protected long living

per horizon) down to 50 cm soil depth for pure spruce and mixed species stands on

T.W. Berger et al. / Forest Ecology and Management 159 (2002) 3±14

foliage, causes slow decomposition of its litter, build up of forest ¯oor and sequestration of nutrients in organic matter. Hence, acidi®cation starts at the top layer, which is documented in signi®cantly lower soil pHs under pure spruce than under mixture of spruce and beech (Table 3). Acidifying effects of species composition were much more pronounced on Flysch (signi®cant differences down to 30 cm soil depth) than on Molasse (upper 5 cm only). The fact that acidifying

9

effects of pure spruce are hardly visible at low pH is not only the consequence of the logarithmic pH-scale but also the consequence of general low base content on Molasse, which does not allow sequestration of large amounts of base cations in the canopy and forest ¯oor, lowering soil pH. How species composition affects C and N sequestration in forest ¯oor and mineral soil (0±50 cm) separately, is derivable from Fig. 3, where mean C and N stores of all 36 study sites

Fig. 3. Organic carbon (kg m 2) and nitrogen (g m 2) stores of the forest ¯oor, mineral soil (0±50 cm) and the whole soil pro®le (Total). Data of 18 pure spruce stands and 18 mixed species stands on two different bedrock materials (Flysch, Molasse) are given in increasing order.

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T.W. Berger et al. / Forest Ecology and Management 159 (2002) 3±14

(18 pairs) are ranked in ascending order. Hence, in general, pairs of pure spruce and mixed species stands are not plotted at the same x-coordinates. In all the cases, C and N stores of pure spruce stands are higher, never crossing the line of the mixed species stands. While carbon accumulation is closely linked to the bedrock material (higher ranks for acidic soils on Molasse), no such trend is visible for nitrogen (compare results of Table 2). At both extremes of the range, which correspond to strongly acidic and neutral, differences of carbon stores are declining. As pointed above, we expected increasing and not decreasing C and N stores in the lower mineral soil horizons due to admixture of beech, because natural mixed forests of F. sylvatica and P. abies root to deep mineral soil horizons. It might be argued, that this effect will be seen in horizons below 50 cm soil depth, which were not taken into consideration for this study. Schmid and Kazda (2000) studied the distribution of Norway spruce roots in monocultures and in mixtures with European beech for one representative pair of our study sites both on Flysch and Molasse. Single species stands of spruce and beech had a bulk of their roots in the upper 40 cm soil; more than 90% of roots of both species was found in the soil above 60 cm. Total number of roots down to 1 m depth was nearly the same in monospeci®c stands of spruce and beech as in the mixed stand of both species. However, the distribution pattern was changed in the mixed species stand: root systems of spruce were then concentrated in the upper soil, while beech roots occurred predominantly in deeper soil horizons. Hence, for our study it is justi®ed to conclude that the measured difference of Ca and N stores between species composition in the upper 50 cm of the soil pro®le will not be balanced by including deeper soil horizons (i.e., down to 1 m soil depth). This is in accordance to Schlenker et al. (1969) and Miehlich (1970) who found higher C stores for the whole 1 m soil pro®le under a ¯at rooting spruce stand than under a deep rooting beech/oak stand, where humus was concentrated in the upper soil under spruce and in the lower soil under the broadleaf trees. Also Chen and Glatzel (1988) reported that higher C stores under spruce than under beech down to 50 cm soil depth were attributable to differences in the upper 20 cm soil pro®le only, while C stores of the 20±50 cm pro®le were similar.

The above explanations would explain similar C and N stores under spruce stands and under mixed species stands, but not increased C and N stores in lower mineral soil horizons under spruce. Hence, other hypotheses are put forward: (a) due to lowered soil pH and consequently reduced soil microbial activity under spruce (see pH, Ca and Al differences of Table 3) decomposition is retarded, increasing total C stores; (b) since spruce is more sensible to waterlogging than beech, higher turnover rates of root growth (dying during anaerobic condition followed by quick recovery during aerobic conditions) cause the observed differences; (c) beech incorporates more carbon into its living biomass, reducing soil C stores; (d) additional C and N input under spruce is caused by the decaying old deep root system of former mixed species stands; (e) soil compaction below the ¯at root system of spruce reduces soil aeration (retarding C decomposition) and increases bulk densities of the lower mineral soil (Friedrich, 1992; Berger and Hager, 2000) causing higher C and N stores under pure spruce. 3.3. Factors controlling soil carbon and nitrogen stores 3.3.1. Bivariate correlations Carbon and nitrogen stores as well as C/N ratios were regressed against the following parameters individually: total stores of phosphorus and sulfur; exchangeable (BaCl2) stores of calcium, magnesium, potassium, sodium, manganese, iron and aluminum; pH, stand age, and admixture of spruce. Selected bivariate correlation coef®cients are given in Table 4 for the top soil (forest ¯oor and 0±5 cm mineral soil) because top soil layers showed most clearly feedback effects from stand composition (see Table 3), and for the total soil pro®le (forest ¯oor and 0±50 cm mineral soil), because the whole rooted zone must be taken into consideration for managing carbon and nitrogen. Increasing stand age increased C and N sequestration in the forest ¯oor on acidic soils on Molasse, since spruce, having a highly protected long living foliage, causes slow decomposition of its litter and build up of forest ¯oor over time. The sum of both exchangeable base and acid cations were positively correlated with C and N stores in the forest ¯oor on both bedrock materials, which is not surprising, since organic carbon

T.W. Berger et al. / Forest Ecology and Management 159 (2002) 3±14

11

Table 4 Bivariate correlation coef®cients between C and N stores (g m 2) as well as C/N ratios and sum of exchangeable base cations (Ca, Mg, K, Na) and acid cations (Mn, Al, Fe) (g m 2), stand age (years), admixture of spruce (% of growth volume) and total stores of carbon, nitrogen, phosphorus and sulfur (g m 2) (signi®cant correlation coef®cients …p < 0:05† are printed in bold letters) C

N

C/N

Flysch

Molasse

Flysch

Molasse

Forest floor Base cations Acid cations Stand age Spruce (%)

‡0.64 ‡0.81 ‡0.21 ‡0.55

‡0.79 ‡0.80 ‡0.50 ‡0.43

‡0.59 ‡0.87 ‡0.32 ‡0.53

‡0.79 ‡0.81 ‡0.50 ‡0.39

0.18 0.51 0.29 0.26

‡0.46 ‡0.37 ‡0.32 ‡0.37

Top mineral soil (0±5 cm) Base cations Acid cations Stand age Spruce (%)

0.23 ‡0.51 ‡0.17 ‡0.59

‡0.54 ‡0.35 ‡0.30 ‡0.30

‡0.09 ‡0.22 ‡0.18 ‡0.30

‡0.66 ‡0.15 ‡0.26 0.17

0.35 ‡0.39 0.03 ‡0.53

‡0.01 ‡0.37 ‡0.15 ‡0.60

Total soil profile Base cations Acid cations Stand age Spruce (%) Phosphorus Sulfur Carbon Nitrogen

0.02 ‡0.38 0.00 ‡0.66 ‡0.62 ‡0.70 ± ‡0.70

‡0.39 ‡0.78 ‡0.36 ‡0.53 0.30 ‡0.56 ± ‡0.44

‡0.45 ‡0.32 0.01 ‡0.32 ‡0.71 ‡0.92 ‡0.70 ±

‡0.23 ‡0.35 0.09 ‡0.17 ‡0.17 0.20 ‡0.44 ±

0.64 0.10 0.03 ‡0.29 0.32 0.49 ‡0.12 0.61

‡0.18 ‡0.46 ‡0.41 ‡0.37 0.43 ‡0.75 ‡0.61 0.45

builds up part of the soil exchange complex. However, base and acid cations tended to increase the C/N ratio on Molasse but to decrease this ratio on Flysch. In the top mineral soil base cations stores were positively correlated with carbon stores on Molasse, indicating that organic compounds play the dominant role of the soil exchange complex on acidic soils; on Flysch, this relationship seemed to be inverse probably due to higher rates of carbon decomposition with increasing base cation stores. The following facts were recorded for the total soil pro®le: admixture of spruce was positively correlated with total carbon content and tended to increase total nitrogen stores on both bedrock materials. In general, correlations between C and N were higher on Flysch (R ˆ 0:70; p < 0:01) than on Molasse (R ˆ 0:44; p < 0:10). The lack of signi®cant correlations between soil nitrogen and other parameters on Molasse (e.g., C, P, S; Table 4) is probably caused by higher heterogeneity of root distributions (root clumping and root free zones) in nutrient poor soils than in nutrient rich soils (Puhe, 1994). Signi®cant correlation between the sum of acid cations and C stores on Molasse (R ˆ 0:78;

Flysch

Molasse

p ˆ 0:000) strengthens the role of acidic soil conditions for accumulation of organic carbon. On Flysch, base cations favored N storage (R ˆ 0:45; p ˆ 0:06). While base cations decreased the C/N ratio on Flysch (preferentially increase of N over C), acid cations increased the C/N ratio on Molasse (preferentially increase of C over N). This is consistent with the fact that the C/N ratio is primarily controlled by nitrogen on Flysch (R ˆ 0:61; p < 0:01) but by carbon on Molasse (R ˆ ‡0:61; p < 0:01). 3.3.2. Stepwise regressions In addition to the above bivariate correlations, stepwise regressions were performed to select the driving forces of total carbon and nitrogen stores (dependent variables; Table 5). Stepwise variable entry or removal (SPSS Inc., 1993) examines the variables in the block at each step for entry or removal, taking inter-correlation between variables into consideration. Carbon stores of the whole soil pro®le were best predicted by the independent variables admixture of spruce, stores of phosphorus, sodium and calcium as well as stand age on Flysch (adjusted R2 ˆ 0:88;

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T.W. Berger et al. / Forest Ecology and Management 159 (2002) 3±14

Table 5 Results of stepwise regressions selecting driving forces (independent variables) of total carbon and nitrogen stores (forest ¯oor and 0±50 cm mineral soil) on the bedrock materials Flysch and Molasse (signi®cance of adjusted coef®cients of determination (R2) and partial regression coef®cients) Dependent variable Carbon

Nitrogen

Substrate

R2 ***

Independent variables

Coefficients

(Constant) Admixture of spruce Phosphorus Sodium Calcium

5174.0*** 1571.4*** 31.8*** 52.7*** 2.3**

Stand age (Constant) Aluminum

15.1* 3463.2* 82.6***

Flysch

0.88

Molasse

0.59***

Flysch

0.87***

(Constant) Sulfur Base cations

Molasse

N.S.

(No model)

122.9* 4.9*** 0.11*

*

p < 0:05. p < 0:01. *** p < 0:001. N.S.: not signi®cant, p > 0:05. **

N stores were not included in runs for C); on Molasse, the only given predictor was aluminum storage …R2 ˆ 0:59†, indicating that acidic soil conditions favor accumulation of organic carbon. These results support the role of cation bridges for organomineral interactions, mainly Ca, in neutral and alkaline soils, Al in acidic soils and sorption of organic material on iron oxide surfaces (Oades, 1988). No model was given for N stores on Molasse, probably caused by higher heterogeneity of root distributions in nutrient poor soils as discussed earlier. However, on Flysch, total nitrogen stores were best explained by stores of sulfur and the sum of base cations (R2 ˆ 0:87; C stores were not included in this run), strengthening the above ®nding that base cations are key factors controlling nitrogen retention and release of forest ecosystems. 4. Conclusions Soil data of secondary pure spruce stands and adjacent mixed stands of spruce and beech on comparable sites were used to study effects of species composition and other driving forces on carbon and nitrogen stores. Admixture of spruce versus beech increased carbon and nitrogen stores as well as the

corresponding C/N ratios within the upper 50 cm of the soil. Hence, increasing nitrogen storage at all means might be a dangerous strategy, because a breakdown or conversion of these secondary spruce stands could lead to a vast ef¯ux of nitrate. In nutrient rich soils increasing base cation stores increased nitrogen stores, narrowing the C/N ratios. If this relationship proves to be causal, soil acidi®cation would eventually decrease the nitrogen storage capacity of the studied soils. Acknowledgements This research was conducted as part of the Special Research Program ``Forest Ecosystem Restoration'' (SFB 008), funded by the Austrian Science Foundation (FWF). References Adger, W.N., Brown, K., Shiel, R.S., Whitby, M.C., 1992. Carbon dynamics of land use in Great Britain. J. Environ. Manage. 36, 117±133. Anderson, J.M., 1991. The effects of climate change on decomposition processes in grassland and coniferous forests. Ecol. Appl. 1, 326±347.

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