Carbon and nitrogen in forest floor and mineral soil ...

Forest Ecology and Management 255 (2008) 35–48 www.elsevier.com/locate/foreco

Carbon and nitrogen in forest floor and mineral soil under six common European tree species Lars Vesterdal *, Inger K. Schmidt, Ingeborg Callesen 1, Lars Ola Nilsson, Per Gundersen Forest & Landscape Denmark, University of Copenhagen, Hørsholm Kongevej 11, DK-2970 Hørsholm, Denmark Received 14 March 2007; received in revised form 21 August 2007; accepted 21 August 2007

Abstract The knowledge of tree species effects on soil C and N pools is scarce, particularly for European deciduous tree species. We studied forest floor and mineral soil carbon and nitrogen under six common European tree species in a common garden design replicated at six sites in Denmark. Three decades after planting the six tree species had different profiles in terms of litterfall, forest floor and mineral soil C and N attributes. Three groups were identified: (1) ash, maple and lime, (2) beech and oak, and (3) spruce. There were significant differences in forest floor and soil C and N contents and C/N ratios, also among the five deciduous tree species. The influence of tree species was most pronounced in the forest floor, where C and N contents increased in the order ash = lime = maple < oak = beech spruce. Tree species influenced mineral soil only in some of the sampled soil layers within 30 cm depth. Species with low forest floor C and N content had more C and N in the mineral soil. This opposite trend probably offset the differences in forest floor C and N with no significant difference between tree species in C and N contents of the whole soil profile. The effect of tree species on forest floor C and N content was primarily attributed to large differences in turnover rates as indicated by fractional annual loss of forest floor C and N. The C/N ratio of foliar litterfall was a good indicator of forest floor C and N contents, fractional annual loss of forest floor C and N, and mineral soil N status. Forest floor and litterfall C/N ratios were not related, whereas the C/N ratio of mineral soil (0–30 cm) better indicated N status under deciduous species on rich soil. The results suggest that European deciduous tree species differ in C and N sequestration rates within forest floor and mineral soil, respectively, but there is little evidence of major differences in the combined forest floor and mineral soil after three decades. # 2007 Elsevier B.V. All rights reserved. Keywords: Tree species; Carbon; Nitrogen; C/N ratio; Forest floor; Fractional annual loss; Mineral soil; Common garden design

1. Introduction The influence of tree species on forest soil properties has for a long time been studied by ecologists (Mu¨ller, 1887; Zinke, 1962; Binkley, 1995). The interest has mainly been focused on soil fertility parameters and possible environmental problems, e.g. following deposition of nitrogen and heavy metals. Recently, the role of soil carbon (C) pools for mitigation of greenhouse gases has highlighted the need for more knowledge on tree species effects (Jandl et al., 2007). Forest management, including a change in tree species, has been accepted as a measure for mitigation of atmospheric CO2 in national greenhouse gas budgets. However, quantitative estimates of * Corresponding author. Tel.: +45 35281672; fax: +45 35281517. E-mail address: [email protected] (L. Vesterdal). 1 Present address: Risø National Laboratory, Technical University of Denmark, Frederiksborgvej 399, P.O. Box 49, DK-4000 Roskilde, Denmark. 0378-1127/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2007.08.015

tree species effects on soil C pools are still scarce. Several other factors such as soil type and climate influence changes in soil C pools, and experimental plots that limit the influence of such factors are extremely rare. Another important function of soils is their ability to retain nitrogen (N). Soil N pools and C/N ratios are important parameters for assessment of tree species effects on ecosystem functioning. Nitrogen pools and C/N ratios have recently been suggested as indicators of C sequestration potential in soils (Akselsson et al., 2005; de Vries et al., 2006), and soil C/N ratios are recognized as good indicators of nitrate leaching to ground and surface waters (Gundersen et al., 1998; Dise et al., 1998). The variability in soil C/N ratios and N retention can be closely linked with variation in tree species composition (Lovett et al., 2002). Several studies have reported the influence of tree species on microbial processes related to C and N cycling (Blagodatskaya and Anderson, 1998; Smolander and Kitunen, 2002; Menyailo et al., 2002).

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L. Vesterdal et al. / Forest Ecology and Management 255 (2008) 35–48

Many European countries currently experience a change in forest policy towards use of native tree species adapted to local climate for use in continuous cover forestry with natural regeneration (Larsen and Nielsen, 2007). This often translates into conversion from coniferous species to deciduous species. However, little specific knowledge is available on deciduous species to support conclusions regarding the influence of a species change on soil C and N pools. Most of the research has so far aimed at evaluating general soil differences between coniferous trees and selected deciduous trees (Fried et al., 1990; Wilson and Grigal, 1995; Alriksson and Eriksson, 1998). In central and western parts of Europe beech and spruce are main species, and more C has been reported in soils under spruce than under beech (Nihlga˚rd, 1971; Berger et al., 2002). The knowledge of soil C and N pools under common European broadleaves other than beech is more scarce and inconclusive (Nordeń, 1994; Neirynck et al., 2000; Hagen-Thorn et al., 2004) whereas studies in North America indicated strong differences in soil C and N between beech, maple and oak species (Finzi et al., 1998; Lovett et al., 2004). Soil C and N contents are determined by differences between rates of input and rates of output from soils. The tree species is one of several possible factors driving input and output rates of C and N. Most ideas of tree species influence have been based on comparative studies of tree species growing under different site conditions (Binkley, 1995). Therefore, influences of tree species could be confounded with prior differences in soil conditions such as parent material or land use. Common garden experiments provide an opportunity to minimize such confounding effects as the same tree species are planted in adjacent blocks so that climate, parent material, time, hydrology and previous land use are almost the same. However, common garden experiments are rare (Binkley and Giardina, 1998; Hobbie et al., 2006) and often without replication (Augusto et al., 2002; Oostra et al., 2006). Consequently, little research has been able to address tree species effects by use of balanced common garden designs where species differences are not confounded with spatial variability. In most cases, however, generalizations are hampered by no, limited or unbalanced tree species replication (Nordeń, 1994; Augusto et al., 2002; HagenThorn et al., 2004) or by soil or other site properties being confounded with the possible species effect (Neirynck et al., 2000; Lovett et al., 2004). This highlights the need for replicated common garden experiments in order to address the influence of tree species on soil C and N pools. Previous studies on soil properties seldom focused on both forest floor and mineral soil. Large variability in forest floor C and N content between deciduous and coniferous species has been reported (Ovington, 1954; Vesterdal and RaulundRasmussen, 1998). Studies of C and N contents in the top mineral soil have shown less effect of individual broadleaved trees in mixed stands (Nordeń, 1994) and also in common garden design (Hagen-Thorn et al., 2004). Tree species influence is often first detectable in forest floors whereas mineral soil differences emerge later (Vesterdal et al., 2002). A recent study indicated that deciduous species with large forest floor C pools stored less C in top mineral soils and vice versa

(Oostra et al., 2006). Conclusions on total soil C and N pools may therefore not be supported by previous separate studies of forest floors or mineral soils. In this study our objective was to explore the influence of some common European tree species on both forest floor and mineral soil C and N contents and C/N ratio. We studied litterfall, forest floor and top mineral soil in a 30-year-old common garden experiment with five European deciduous species and Norway spruce replicated at six sites. This unique experiment allowed us to evaluate whether consistent differences developed in soil C and N attributes below these six tree species across different soil types. The specific aims were to evaluate (1) if consistently different C and N contents and C/N ratios had developed after three decades for both forest floor and mineral soil, and (2) the influence of tree species on forest floor C and N turnover. 2. Materials and methods 2.1. Common garden design Soil C and N was evaluated in a common garden design based on monoculture stands of six tree species replicated at six sites. The six tree species included five common European broadleaves: beech (Fagus sylvatica L.), pedunculate oak (Quercus robur L.), lime (Tilia cordata L.), sycamore maple (Acer pseudoplatanus L.) and ash (Fraxinus excelsior L.). Norway spruce (Picea abies (L.) Karst.) represented conifers and served as a reference to previous studies on this species. There was no replication of tree species within each site, i.e. there was one stand of each species present in each site. Five of the six sites belonged to an experiment on performance of various broadleaves established by Forest & Landscape Denmark in 1973 (Mattrup, Odsherred, Vallø, Viemose, Wedellsborg), while the last site (Kragelund) had an unreplicated tree species experiment established by Silkeborg Plantation Association in 1961. The common garden design is almost complete; at the Vallø site, the ash stand was missing as its establishment failed due to deer browsing. Table 1 summarizes the most important site properties. Previous land use was beech forest except at Kragelund and Mattrup where stands were planted on former arable land. The sites were distributed throughout Denmark, but climatic variation is fairly limited within the small and flat country. Annual mean precipitation varied the most (579–783 mm) while annual mean temperature just ranged from 7.5 to 8.1 8C. Except the site Kragelund, all sites had soils developed from deposits from the most recent glaciation and therefore were classified according to Soil Survey Staff (1998) as Hapludalfs or Argiudolls with coarse-loamy to fine-loamy texture and relatively high pH and base saturation. The Kragelund site had a more sandy soil (Hapludult) with low base saturation as it was developed from aeolian sand deposits covering the glacial till. All stands are planted on almost level or weakly undulating ground. In the five sites belonging to the Forest & Landscape Denmark tree species experiment, individual tree species plots were approximately 0.25 ha and had been thinned


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Table 1 Selected climate and soil properties at the six sites Site

Location

Kragelund Mattrup Odsherred Vallø Viemose Wedellsborg

568100 N, 558570 N, 558500 N, 558250 N, 558010 N, 558240 N,

98250 E 98380 E 118420 E 128030 E 128090 E 98520 E

Precipitationa (mm yr1)

Annual temperature (8C)

Soil typeb (USDA)

Clayc (%)

Silt (%)

Sand (%)

pHCaCl2 c

BSe (%)

Extractable Pc (mg kg1)

Previous land use

703 783 579 625 603 658

7.5 7.5 8.0 7.7 8.1 7.8

Arenic Hapludult Lamellic Hapludalf Oxyaquic Hapludalf Oxyaquic Hapludalf Typic Hapludalf Oxyaquic Argiudoll

8 17 11 17 25 30

7 9 11 13 20 30

85 74 78 70 55 40

4.4 4.6 4.2 5.4 4.4 6.8

18 76 68 98 84 100

30 34 281 187 77 222

Agriculture Agriculture Beech Beech Beech Beech

a

Means 1961–1990 from nearby stations (Danish Meteorological Institute). Soil classification is based on Soil Survey Staff (1998). c Clay: 0.9), indicating faster rates of turnover with higher foliar litterfall N status (Fig. 2c–d). 3.3. Mineral soil C and N While forest floor C and N contents were strongly and mainly influenced by tree species, this was opposite for mineral soil C and N attributes which were most affected by the site (P < 0.001). Focusing on tree species differences, bulk density

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Fig. 1. Forest floor C and N contents and C/N ratios. (a) C content, (b) N content, (c) C/N ratio of the total forest floor, (d) C/N ratios of foliar and non-foliar fractions of forest floors. Error bars indicate S.E.M. (n = 6 except ash n = 5). Bars with different letters indicate significantly different (P < 0.05) species means based on Tukey–Kramer tests.

differed significantly only in the 5–15 cm layer (P = 0.012) and was higher under beech than under ash (Table 4). Mineral soil C concentrations were not quite significantly affected by tree species in 0-5 cm (P = 0.098) and 5–15 cm (P = 0.08), but differences were marginally significant in the 15–30 cm segment (P = 0.05). Ash and lime tended to have highest and beech and spruce tended to have lowest C concentrations in the 15–30 cm depth segment. Nitrogen concentrations differed significantly among tree species in 5–15 cm (P = 0.018) and 15–30 cm (P = 0.033) layers. In 5–15 cm, ash had higher soil N concentration than beech, lime and spruce, and in 15–30 cm ash and lime had higher N concentration than beech and spruce. The contents of C and N were significantly affected by tree species only in 15–30 cm (P < 0.03) and were significantly higher under ash and lime than under spruce (Table 4). Ash also had significantly more N in 15–30 cm than beech. Mineral soil C/N ratios were significantly affected by tree species in 0–5 cm (P < 0.001) and 5–15 cm (P = 0.008). In 0– 5 cm, spruce had significantly higher C/N ratio than all the broadleaves (Table 4). Among broadleaves, ash and maple had lower C/N ratios than beech and oak, and ash also had significantly lower C/N ratio than lime. In 5–15 cm, ash and maple had lower C/N ratios than spruce. In the combined mineral soil (0–30 cm) there was no significant effect of tree species on C content, but a significant

effect of tree species on N content (P = 0.02) and C/N ratio (P < 0.001). Ash had significantly more N in 0–30 cm than spruce and beech, and C/N ratios under ash and maple were significantly lower than under spruce (Fig. 3). The C/N ratio of the whole sampled soil compartment differed significantly among tree species (P < 0.001); spruce had a higher C/N ratio than all broadleaves, and ash had significantly lower C/N ratio than beech, lime and oak. Carbon and N contents of the studied soil as a whole were not significantly affected by tree species (P = 0.1). The differences in forest floor C and N contents among tree species appeared to be offset by differences in mineral soil C and N. This was supported by significant negative relationships (Fig. 4) between forest floor C and mineral soil C in 15–30 cm (P = 0.028, R2 = 0.74) and between forest floor N and mineral soil N in 15–30 cm (P = 0.019, R2 = 0.78). Forest floor N content was also negatively related to mineral soil N in the combined 0–30 cm layer (P = 0.040, R2 = 0.69). The differences in mineral soil N status among tree species (Table 3) were clearly related to litterfall N status. The best significant relationships between litterfall and mineral soil N status in sub-layers within 0–30 cm are shown in Fig. 5 Soil C and N contents and C/N ratios were also strongly influenced by site-related factors, and standard errors in Fig. 4 and Table 4 may for the most part be attributed to variability


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Fig. 2. Relationships between foliar litterfall C/N ratio and (a) forest floor total C content, (b) forest floor total N content, (c) fractional annual loss (k) of forest floor foliar C, (d) fractional annual loss of forest floor foliar N. Each point is a species mean based on six stands except for ash (n = 5) and spruce (n = 2). Error bars indicate S.E.M. A: ash, B: beech, L: lime, M: maple, O: oak, S: spruce.

Table 4 Mineral soil bulk density, C and N concentrations, C and N contents, and C/N ratios in three layers of the mineral soil Bulk density (g cm3)

Carbon (mg g1)

Nitrogen (mg g1)

Carbon (Mg ha1)

Nitrogen (Mg ha1)

C/N

0–5 cm Ash Beech Lime Maple Oak Spruce

0.80 0.89 0.80 0.88 0.77 0.87

(0.06) (0.07) (0.07) (0.06) (0.08) (0.08)

36.7 36.3 39.6 33.7 43.0 43.7

(4.5) (4.6) (5.5) (3.9) (6.2) (8.1)

2.87 2.44 2.73 2.48 2.86 2.58

(0.40) (0.30) (0.34) (0.31) (0.37) (0.47)

14.1 15.3 14.9 14.2 15.5 17.0

(1.3) (1.4) (1.4) (1.0) (1.5) (2.3)

1.10 1.04 1.04 1.05 1.04 1.01

(0.13) (0.12) (0.10) (0.10) (0.12) (0.15)

13.0 15.1 14.5 13.7 15.0 17.7

(0.6)d (0.7)b (0.8)bc (0.5)cd (0.7)b (1.3)a


1.11 1.26 1.14 1.18 1.16 1.23

(0.06)b (0.04)a (0.04)ab (0.06)ab (0.05)ab (0.07)ab

25.9 19.1 19.9 21.5 23.5 19.5

(4.2) (2.0) (2.0) (3.2) (2.6) (3.4)

2.04 1.42 1.41 1.66 1.66 1.35

(0.40)a (0.20)b (0.20)b (0.28)ab (0.23)ab (0.28)b

27.3 23.1 22.0 23.9 26.4 22.6

(3.4) (2.2) (2.3) (2.6) (2.7) (3.1)

2.13 1.71 1.55 1.85 1.88 1.56

(0.34) (0.24) (0.23) (0.28) (0.29) (0.28)

13.3 14.2 14.8 13.5 14.7 16.1

(0.9)b (1.2)ab (1.2)ab (0.9)b (1.1)ab (2.0)a


1.43 1.48 1.41 1.40 1.40 1.44

(0.05) (0.05) (0.03) (0.07) (0.04) (0.05)

14.8 10.9 15.0 13.4 13.5 10.5

(2.4) (0.9) (2.9) (1.8) (1.0) (1.8)

1.23 0.83 1.11 1.04 1.02 0.80

(0.27)a (0.13)b (0.24)a (0.17)ab (0.17)ab (0.17)b

29.6 22.7 30.2 26.1 27.1 21.7

(4.4)a (2.4)ab (5.3)a (2.3)ab (2.6)ab (3.3)b

2.49 1.75 2.25 2.04 2.07 1.66

(0.49)a (0.33)bc (0.46)ab (0.30)abc (0.38)abc (0.33)c

12.7 14.2 14.5 13.6 14.3 13.6

(1.3) (1.6) (1.5) (1.2) (1.5) (1.3)

Species

Values are means (S.E.M.) of six samples (except ash, n = 5). Values within a soil layer followed by different letters are significantly different (P < 0.05) based on Tukey–Kramer tests.

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Fig. 3. Mineral soil C and N content (Mg ha1), and C/N ratio in 0–30 cm. Error bars indicate S.E.M. (n = 6 except ash n = 5). Bars with different letters indicate significantly different (P < 0.05) species means based on Tukey–Kramer tests.

among sites. In an analysis of covariance with species as nominal variable and various site-related mineral soil properties as covariate, forest floor C and N contents were significantly negatively related to pH (P < 0.01), exchangeable Ca (P < 0.001) and clay content (P < 0.05) in 50–100 cm. The relationships between mineral soil C and N and soil properties such as P content and clay content were even stronger. There were positive simple linear relationships between mineral soil extractable P (50–100 cm) and contents of C (P < 0.001, R2 = 0.50) and N (P = 0.002, R2 = 0.26), and positive relation-

ships between clay content (50–100 cm) and mineral soil C (P = 0.022, R2 = 0.15) and N content (P < 0.001, R2 = 0.32). 4. Discussion 4.1. Tree species profiles Three decades after planting the six tree species formed three groups according to litterfall, forest floor and soil C and N attributes: (1) ash, maple and lime, (2) beech and oak, and (3)


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Fig. 4. Relationships between (a) total forest floor C content and mineral soil C content in the 15–30 cm layer and (b) total forest floor N content and mineral soil N content in 15–30 cm layer. Each point is a species mean based on six stands except for ash (n = 5) and spruce (n = 2). Error bars indicate S.E.M. A: Ash, B: beech, L: lime, M: maple, O: oak, S: spruce.

spruce. The tree species profiles in Fig. 6 synthesize the differences in C and N attributes. The attributes of ash, maple and lime indicated strategies of more N-rich litter input, low storage of C and N in forest floors due to high rates of C and N turnover, high N status in mineral soil, and above average

mineral soil C and N storage (for ash). Oak was most outstanding in its lower than average forest floor C and N contents, low forest floor C/N ratio, but still lower than average turnover of forest floor N. The beech profile indicated less Nrich litter input, lower than average rates of C and N turnover,

Fig. 5. The best relationships found between N status for various mineral soil layers and foliar litterfall N status. Each point is a species mean based on 6 stands except for ash (n = 5) and spruce (n = 2). Error bars indicate S.E.M. A: Ash, B: beech, L: lime, M: maple, O: oak, S: spruce.

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4.2. Distribution of C and N between forest floor and mineral soil

Fig. 6. Species profiles of relative values of C and N in litterfall, forest floors, and mineral soils. The y-axes represent the species mean for each variable relative to the mean across all six species, i.e. the value of an attribute for each species as a relative derivation from the mean of all species. The species are listed from top to bottom according to the characteristics of their profile. Abbreviations on the x axis are as follows: LFC, litterfall C (Mg ha1 yr1); LFN, litterfall N (kg ha1 yr1); LFC/N, litterfall C/N ratio; FFC, forest floor C, Mg ha1; FFN, forest floor N (kg ha1); kC, fractional annual loss of C (yr1); kN, fractional annual loss of N (yr1); MINC, mineral soil C (Mg ha1); MINN, mineral soil N (Mg ha1); MINC/N, mineral soil C/N ratio.

and low mineral soil C and N contents. This altogether suggests a strategy of slower C and N cycling leading to higher storage of C and N in forest floor than mineral soil in beech compared to ash, maple and lime. Spruce was the most extreme species in all C and N attributes, showing a profile of less N-rich litter, an even greater tendency to accumulate C and N in forest floor instead of mineral soil than beech, and N-poor mineral soil (high C/N ratio). These species profiles are much in line with reports from European studies (Reich et al., 2005; Oostra et al., 2006) and North American studies (Finzi et al., 1998; Dijkstra and Fitzhugh, 2003; Lovett et al., 2004) of the genera Fraxinus, Acer, Quercus and Fagus.

The influence of tree species was most pronounced in the forest floor where C contents differed 10-fold and N contents differed 20-fold between spruce and ash (Fig. 1a–b). Tree species influenced mineral soil significantly only in some of the sampled mineral soil layers, but species with little forest floor C and N content had more C and N in the mineral soil (Fig. 4). This opposite trend probably offset the differences in forest floor C and N with no significant difference between tree species in C and N contents for the combined forest floor and mineral soil. The larger C and N stocks in mineral soil could also have diluted the pattern observed in forest floors. However, such opposite allocation of C and N in soil was also noted by Finzi et al. (1998) in a study of hemlock and North American species of maple, ash, beech, and oak, where the least forest floor C and N but the most mineral soil C and N were found under maple and ash. In a Swedish unreplicated tree species experiment oak and ash had more C in mineral soil than spruce and beech, while the pattern was reversed for the forest floor (Oostra et al., 2006). These results altogether suggest that distributions of C and N differ substantially within common European broadleaf species. We have no specific information to address the causes behind the observed pattern and can only suggest a few possible mechanisms behind. Earthworms are known to incorporate material from the forest floor into deeper soil horizons (Devliegher and Verstraete, 1997), and they may strongly affect soil C and N dynamics (Bohlen et al., 2004). A recent study of beech, lime, maple, oak, spruce and nine other tree species in a Polish common garden experiment revealed that maple and lime had high and spruce had low earthworm abundances, which was attributed to differences in litter Ca (Reich et al., 2005). Earthworm presence was noted at our sites during sampling, but was not quantified. Leaching of DOC from the forest floor is another pathway for C to enter deeper soil horizons (Fro¨berg et al., 2006). DOC leaching fluxes are usually greatest just below the forest floor and often associated with microbial biomass and activity (Smolander and Kitunen, 2002). However, we found no significant species differences in concentrations or contents of C and N in the 0–5 cm layer of the mineral soil. DOC input to the mineral soil was found to be high in stands with large amounts of forest floor C such as Norway spruce (Andersen et al., 2004). This contrasts with the tendency of less mineral soil C under spruce and beech in our study. DOC inputs were probably not the most important factor for the species differences in C distribution. Lastly, C and N may also be incorporated into the mineral soil at a higher rate in certain tree species due to different root distribution in the soil profile. Root litter may contribute to the soil C pool with an amount of C that equal that in foliar litterfall (Vogt et al., 1986; Rasse et al., 2005). Root systems of Norway spruce have a preference for forest floors (Puhe, 2003) and may therefore provide an input of root litter to this soil compartment. In the mineral soil, Oostra et al. (2006) found lower root biomass ( oak > maple, lime > ash (Melillo et al., 1982; Cotrufo et al., 1998; Lovett et al., 2004; Sariyildiz and Anderson, 2005; Kalbitz et al., 2006). The lignin/

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N ratio would probably also have been closely related to forest floor turnover and accumulation. Nevertheless, our results do support that high litterfall C/N ratios are indicative of large forest floor C sequestration and vice versa within the studied tree species. It is conceivable that the different relationships reported between litter N status and rate of turnover are due to strong interaction with the soil type and its inherent decomposer community. Much of the evidence for low mass loss of N-rich litters has come from boreal forests on relatively poor soil types (Berg et al., 2001). Five of our six sites were nutrient-rich sites, and the only site with more nutrient-poor soil was former arable land (Table 1). Our findings can therefore not be extrapolated to very nutrient-poor soil types. Future research in this field should better address site-related interactions between litter N status and forest floor accumulation. Differences in forest floor C/N ratios among the six tree species were not positively related to their litterfall N status. At the time of sampling, very little litter other than structural remnants with higher C/N ratio was present in forest floors of ash, maple, and lime as it had been lost by decomposition or incorporation in mineral soils. In beech and spruce stands, with more developed forest floors, the litter typically experiences relatively more N immobilization (greater decrease in C/N ratio) before it is decomposed (Gloaguen and Touffet, 1982). A comparison of forest floor C/N ratio among these different species is therefore questionable given the different stages of decomposition for the material. 4.4. Tree species differences in mineral soil C and N attributes The effect of tree species on C content of the mineral soil was weaker than that on forest floor C content. Our findings in a replicated common garden design confirm results from previous studies that species-induced differences in C contents are easily and relatively soon seen in forest floors but much less so in the mineral soil (Finzi et al., 1998; Neirynck et al., 2000; Vesterdal et al., 2002; Dijkstra and Fitzhugh, 2003). In terms of stability, C sequestered in the mineral soil is more desirable than C sequestration in forest floors which are more vulnerable to decomposition following disturbances and to burning during forest fires (Jandl et al., 2007). It is therefore relevant to identify tree species that accommodate C sequestration in the mineral soil rather than in forest floors. We did find more C in 15–30 cm under ash and lime than under spruce (Table 4) and an indication that species differences in total C contents might be underway (Fig. 3). It is conceivable that the studied tree species will develop larger differences in soil C content over a full rotation, but studies of soil conditions under older single trees have so far found little evidence of different C contents under European broadleaves (Nordeń, 1994; Neirynck et al., 2000). It is therefore unlikely that it will be possible to monitor any clear effects of management-related changes in tree species for use in reporting under the Kyoto Protocol, not even a drastic change from spruce to ash or maple. The six tree species exhibited stronger differences in mineral soil N content and especially C/N ratios after three decades.

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The significant relationships between litterfall N and mineral soil N (Fig. 5) strongly suggest that differences in soil N status can be attributed to litterfall N status of the individual species. Incorporation of N-rich litter under ash, maple and lime has likely increased soil N status relative to beech and spruce. Belowground litter input from roots probably also contributed to the differences developed. In one of the few other extensive European studies of mineral soil N attributes under broadleaves, Hagen-Thorn et al. (2004) found lower C/N ratios and higher N content in ash compared to beech and spruce, but the differences were not significant. In Belgium, Neirynck et al. (2000) also observed that maple had lower mineral soil C/N ratio than beech and attributed this to the faster turnover rate of maple litter and subsequent incorporation of N in mineral soil. From North American studies of ash, beech, maple and oak species, there is also evidence that N pools are greater and C/N ratios lower in mineral soils under ash, and maple (Finzi et al., 1998; Lovett et al., 2004). Studies of different tree species have suggested that uptake of nutrients and allocation to tree biomass can influence nutrient content in the soil (Alban, 1982; Eriksson and Roseń, 1994). We have no information on N accumulation in biomass, but it is possible that the high N content in mineral soil under ash (Fig. 3) was also due to lower productivity (Table 2) and thus less depletion of mineral soil N. 4.5. The C/N ratio as indicator of N status for the six tree species It is a challenge in present forest environmental research to identify useful parameters as indicators of N saturation (Gundersen et al., 2006). The C/N ratio of forest floors has been suggested as a good indicator of ecosystem N saturation and consequently N leaching. The evidence for this has primarily come from coniferous forest stands with mor humus layers (Gundersen et al., 1998; Dise et al., 1998) and not from deciduous stands with mull-like forest floors as in this study. We expect that the forest floor C/N ratio would be of little indicative value for N leaching between the six studied species since N is differently distributed in soils under the six species. Forest floor C/N ratio has recently been found to be less reliable as predictor of nitrate leaching in deciduous forests (Kristensen et al., 2004; Lovett et al., 2004; Nilsson et al., 2006). We suggest that the C/N ratios of the upper mineral soil better reflect litterfall N status (Fig. 5) and the possibility for N retention in deciduous forests, but other parameters should evidently also be evaluated. 4.6. Influence of site properties Site properties strongly affected mineral soil C and N attributes. Soil properties such as P concentration and clay content explained some of the site-related variability. The positive relationship between mineral soil C and N contents and clay content may be attributed to stabilization of organic matter by formation of stable complexes with clay minerals (Torn et al., 1997; Hagedorn et al., 2003). Forest floor C and N attributes were most affected by tree species, but consistent with another Danish common garden study (Vesterdal and Raulund-Rasmussen,

1998) forest floor C and N contents were negatively related to mineral soil fertility parameters such as pH, exchangeable Ca and clay content. Within the range of Danish soil types, these properties tend to be associated with high rates of litter decomposition (Vesterdal, 1999), which in turn result in low forest floor C and N contents. Vesterdal and Raulund-Rasmussen (1998) reported that C and N contents in oak forest floors were less affected by soil fertility than spruce and beech, but in this study with less variability in soil fertility, tree species differences were consistent along the gradient in soil fertility. 5. Conclusion Three decades after planting the six tree species formed three groups according to litterfall, forest floor and soil C and N attributes: (1) ash, maple and lime, (2) beech and oak, and (3) spruce. There were significant differences in soil C and N contents and C/N ratios, also within the five deciduous tree species. The influence of tree species was most pronounced in the forest floor, where C and N contents increased in the order ash = lime = maple < oak = beech spruce. Tree species influenced mineral soil only in some of the sampled soil layers. Species with little forest floor C and N content had more C and N in the mineral soil. This opposite trend probably offset the differences in forest floor C and N with no significant difference between tree species in C and N contents of the whole soil profile. The effect of tree species on forest floor C and N content was primarily attributed to large differences in turnover rates as indicated by fractional annual loss of forest floor C and N. The C/ N ratio of foliar litterfall was a good indicator of both forest floor C and N contents, fractional annual loss of forest floor C and N, and mineral soil N status. Forest floor and litterfall C/N ratios were not related, whereas the C/N ratio of mineral soil better indicated N status under deciduous species on rich soil. The results suggest that European deciduous tree species differ in C and N sequestration rates within forest floor and mineral soil, respectively, but there is little evidence of major differences in the entire soil profile after three decades. Acknowledgements This study was funded by the Danish Agricultural and Veterinary Research Council (project no. 23-03-0195) and the European Commission in the 5th Framework Program CNTER (contract no. QLK5-2001-00596). We are grateful to Bjo¨rn Berg, Jesper Riis Christiansen, Rasmus Dalhoff Andersen, Christina Kjærby, Morten Alban Knudsen, Mads M. Krag and Lena Byrgesen for help with field sampling and laboratory analyses. Bruno Bilde Jørgensen kindly provided the data on stand biomass. We thank Karin Hansen and Bo Elberling for helpful comments on the manuscript. References Akselsson, C., Berg, B., Meentemeyer, V., Westling, O., 2005. Carbon sequestration rates in organic layers of boreal and temperate forest soils—Sweden as a case study. Global Ecol. Biogeogr. 14, 77–84.

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