Limit values for plant litter decomposing in two contrasting soils—influence of litter elemental composition

Acta Oecologica 24 (2003) 295–302 www.elsevier.com/locate/actoec

Original article

Limit values for plant litter decomposing in two contrasting soils—influence of litter elemental composition Björn Berg a,*, Amalia Virzo De Santo b, Flora Angela Rutigliano c, Angelo Fierro b, Gunnar Ekbohm d a

Lehrstuhl für Bodenökologie, BITÖK, Postfach 101251, Universität Bayreuth, Dr. Hans Frisch Strasse 1-3, 95440 Bayreuth, Germany b Dipartimento Biologia Vegetale, Universita di Napoli, Via Foria 223, 80139 Naples, Italy c Dipartimento di Scienze Ambientali, Seconda Universita di Napoli, via Vivaldi 43, 81100 Caserta, Italy d Department of Statistics, P.O. Box 7013, Swedish University of Agricultural Sciences, 75007 Uppsala, Sweden Received 19 December 2002; accepted 20 August 2003

Abstract The decomposition dynamics of four types of needle litter and three types of leaf litter were followed for 3 years at two very contrasting coniferous forest systems, a nutrient-rich silver fir (Abies alba Mill.) forest in south Italy (Monte Taburno) and a nutrient-poor Scots pine (Pinus sylvestris L.) forest in central Sweden (Jädraås). Decomposition of the same litter type at the two sites did not differ in the early stages but proceeded further at the nutrient-rich forest site than at the nutrient-poor one. Limit values for decomposition were calculated and the differences for the same litter type between the two contrasting coniferous systems were investigated. At both sites six of the seven litter types gave significant (asymptotic) limit values for decomposition, which varied with litter type. For four litter types out of six the limit values differed significantly between the two sites and were always higher at the nutrient-rich site (Monte Taburno). Using all available data for litters incubated at the two sites revealed that at the nutrient-poor site (Jädraås) there was a significant negative relationship between litter N levels and limit values and there was also a significant negative relationship between initial concentrations of heavy metals (e.g. Zn, Cd, Cu) and limit values. In contrast, at the site Monte Taburno, rich in nutrients and in heavy metals, there was no such relationship. © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Litter; Decomposition; Nutrients; Heavy metals; Limit value; Humic surface horizon

1. Introduction Plant litter in temperate and boreal coniferous forests is decomposed through microbial activity and the quantitative contribution of microorganisms to decomposition is considered to be above 95% (Persson et al., 1980), with soil animals being responsible for the remaining maximum 5%. Thus the environmental factors most important in regulating the turnover rate of litter should be those that regulate the activity of microorganisms, i.e. soil temperature, soil moisture content, and the availability of nutrients and of the energy source. Part of the plant litter accumulates as remains that are recalcitrant or decompose extremely slowly. Couteaux et al. (1998) estimated the decomposition rate of the stable fraction of Scots pine needle litter to be of the magnitude of 1% in * Corresponding author. E-mail address: [email protected] (B. Berg). © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. doi:10.1016/j.actao.2003.08.002

30–300 years. An approach to estimating the size of such a slowly decomposing fraction is to use the limit value for accumulated mass loss (Berg et al., 1995, 1996). The limit values in different litter types were calculated as asymptotic values towards which the decomposition proceeds (Howard and Howard, 1974). An accumulation of soil organic matter (SOM) would take place at a rate dependent on the magnitude of the limit value, reflecting the magnitude of litter remains (Berg et al., 1995), as well as on the magnitude of litter fall. Berg et al. (1996) and Berg (1998, 2000a,b) related the limit values to litter concentration of N, Mn, and Ca which are nutrients regulating the lignin-degrading microflora (Eriksson et al., 1990). Lignin and modified lignin-like humification products make up an important fraction of the recalcitrant part of the litter. As litter decomposes, lignin concentrations increase as do those of N (Berg et al., 1997). Nitrogen accumulation in

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litter, at least in the early decomposition stages, may be enhanced by soil N levels (Virzo De Santo et al., 1998). The litters that accumulate more N might decompose more slowly in the later stages (Fog, 1988) or the decomposition process may go less far, i.e. reach a lower limit value (Berg et al., 1996). The negative effect on lignin decomposition rate of raised N concentrations was seen in a field experiment by Berg and Ekbohm (1991), and in a review, Berg and Matzner (1997) confirmed a general retarding effect of N additions on respiration rates of humus. For needle litter decomposing in an unpolluted forest, Laskowski and Berg (1993) found that the concentrations of heavy metals, such as Fe, Zn, Mn, Pb, and Cd, could reach levels potentially inhibitory to microorganisms in later decomposition stages, viz. at those stages when lignin degradation dominates the litter decomposition (cf. Berg et al., 1991). Virzo De Santo et al. (2002) found that, in the late phase, decomposition of seven different litter types was significantly and negatively correlated with Zn and Cu concentration; however, heavy metals’ accumulation or release from decomposing litter may depend on the gradient of metal concentration between litter and soil, on the pH of the soil, and on the capacity of litter to bind metals. The aim of this study was to explore how limit values for decomposition, and thus the litters’ capacity to sequester C, vary between two contrasting sites and to highlight the relationship of limit values with litter nutrients and heavy metals concentrations, taking into account the influence of differences in soil chemical composition. The study was performed on three types of leaf litters and four types of needle litters incubated in two contrasting coniferous forest stands, viz. a boreal Scots pine (Pinus sylvestris L.) forest, and a temperate silver fir (Abies alba Mill.) forest. In addition, earlier published limit values for the same two sites (Berg et al., 1996; Berg and Ekbohm, 1991) have been used to increase the data base for the evaluation of relationships between limit values, litter nutrients and heavy metals’ concentrations. At both sites an accumulation of SOM had been established (Berg et al., 1993, 1995). The results are discussed on the basis of the difference for essential nutrients and heavy metals’ contents in the soil of the humic surface horizon at the two sites. The study is part of a research project on the influence of litter and soil chemical composition on decomposition. Results concerning the dynamics of heavy metals have already been published (Virzo De Santo et al., 2002). The research aims at contributing more knowledge of factors determining C sequestration in the soil. 2. Materials and methods 2.1. Litters and sites of incubation Needle litter of lodgepole pine (Pinus contorta var contorta), stone pine (Pinus pinea L.), brown and green leaves of Scots pine (Pinus sylvestris, L.), and trembling aspen (Populus tremula L.) as well as green leaves of silver birch (Betula

pubescens Ehrh) were selected for investigating the relationships between limit values and concentrations of litter nutrients and heavy metals. Scots pine needles and leaves of silver birch and trembling aspen were collected at site Jädraås (Axelsson and Bråkenhielm, 1980); lodgepole pine needles were sampled at a research site close to the town of Malung, central Sweden (cf. Berg and Lundmark, 1987). Stone pine litter was collected on Mount Vesuve (south Italy) close to site Terzigno (site description by Virzo De Santo et al., 1993). The needle litter of P. sylvestris was sampled in the autumn of 1993 from the branches of trees in a 25-year-old stand. Brown needles from the needle generation to be shed were taken at the time of abscission; green needles were taken from the second and third needle generations. Brown leaves of P. tremula were sampled at the same time as the P. sylvestris needles. Green leaves of B. pubescens and P. tremula were sampled in early August. Brown needles of P. contorta were sampled in the autumn of 1993 from trees about 25 years old. Brown needles of P. pinea from the needle generation to be shed were taken at abscission from the branches of trees in a 50-year-old stand in June, during the dry period when there was a maximum needle litter fall. Before weighing, the needles and leaves were air-dried at room temperature to about 5–8% moisture. Dry mass was determined on 25 samples at 85 °C and the largest difference in moisture content was less than ±0.5% of the average. Litters were incubated at two sites: (1) a temperate silver fir forest at an altitude of 1100 m a.s.l. on Monte Taburno (41°06′ N; 14°36′ E, 42 km NE of Naples, Campania, Italy), and (2) a boreal Scots pine forest at the village Jädraås in central Sweden (60°49′ N; 16°30′ E, ca 200 km NNW of Stockholm). Both sites are located far from heavy metal pollution sources. At high elevation, the average annual precipitation for Monte Taburno is 2166 mm, and the long-term average temperature is 7.9 °C, based on the records from the Montevergine weather station 20 km south of the site and at the same altitude. Average annual evapotranspiration (AET) was 561 mm (Berg and Meentemeyer, 2002). The silver fir stand, which is about 100–120 years old and was planted on a soil formerly covered by common beech (Fagus sylvatica L.), has been described in previous papers (Caputo, 1966– 1967; Virzo De Santo et al., 1993). The soil, a Typic Hapludand, medial, mesic, formed a deep profile on the pyroclastic parent material. The humic surface horizon (top 5 cm) has a pH value of 5.97, a C concentration of 11.05% and an N concentration of 0.85%, giving a C-to-N ratio of 13. Carbon concentration in the soil profile is given in Table 1. The Scots pine stand (about 145 years old) was that of the former Swedish Coniferous Forest Project research site Jädraås, located at an altitude of 185 m a.s.l. on a flat area of deep glacifluvial sand sediments. The mean annual precipitation at a nearby village is 609 mm, and the mean annual temperature is 3.8 °C. AET was 472 mm (Berg and Meentemeyer, 2002). The forest has been described in earlier papers (Berg et al., 1982, 1995). The soil is a Typic Haplocryod,

B. Berg et al. / Acta Oecologica 24 (2003) 295–302 Table 1 Organic carbon content (mean values ± standard error) along the soil profile of the Monte Taburno forest Corg (% dw) 9.65 (±0.05) 5.75 (±0.04) 5.77 (±0.04) 4.03 (±0.02) 2.96 (±0.002) 3.00 (±0.02) 2.74 (±0.01) 1.77 (±0.06) 2.72 (±0.06) 5.10 (±0.01)

80 P.t. (g)

P.t. (b)

B.p. (g)

60

Accumulated mass loss (%)

Soil depth (cm) 0–10 10–20 20–30 30–40 40–50 50–60 60–70 70–80 80–90 90–100

297

40 20 Leaves 0 P.s. (g) 60

P.s. (b)

P.p. (b) P.c. (b)

40 20

with a weakly developed Ae horizon (bleached horizon; 2–7 cm) and a typical mor humus. A very loose L horizon (A00), interwoven with living mosses and lichens, covers an F/H horizon (A01–A02) of 5–10 cm. The pH range is 3.9– 4.2 in the F/H horizon and 4.6–4.8 in the upper mineral soil. The parent mineral material as well as the whole soil is considered to be very poor in essential nutrients. In the humic surface horizon (top 5 cm), the C concentration is 44.8% and the N concentration 1.06%, giving a C-to-N ratio of 42.3.

Needles 0

0

200

400

600

800

1000

1200

Time (days)

Fig. 1. Decomposition of seven litters types incubated in a silver-fir forest (Monte Taburno—filled symbols) and in a Scots pine forest (Jädraås—open symbols). Populus tremula (P.t.), Betula pubescens (B.p.), Pinus sylvestris (P.s.), Pinus pinea (P.p.), and Pinus contorta (P.c.). (g) stands for green and (b) for brown.

2.4. Chemical analyses of litter and soil 2.2. Litter incubation, sampling and mass loss determination For each type of litter, samples of 0.6–1.0 g were enclosed separately in litter bags (8 × 8 cm for needle litter and about 10 × 15 cm for leaf litter) made of terylene net with a mesh size of about 1 mm. At both sites, the bags were placed on the litter (L) layer in a measurement plot (1 × 1 m) in each of 25 plots in a randomized design. They were fastened to the ground by 10–15 cm-long stainless steel pegs through a 1 cm-wide edge on the bags. The incubations at site Monte Taburno were made on 11 May 1994 and those at site Jädraås on 9 November 1994. At both sites, samplings took place mainly three times annually (Fig. 1). On each sampling occasion, we collected one sample of each litter type from each of the 25 plots and samples were transported directly to the laboratory. Plant remains, such as mosses, lichens and lingonberry (Vaccinium vitia-idaea L) were removed, after which the loss of dry mass was determined by drying the samples to a constant mass at 85 °C (for 24 h). Mean values of mass loss were calculated for each sampling. The 25 samples of each type of litter were combined for chemical analyses. 2.3. Sampling of soil At both sites soil from the humic surface horizon was sampled to a depth of ca 5 cm using a sampler of ca 10 cm diameter at Monte Taburno and ca 12 cm at Jädraås. Sampling took place at eight spots within the same area as the bags were incubated.

Soil and litter samples were analyzed for N, P, S, K, Ca, Mg, Mn, Fe, Zn, Cd, Cu, and Pb. Soil (2 mm mesh) and litter samples were ground in a laboratory mill equipped with a filter allowing particles of less than 1 mm to pass. The samples were digested in a Milestone (mls 1200) Microwave Laboratory System with a mixture of hydrofluoric and nitric acid (HF 50% v/v: HNO3 65% v/v = 1:2). Element concentrations of digested samples were measured by atomic absorption spectrometry (AAS; SpectrAA 20 Varian) using standard solutions (STD Analyticals, Carlo Erba), diluted in the same acid matrix for extraction. Fe, Mn, and Zn concentrations in the digestion extracts were measured using flame AAS; Cu, Cd, and Pb concentrations in the digestion extracts were measured by graphite furnace AAS. The extractable fraction of elements in soil was determined as follows: K, Mg, and Ca were measured in 0.4 M BaCl2 extracts at pH 8.1 ± 0.1; Mn, Zn, Fe, Cu, Pb, and Cd were determined in 0.02 M EDTA and 0.5 M CH3CO2NH4 extracts at pH 4.65 ± 0.05, according to the method of Lakanen and Erviö (1971). Phosphorus was determined using the method described by Bray and Kurtz (1945). Carbon in the Monte Taburno soil profile was determined as loss on ignition (a method that gives an overestimate as compared to wet digestion), but we corrected the values by using a factor derived from data obtained from analyses made on the same soil following two different methods. All analyses were carried out on three subsamples. The initial litter samples were analyzed for water solubles and sulfuric acid lignin, according to Bethge et al. (1971).

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2.5. A model for decomposition

2.7. Statistics

To describe the decomposition pattern and calculate the limit values, we used the model of Berg and Ekbohm (1991) (modified from Howard and Howard, 1974) and applied it to the decomposition data reported in Fig. 1:

The relationship between limit values and single or multiple nutrients or heavy metals was investigated through linear regression. In order to increase the data base we included earlier collected data in the present analysis.

m.l. =m(1 – e–kt/m)

(1)

where m.l. is the accumulated mass loss (in percent) and t is time in days. The parameter m represents the maximum accumulated mass loss (asymptotic level), and the parameter k is the initial decomposition rate (the derivative of the function at t = 0). The basis for our choice of the above expression was that we regarded the substrates as consisting of a nondecomposable component and a component decomposing exponentially. For estimating the parameters of model (1) we used the NLIN procedure of SAS (SAS, 1979). For estimating and comparing the parameters for all litter types, the dummyvariable technique was utilized. Since 24 parameters were simultaneously estimated using 136 observations the residual variance was estimated with 112 degrees of freedom. 2.6. External data At site Jädraås studies had been carried out earlier with the same methods and similar litters (Berg and Ekbohm, 1991). Also at site Monte Taburno other decomposition studies had been carried out (Berg et al., 1996). Chemical composition and limit values for these litter types are given in Table 2.

3. Results 3.1. Comparison of litter and soil chemical composition The seven litters differed in concentrations of nutrients, heavy metals, water solubles, and lignin (Table 3). The chemical composition of the humic surface horizon at the two sites was very different both with respect to total concentrations and available fractions of nutrients and heavy metals (Table 3). The chemical analysis showed a humic surface horizon, relatively rich in essential nutrients at Monte Taburno and relatively nutrient-poor at Jädraås. The ratios for concentrations at Monte Taburno to those at Jädraås (Table 3) may illustrate the difference between sites. For P, Mn, Fe, Mg, Ca, and Cu, the total concentrations were at least twice as high at Monte Taburno with Cu, P, Mg, and Ca being exceptionally high with ratios between 6.8 and 4.9. In the other cases (Zn, K, Cd, and Pb) the ratios ranged between 1.1 and 1.9. The ratio for N was 0.8 when comparing the concentration in the soil layers. However, when comparing N concentrations in the organic part of the humus layer, the ratio was 3.0 indicating that although Monte Taburno had a

Table 2 Initial chemical composition of litters from earlier studies incubated at the sites Jädraås and Monte Taburno. Cf. Section 2.6. Data from Berg and Ekbohm (1991) and Berg et al. (1996) Concentration of element

Jädraås P. contorta (g) P. contorta (b) P. sylvestris (g) P. sylvestris (b) A. incana (g) B. pubescens (g) B. pubescens (b) Monte Taburno F. sylvatica (1981) F. sylvatica (1981) F. sylvatica (1982) F. sylvatica (1982) A. alba (1981) A. alba (1983) A. alba (1988)

Limit value (%)

(mg g-1) N P

S

K

Ca

Mg

Mn

(µg g-1) Fe

Zn

Cd

Cu

Pb

10.5 3.9 15.1 4.8 30.7 17.4 7.7

0.82 0.34 1.31 0.33 1.37 1.80 1.05

1.17 0.62 1.13 0.55 6.12 1.32 0.80

3.8 0.6 4.7 1.1 15.6 7.0 4.7

4.0 6.4 2.8 4.4 12.3 5.6 11.8

0.93 0.95 0.82 0.49 2.32 1.99 3.3

0.82 1.79 0.26 0.79 0.10 0.36 1.23

– – 50 57 – 53 61

– – 43 51 – 140 340

– – 0.3 0.2 – 0.2 0.8

– – 2.8 1.4 – 6.4 3.4

– – 1.0 2.5 – – 2.6

82 100 68 89 51 54 57

16.8

1.01

1.48

1.2

14.8

1.86

0.05

–

–

–

–

–

64

16.8

1.01

1.48

1.2

14.8

1.86

0.05

–

–

–

–

–

48

9.8

1.10

–

6.4

9.7

2.90

–

–

–

–

–

–

52

10.5

1.00

–

6.1

9.5

2.90

–

–

–

–

–

–

67

13.2 12.3 13.6

1.23 1.00 0.84

1.32 – 1.49

4.7 5.0 1.5

12.3 10.2 29.0

1.03 1.00 0.83

0.06 – 0.07

– – –

– – –

– – –

– – –

– – –

54 52 48

b, brown litter; g, green litter; –: not determined

B. Berg et al. / Acta Oecologica 24 (2003) 295–302

299

Table 3 Initial chemical composition of litters incubated at the sites Jädraås and Monte Taburno in the present study. Total concentration (tc) and available fraction (af) of nutrients and heavy metals of the humic surface horizon, as well as the ratios between Monte Taburno and Jädraås values for both tc and af elements are also reported Concentration of components (mg g–1) Wat. sol. Lignin N Litter P. contorta (b) 143 P. pinea (b) 205 P. sylvestris (g) 218 P. sylvestris (b) 122 P. tremula (g) 319 B. pubescens (g) 267 Soil (humic surface horizon) Taburno tcb Taburno afb Jädraås tcb Jädraås afb Relative composition of nutrients Taburno tc/Jädraås tc Taburno af/Jädraås af

376 312 239 277 230 179

P

S

K

Ca

Mg

Mn

(µg g–1) Fe

Zn

Cd Cu Pb

3.1 3.0 12.1 3.6 24.2 24.3

0.29 0.57 1.36 0.20 2.12 1.96

0.44 1.36 0.81 0.44 1.87 1.54

0.5 5.9 5.9 0.5 14.2 9.0

8.7 7.1 3.9 5.6 8.4 9.5

1.06 2.40 0.79 0.34 2.29 3.37

2.03 0.19 0.53 1.19 0.10 0.76

53 299 64 79 44 66

85 49 49 48 107 223

0.6 0.1 0.1 0.1 0.3 0.4

2.8 5.0 4.7 2.6 8.8 7.1

8.5 (38.2)a

2.84 0.01 0.47 0.06

– – – –

17.7 0.2 10.9 0.13

20.0 7.73 3.2 0.79

4.76 0.23 0.98 0.06

0.76 0.12 0.37 0.17

6.5b 0.3b 9.4b 0.5b

0.11 0.03 0.06 0.02

0.9 0.2 0.7 0.1

62.6 9.7 12.9 24.2 9.2 8.9 1.02 1.1

6.00 0.24

– –

1.6 1.8

6.2 9.8

4.9 3.9

2.1 0.7

2.8b 0.7b

1.9 1.3

1.3 6.8 1.1 2.8 13.6 1.2

10.6 (12.8)a

0.8 (3.0)a

1.0 3.0 1.0 2.0 n.d. 1.0

b, brown litter; g, green litter; –, not determined; n.d., not detectable. a mgN g–1 SOM. b mg g–1.

lower N level in the humus layer as such the concentration in the biologically active part was considerable higher than at site Jädraås (cf. C-to-N ratios in Section 2.1). As regards the available fraction, P exhibited a much lower concentration at Monte Taburno (Table 3) consistent with the high P retention capacity of andic soils. The available fractions of Mn and Fe were lower at Monte Taburno, while K, Ca, Mg, Zn, Cd, Cu, and Pb showed ratios Monte Taburno/Jädraås ranging between 1.2 and 13.6 (Table 3).

accumulated mass loss of all leaf litters was clearly lower at site Jädraås than at Monte Taburno and no relevant difference between leaf litter types was evident within either of the sites. Similarly, all types of needle litter incubated at Monte Taburno showed a higher accumulated mass loss as compared to the same litter types incubated at Jädraås; however, green needles of P. sylvestris, the needles most rich in nutrients, reached at Jädraås an accumulated mass loss comparable to that of the needle litters incubated at Monte Taburno.

3.2. Decomposition of leaf and needle litters and comparison of limit values at two contrasting sites

A direct comparison of limit values was made among the litter types incubated at the two different sites (Table 4) except for P. contorta litter which did not give a significant limit value and thus was excluded from further comparisons. Of the six remaining litter types four (green needles and leaves of Scots pine, silver birch, and aspen as well as brown aspen leaves) were significantly different between the sites and two (brown needles of Scots pine and stone pine) were not. In the case of significant differences, limit values were higher at the more nutrient-rich Monte Taburno site than at

At both sites, the leaf litters (Fig. 1) reached an accumulated mass loss of about 40% during the first 3 months of incubation; in the same period needle litters showed an accumulated mass loss of about 20%, and for lodgepole pine an even lower value (about 10%). With the exception of green birch leaves the first-year mass loss for a given litter type was not different between sites. In the following 2-year period the

Table 4 Limit values for decomposition (%) of some different litter types in two contrasting coniferous ecosystems. Standard error within parenthesis. Significant differences (t–test) between sites for the same litter type are indicated by stars (*) for P < 0.05, and (**) for P < 0.01; n.s. stands for not significant differences. For each litter type the number of samplings was 11 for Monte Taburno and 12 for Jädraås Litter type P. pinea (b) P. sylvestris (g) P. sylvestris (b) P. contorta (b) P. tremula (g) P. tremula (b) B. pubescens (g)

Limit values (%) Monte Taburno 67.76 (8.06) 85.05 (9.62) 74.34 (5.58) No limit value 79.71 (3.68) 77.63 (3.90) 69.56 (2.13)

Jädraås 77.76 (10.85) 72.76 (3.45) 82.86 (9.89) No limit value 62.74 (2.10) 63 (2.12) 62.28 (2.32)

Significance of differences n.s. * n.s. –– ** ** *

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Table 5 Coefficients of determination (R2) between the estimated limit values and initial concentrations of N, P, S, K, Ca, Mg, Mn, Fe, Zn, Cd, Cu, and Pb for litters incubated at each of two contrasting sites investigated and for the two sites combined. Data used are from both the present study only and from the present study combined with earlier collected data (Berg and Ekbohm, 1991; Berg et al., 1996) (see Tables 2 and 3). The coefficients of determination have been calculated using available data for all the litters that have been incubated at the two sites. Negative relationships are indicated by (–) Element N P S K Ca Monte Taburno (n=6) 0.022 0.050 0.030 0.024 0.014 Jädraås (n=6) (–) 0.508 (–) 0.481 (–) 0.615 0.587 0.442 Monte Taburno and Jädraåa data combined (n=12) (–) 0.106 (–) 0.082 (–) 0.237 0.124 0.162 All data from Monte Taburno (n=13)a (–) 0.001 0.031 (–) 0.089 0.216 (–) 0.296 All data from Jädraås (n=13)b (–) 0.481* (–) 0.509** (–) 0.309* 0.566** 0.277

Mg

Mn

Fe

Zn

Cd

Cu

Pb

(–) 0.281

0.022

(–) 0.397

(–) 0.091

(–) 0.002

0.015

(–) 0.375

(–) 0.479

0.281

(–) 0.260

(–) 0.596

(–) 0.787*

(–) 0.900** 0.682*

(–) 0.329

0.052

0.001

(–) 0.280

(–) 0.272

(–) 0.213

0.044

0.004

0.201

–

–

–

–

–

(–) 0.469*

0.388*

0.108

(–) 0.457*

(–) 0.376*

(–) 0.410*

0.262

* P < 0.05; ** P < 0.01. a S and Mn in this set have n = 10 . b Fe, Zn, Cd, and Cu in these data set have n = 10 for Pb n = 9.

the nutrient-poor site (Jädraås) (Table 4), meaning that the decomposition could be expected to proceed further at Monte Taburno. For the Monte Taburno data (n = 6) no single nutrient or heavy metal gave a significant relationship to limit values. The highest R2 value obtained (0.397, P = 0.181) was that with Fe and was negative (Table 5). We may note that the R2 value for limit values vs. N concentration was as low as 0.022 at this site with very high N concentrations in SOM (C-to-N ratio = 13; Table 3). For site Jädraås (n = 6) significant negative relationships (Table 5) were seen between limit values and concentrations of Cd (R2 = 0.787; P < 0.05), and Cu (R2 = 0.900; P < 0.01), and a positive relationship to Pb concentration (R2 = 0.682; P < 0.05). However, the initial Pb concentrations were just above detection level (0–3 µg g–1, Table 3). A negative relationship to Zn was significant at the level of P < 0.1 (Table 5). Although there was no significant relationship to N concentration we may compare the magnitude of the R2 value in this case (0.508) and a negative coefficient to the complete absence of a relationship for the corresponding Monte Taburno data. The combined data sets for the two sites (Jädraås and Monte Taburno, n = 12) did not give any relationship to initial nutrient concentrations (Table 5), which may further indicate a difference between sites. To trace relationships, data from different earlier investigations at the same two sites were combined with the present data set (Berg and Ekbohm, 1991; Berg et al., 1996). Combining several data sets (n = 13) for Jädraås (Tables 3 and 5) gave significant and negative relationships for N, P, S, Mg, Zn, Cd, and Cu and positive ones to K and Mn (Table 5). As for N, Mn, and Ca, which are nutrients regulating the lignin-degrading microflora (Eriksson et al., 1990) a model for a multiple relationship of N, Ca, and Mn to limit values gave an R2 of 0.903 (P < 0.001). When combining data for site Monte Taburno (n = 13), we found no significant relationship to any nutrient concentration

(Table 5) and the R2 values were so low that they indicated that generally no relationship existed to any of the elements investigated (N, P, S, K, Ca, Mg, and Mn). To obtain a wide enough range in nutrient concentrations we have used both green and brown needle and leaf litter for our experiment. In general, the green leaves hold not only higher levels of the main nutrients but also higher levels of solubles. We could expect that a higher level of solubles e.g. carbohydrates should mean that a larger part of the litter would decompose, thus that the limit value should be higher. At site Jädraås earlier studies indicate the opposite. For lodgepole pine, Scots pine and silver birch Berg and Ekbohm (1991) found that the green leaves and needles had lower limit values than the brown ones. This may mean that there are two counteracting effects and if limit values were estimated, e.g. using the nonsoluble matter only, that the effect of N should be stronger.

4. Discussion In the early phase of decomposition of the studied litters, the lack of differences in accumulated mass loss between such contrasting sites as Monte Taburno and Jädraås indicates that climate and/or soil chemical composition does not play any important role. As a contrast, decomposition in the late stages proceeds further at Monte Taburno. Limit values differed at the two contrasting sites; they were significantly higher at Monte Taburno and still significantly different from 100%. That accumulation of organic matter was taking place at this site is supported by the amount of SOM actually stored (a ca 1-m-deep humus layer, after Berg et al., 1993; cf. Table 1). Limit values at Monte Taburno, with a soil rich in nutrients and heavy metals, were not related to litter chemical composition. In contrast, at the poorer site Jädraås, positive and significant correlations were found between limit values and

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nutrients and negative correlations between limit values and Cd, Zn, and Cu concentrations. These results suggest that the two sites are characterized by different populations of decomposer microorganisms.

at Monte Taburno than at Jädraås (Virzo De Santo et al., 2002).

There was a relationship between limit values and litter N concentrations only at the N-poor site Jädraås (Table 5) and no relationship was found at the N-rich site. The relatively high level of N in the organic matter of the soil at Monte Taburno (C-to-N ratio = 13; Table 3) may have a selective influence on the microflora and it is possible that lignindegrading organisms with an activity not hampered by high N levels have a competitive advantage. A support for such an hypothesis is given by Eriksson et al. (1990) who compared the influence of N on the repression of the lignolytic enzyme system in fungi isolated from both N-poor and N-rich environments. This could mean that due to functionally different microfloras the magnitude of the limit value at Monte Taburno is more or less independent of litter N levels. A site effect on the limit value was seen by Prescott (1996) who found a more complete decomposition of the leaf litter of Betula papyrifera Marsh. incubated on a nutrient-rich soil as compared to the same litter incubated on a nutrient-poor soil.

5. Conclusions

At Monte Taburno there was a complete lack of relationship between limit values and heavy metals. In contrast, there was a negative relationship between limit values and litter Cd, Cu, and Zn at site Jädraås. It is likely that decomposer microorganisms at Monte Taburno were adapted to higher levels of heavy metals and/or were able to tolerate high concentrations of heavy metals due to the higher pH and the abundance of nutrients in the soil. We cannot exclude that climate has an effect on the level of the limit value. We have thus found that limit values for Scots pine needle litter are negatively related to the sites’ actual evapotranspiration (AET) (B. Berg, unpublished). This would mean that under warmer and wetter conditions the decomposing litter develops a larger fraction of recalcitrant matter. That is in agreement with the finding of Dalias et al. (2001) that litter incubated under a higher temperature had a lower respiration rate than the same litter type incubated at a site that was colder and drier. These observations support that the difference in limit values observed in the present investigation may be related to the soil properties. Virzo De Santo et al. (2002), in a previous paper dealing with decomposition of the same litters at the two sites, showed that Cd accumulates in decomposing litters at both sites. In contrast, Cu is accumulated in litter at Monte Taburno and released at Jädraås, likely depending on physico-chemical processes promoted by the high Cu level in the soil of Monte Taburno and the low pH of the soil of Jädraås, respectively. Although the concentrations of Cu in far-decomposed litter were lower at Jädraås than at Monte Taburno a negative correlation between Cu concentration and the limit value was found only at Jädraås. Also Fe and Pb showed higher levels of accumulation in litter decomposing

Provided that litter decomposition has limit values, thus leaving recalcitrant remains, a buildup of SOM should occur unless disturbance by e.g. fire destroys the accumulated material. The calculated limit values indicate that an accumulation of SOM may take place both in the soil of the undisturbed boreal forest and in that of the temperate coniferous fore st. Wardle et al. (1997) described boreal humus layers with humus accumulating for up to ca 3000 years leaving on the average 49 kg of humus per meter square. This kind of mor humus is nutrient-poor as illustrated by an N concentration of 1.0–1.4% (Wardle et al., 1997). The estimated annual humus accumulation rate was similar to that at the younger, 145-year-old Jädraås site used in the present study with mor humus of a similar N level (ca 1.2%). At the contrasting site Monte Taburno a heavy accumulation of SOM had taken place on top of the pyroclastic parent material covering the limestone ground reaching a depth of ca one meter but the time for this accumulation was unknown. Thus, although the sites are very different from the point of view of climate and soil nutrient composition an accumulation of SOM has been clearly documented in both cases (Berg et al., 2001). The results of this study show that the limit values at the two sites have different levels, i.e. they are higher at the nutrient-rich Monte Taburno site. Moreover, limit values are related to litter chemical composition only at the nutrientpoor site Jädraås. Thus accumulation of organic matter is found not only over a wide spectrum of litter chemical composition (Berg and Ekbohm, 1991; Berg et al., 1995, 1996, 2001; Berg, 1998, 2000a) but also, as indicated by the results of this research, over soil types of very varying concentrations of nutrients and heavy metals. That means that although the limit-value may be considered a general phenomenon, the regulating factors are different.

Acknowledgements Financial support for this work was provided by German Ministry for Education, Science, Research, and Technology (BMBF, Grant No PT BEO-51-0339476) to Dr Björn Berg, while working as a guest scientist at BITÖK, University of Bayreuth and from the European Union (EU) project CN-ter, QLK5-2001-00596, as well as by the Consiglio Nazionale delle Ricerche to Virzo De Santo (9704310.CT04 and 98.00530.CT04) and to B. Berg for a short term mobility grant (1999). The study was started in the EU project VAMOS (Variation du reservoir de Matiere Organique du Sol).

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