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Silva Balcanica, 14(1)/2013

CARBON STORAGE IN SELECTED EUROPEAN CHESTNUT (CASTANEA SATIVA MILL.) ECOSYSTEMS IN BELASITSA MOUNTAIN,

SW BULGARIA

Miglena Zhiyanski, Maria GlushkovaForest Research Institute – SofiaBulgarian Academy of Sciences

Abstract

This work is focused on carbon storage of various components (above-ground biomass, forest floor and soil) of selected European chestnut (Castanea sativa Mill.) forest ecosystems developed on Chromic Luvisols in Belasitsa Mountain, SW Bul-garia. In 2010 two mixed chestnut forests and one pure stand were sampled. Within each experimental site sampling plot was defined and the characteristics of stands were measured. Estimated biomass was calculated per hectare then the values ob-tained were converted to carbon stock. The carbon content of forest floor and differ-ent soil depths (0–10 cm, 10–30 cm, and 30-50 cm) was estimated in 6 replicates per plot. All soil properties were determined in accordance with the standardized meth-ods. Variations were obtained for soil carbon stock in studied chestnut ecosystems. More carbon is sequestered in chestnut biomass of older forest CF1 (31.1 t C ha-1) compared with the other two stands CF2 and CF3 (14.4 – 19.6 t C ha-1). Concerning carbon stored in soil system the picture is different. The mean value of carbon stored in forest floor layers of all studied ecosystems was 19.8 t C ha-1 while the highest stock was determined for the pure chestnut stand CF3 – 23.8±0.8 t C ha-1. In soils carbon is mainly accumulated in the superficial 0 - 10 cm and decreased toward deeper layers. In the mixed forests CF1 and CF2 the carbon stock in this soil layer was estimated at 20.1 t C ha-1 and 21.2 t C ha-1, respectively. The values obtained were lower in comparison with the same layer under the pure chestnut stand CF3 (34.1 t C ha-1). The carbon stock in both forest floor and 0-50 cm soil was higher in CF3 (84.04 t C ha-1) compared with CF1 (59.77 t C ha-1) and CF2 (50.50 t C ha-1).

Following the estimations of carbon stock including above ground biomass, the total stock in the studied chestnut forests could be ordered as follows: CF3 (105.8 t C ha-1) > CF1 (102.1 t C ha-1) > CF2 (76.3 t C ha-1). The pure chestnut forest CF3 characterized with the highest total carbon stock per hectare and only 20.6 % from it is accumulated in the aboveground tree biomass. This confirms the high po-tential of carbon sequestration in soil system under pure and mature chestnut forests. At the same time the carbon in older mixed chestnut ecosystems dominated by beech in tree composition (CF1) was also high but 33.8 % of carbon is accumulated in the aboveground chestnut biomass, while 21.0% of carbon is sequestered by chestnut

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trees in the other mixed stand CF2, dominated by chestnut. In CF3 the accumulation is mainly in forest floor and in mineral soil (especially in the superficial soil layer), which shows favourable conditions for the incorporation of organic substances in soil system under pure chestnut stands in mature growing phase.

Key words: Carbon storage, soil, forest floor, tree biomass, Castanea sativa, forest ecosystems, Bulgaria

INTRODUCTION

Carbon balance is one of the most important biogeochemical parameters in natural systems, since it determines the flow of organic matter and controls the con-tent of CO2 in the atmosphere. The increased concentration of CO2 in the atmosphere due to anthropogenic activities such as emissions from fossil fuels (about 30 Gt CO2 worldwide in 2007) (IEA, 2009) and from land use change, mainly deforesta-tion in tropical regions (~ 5.8 Gt CO2 per year worldwide in the 1990’s) (Denman et al., 2007) emphasises the role of forest for carbon sequestration and buffering. During the last century, the carbon dioxide (CO2) concentration in the atmosphere has increased from 280 to 367 parts per million (IPCC SRES, 2000; IPCC, 2001). About half of the emissions of fossil fuels are released in the atmosphere and the rest are absorbed by the oceans and land surface. Most analyses to date of options for mitigating the risk of global climate change have focused on reducing emissions of carbon dioxide and other greenhouse gases. Much less attention has been given to the potential for storing (or ‘sequestering’) significant amounts of carbon in forests and other ecosystems as an alternative means of offsetting the effect of future emis-sions on GHG concentrations in the atmosphere.

Castanea sativa Mill. is a multipurpose species because of various useful properties and a wide range of valuable products (chestnut fruits, honey, timber, etc.). Previous studies have revealed that chestnuts grow faster and larger than other hardwood species allowing them to retain more carbon over a shorter period of time (Jacobs et al., 2009). Chestnut trees can sequester as much as five times the CO2 of any other hardwood tree, and they grow quickly, inhaling massive amounts of carbon dioxide in the early phase of their life, making them a viable near-term carbon stor-age solution. And since the tree is most often harvested for high-quality hardwood products such as furniture, the sequestered carbon would remain trapped in the ob-jects for longer periods of time than in wood used for paper manufacture (Jacobs et al., 2009). Nevertheless despite the importance of its multiple uses, chestnut is insufficiently studied concerning carbon sequestration and is not effectively used as a managed forest species in Bulgaria, being categorised as a ‘negligible’ genetic resource. Peev (2008) and Velichkov et al. (2010) reported that in Bulgaria, the for-est habitat of C. sativa (defined under Directive 92/43/EEC) comprises stands with a total area of 3315 ha. In Belasitsa Mountain the protected zone within the Natural

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Park covers 2085 ha out of 2560 ha totally protected forests, thus enhancing the habitat favourable conservation status in the site, which is of primary importance for its conservation at a country scale (Velichkov et al., 2010). According to Velichkov et al. (2011) in Belastitsa Natural Park the mixed chestnut forests increased their area from 604 ha in 1964 to 1030 ha in 2009, while the pure chestnut stands decreased from 1319 ha to 648 ha for the same period. The authors underlined that in the mixed forests European chestnut dominates at diameter of breast height and age (Velichkov et al., 2011).

Forests and other terrestrial ecosystems have great importance in carbon se-questration, absorbing about 3.3 Gt CO2 per year for the period between 1993 and 2003 (Nabuurs et al., 2007). The site conditions and tree species are key factors in determining the forest potential to sequester carbon. In addition, the forestry and forest management activities, through the management decisions made for different forestry systems (harvesting, rotation systems, etc.) play an important role (Lucas et al., 2010). A longer rotation period is proposed as a measure to promote carbon se-questration in forests (Adams et al., 1999; Sohngen, Mendelsohn 2003). The longer rotation period, with a higher proportion of old trees, leads to higher carbon accu-mulation in the ‘reservoirs’ in comparison with short-rotation systems. The soils in mountainous broadleaved forests are characterized by higher soil carbon content as a result of increased quantity of aboveground and belowground biomass and the more intensive mineralization rates compared with coniferous forests (Reicosky, Forcella 1998). Mature and over-matured stands characterized with higher carbon density in the ‘reservoirs’, while young stands have higher capacity as ‘sinks’. Short rotation where harvesting is carried out at the age of maximum growth are linked to maxi-mum biomass production, but not to carbon accumulation. Growth tables (Nedyalk-ov, Shikov, 1983) show that the biomass productivity of forests strongly decreases after the mature phase. However, research in old forests show that especially in these ecosystems, much more carbon is transferred to the soil and increased stocks of carbon in underground biomass and soil are reported (Harmon et al., 1990; Schulze et al., 2000). Old forests can not maintain high stand densities and thus stimulate the decomposition of organic matter in soil, which is associated with an increase of soil organic carbon stocks (Cannell, 1999; Liski et al., 2001; Harmon, Marks, 2002). Gallardo, González-Hernández (2008) reported the maximum soil C content of 530 t C ha-1 in chestnut forests managed as coppices for timber production and concluded that this value is considered to be the maximum C-storage potential of the chestnut soils. At the same time in chestnut orchard soils in Spain reported values were as low as 40 t C ha-1, indicating that the limited soil organic matter allows for a potential C storage capacity of at least 150 t C ha-1 which could be reached under an appropriate management regime (Gallardo, González 2008). In another study of carbon seques-tration in 25 years old coppice of C. sativa the accumulation of C in the tree biomass was 58 mg C ha-1 y-1; the calculated litter decomposition-constants 0.39 y-1; and the aboveground annual-production 5.3 mg C ha-1 y-1. On calculating an annual overall

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balance, inputs of C into this forest ecosystem are always higher than the C outputs and C sequestration reported was 4.6 mg C ha-1 y-1 (Gallardo, González 2005).

The carbon accumulation rate and duration along the full life cycle of stands are determined by a combination of tree species, site characteristics, management practices and climatic conditions (Sakin et al., 2011). These factors are crucial for maintaining maximum accumulation of carbon in mature forests. The important fact is that, especially in these forests, the potential for increasing the net soil carbon accumulation is understood. Under climatic change the variation in environmental conditions presents a need for additional understanding of the adaptation potential of forests, especially where these have special functions in natural reserves. This also is relevant to the carbon sequestration potential of vulnerable and valuable ecosys-tems and studies at the regional level are needed to improve national greenhouse gas inventories, which could be developed as a basis for regional baselines of carbon sequestration projects in the forestry sector (Brown et al., 2002).

The aim of this work was to study the variability and potential of carbon stor-age in selected chestnut forests in Belasitsa Mountain, managed for timber produc-tion, separating carbon in different components of ecosystems – above-ground tree biomass, forest floor and soil.

MATERIALS AND METHODS

The experimental work was carried out in selected European chestnut forests on northern slopes of Belasitsa Mountain, SW Bulgaria. The region is located in the zone of Continental-Mediterranean climatic region. Being a mountainous terrain, the altitude ranges from 450 to 750 m a.s.l. The most extensive areas of natural chestnut populations are located in this region as well as large number of forest plantations and orchards have been created. The chestnut forests in this region are defined as rare habitats with conservation priority under the Habitats Directive of the EU (92/43/EEC).

We selected 3 experimental sites in mature chestnut forests within an altitude gradient between 450 – 750 m. The climate in the region is mountainous with MAT 13.9 ° C and MAP 676 mm (Stanev et al., 1991; Кoleva, Peneva, 1990). The mean age of chestnut trees in studied forests was over 80 years and they could be referred as mature stands (CF1, CF2 and CF3). The sites were chosen to represent different stand structures of formed forests in this region, but developed on the same soil type and site conditions (Table 1). Mixed forests are dominated by beech (Fagus sylvatica L.) in CF1 and by European chestnut (Castanea sativa Mill.) in CF2. In each experi-mental site one sampling plot with an area of 0.15 ha was defined. Six representative soil profiles per plot, randomly located, were studied in 2010. The soils in studied sites could be referred as Chromic Luvisols according to the WRB (IUSS, 2007).

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Table 1. Characteristics of experimental sites in chestnut forests in Belasitsa Mountain, SW Bulgaria (data from FMP, 2009)

Site Name of experi-mental site

Altitude (m)

Composi-tion of stand

Mean ageof chestnut

Total area of the site

(ha)

Slope(º) Aspect

CF1 Petrich 700 Beech 7Chestnut 3 > 120 24.3 12 W

CF2 Samuilovo 450 Chestnut 7Beech 3 > 80 13.2 15 W

CF3 Belasitsa 550 Chestnut 10 > 80 14.0 25 W

At the field the characteristics of the above-ground forest stand, forest floor, and soil in the sampling plots were measured. To estimate the above-ground biomass (AB) of trees a taxation of forest stand was undertaken in order to obtain the param-eters in Table 2. Within each site the tree diameter at 1.3 m above ground (DBH), the mean height and density of stand were measured for each tree in the experimental plot and calculated per hectare. Then the above-ground biomass was estimated using data from growth tables for Platanus orientalis – the fast growing species with simi-lar chestnut-like characteristics, and the bonitation value of the stand according to Nedyalkov, Shikov (1983) and Velichkov et al. (2011), reduced in accordance with the stand density. In this study the aboveground-biomass of trees includes branches and leaves. The dry matter (DM) content of 1m3 wood for different tree species is highly variable: Coniferous - Pinus sylvestris - 530 kg, Picea abies - 450 kg, Abies alba - 450 kg; Broadleaved - Quercus cerris L. - 830 kg, Q. conferta - 870 kg, Q. robur - 750 kg, Fagus sylvatica - 720 kg, Betula sp. - 650 kg, Populus sp. - 450 kg (Yatsenko-Hmelevski, 1954; Vihrov, 1959; Busgen, 1961, Enchev, 1971; Delkov, 1992). For that reason we assumed for this study that the dry matter in 1 m3 of wood is 500 kg (Matthews, 1993) and converted all biomass estimates to carbon using a factor 0.5 Mg DM/Mg C (IPCC 1996). The results presented are in tonnes, having into account that 1 Mg = 1,102 t.

Carbon stock in forest floor layer. The carbon content in forest floor layers was estimated by collecting 6 litter samples with matrix from 25/25 cm area at each plot. Samples were dried for 48 h at 75 ºC and weighed individually. Portions of each dried sample were mixed and ground. The carbon concentrations in resulting powders were measured (for L layer the preparation was as plant materials, while for F and H layers – as soil) according to the modified method of Turin (120 °С, 45 min, with catalyst Ag2SO4) and method of Kononova, Belchikova (Kononova, 1966; Filcheva, Tsadilas, 2002). The carbon content in forest floor was estimated by multi-plying its mass by the measured real value of carbon concentration in dry matter and converted to t C ha-1.

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Table 2. Characterization of forest stands in studied European chestnut ecosystems

Site Regenera-tion type

Total stock of tree

biomass per stand

Total stock of chestnut

biomass

Stand density

(%)

Mean height

(m)

No. trees per plot

0,15 ha

Mean diameter

DBH(сm)

CF1 Natural re-generation

4820 m3

(169 m3 ha-1)

3900 m3

(138 m3 ha-1)

0.5 17 65 47.8

CF2 Artificial regeneration

1710 m3 (103 m3

ha-1)

1100 m3

(64 m3 ha-1)

0.7 15 87 23.8

CF3 Artificial regeneration

1500 m3 (87 m3 ha-1)

1500 m3

(87 m3 ha-1)

0.7 16 72 28.5

Carbon stock in mineral soil. Within each soil profile, from which litter sam-ples were taken, samples from mineral soil were collected. A total number of 18 soil samples per plot were obtained from three different depths: 0–10 cm, 10–30 cm, and 30-50 cm and used to estimate the carbon content. Fresh soil samples were individu-ally dried for 48 h at 105 ºC and weighed. The dried samples were sieved through 2 mm mesh to remove coarse sands while soil aggregates were broken. The sample was ground and soil properties were determined in accordance with the standardized meth-ods in the Laboratory of Forest Soil Science at the Forest Research Institute – BAS: bulk density BD (volumetric method), nitrogen content (Kjeldahl method), carbon content (Turin’s method) (Donov et al., 1974; Kononova, 1966; Filcheva, Tsadilas, 2002), coarse fragments (ISO 11646), particle size distribution (ISO 11277), pH (in H2O) (ISO 10390), bioavailable P and K (ISO 11466) (Cools, De Vos, 2010). The bulk density of the soils was measured in duplicate in each soil layer with cylinder with known volume. The carbon concentrations, combined with bulk density and coarse fraction content, were used to estimate the amount of carbon per unit area.

For each experimental site the following estimations of carbon content were performed:

Forest floor:SOСff (t C ha-1) = C x w, where SOCff – carbon stock in forest floor (kg C m-2) C = content of carbon in (%) from the weight or (g kg-1) w = weight of organic layer in (g).

Soil:SOCplot (t C ha-1) = SOC (g C 100 g-1) × corrected BD (g cm-3) × depth of soil

layer (m) × 100,

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where SOCplot – carbon stock in soil layer BD – bulk density real values corrected with the percent of coarse frac-

tion (> 2 cm).

The total carbon storage in experimental site per unit area was estimated as:

tSOCplot (t C ha-1) = Cff + Cs, where tSOCplot: total carbon content, Cff: carbon content in forest floor and Cs: carbon content in 0-50 cm soil depth (Σ SOCplot for all studied

depths).tSOCsite (tC) = tSOCplot (t C ha-1) x area of experimental site (ha) where tSOCsite: Total soil carbon stock per experimental site.

Ecosystem:Ecosystem (t C ha-1) = Carbon stock of ABchestnut per hectare + tSOCplot.

RESULTS

Carbon storage in chestnut forests Large variations were obtained for carbon stock in the chestnut forest stands

(Table 3). The older chestnut stand CF1 accumulates more carbon in chestnut bio-mass (31.1 t C ha-1) compared with other two stands, where the mean age of chestnut was over 80 years – 14.4 – 19.6 t C ha-1. The total carbon stock, including all trees per stand, also showed differences. The highest carbon stock was determined for CF1 – 38.03 t C ha-1, while for the other stands CF2 and CF3 the values were 23.2 and 19.6 t C ha-1, respectively.

Table 3. Carbon stock in the above-ground biomass in studied forest stands (wood + leaves and branches)

Site Total carbon stock of tree biomass per stand Total carbon stock of chestnut biomass

CF1 1084.5 t C (38.03 t C ha-1) 877.5 t C (31.1 t C ha-1)CF2 384.8 t C (23.2 t C ha-1) 247.5 t C (14.4 t C ha-1)CF3 337.5 t C (19.6 t C ha-1) 337.5 t C (19.6 t C ha-1)

General soil characteristicsThe Luvisols in studied sites from the region of Belasitsa Mountain are devel-

oped on gneiss parent materials. The field observations showed that all soil genetic horizons were formed and the soil profile could be referred as type ААВС (Table 4). The total soil depth in all three sampling plots was over 60 cm, while the lowest soil

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horizon was formed mainly by coarse fractions and weathering materials from the parent rock and started from 43 (45) cm depth. The thickness of superficial mineral soil horizon varied from 19 to 28 cm. The soil texture is considered as sandy clay loam with percent of clay between 25 and 30% and with low clay differentiation through the profile. The soil organic matter content varied between 3.8 and 9.1% for А-horizon. The content of total nitrogen was low and the phosphorus showed high variations. All soil profiles studied are low to average stocked with bio-available phosphorus (P2O5 = 7 ÷ 19 mg 100 g-1 soil). The stock of available potassium is good while the highest values were determined for the A-horizon and decreases with depth. The soil pH was weakly acid with small variations within 5.70-6.00. The depth of carbonates deposits was highly variable and in some cases could be found on the surface as a result of erosion processes. The bulk density (BD) generally in-creased with depth. For superficial soil horizons the BD varied between 0.8 and 1.13 g cm-3, while for deeper layers it varied around 1.1-1.2 g cm-3.

Table 4. General characteristics of soils from selected chestnut ecosystems in Belasitsa Mountain

Soil horizon

depthcm

BDg cm-3

pH (in Н2О)

SOM%

C%

N% C:N

Bio-available forms

mg 100g-1

P2O5 K2OCF1

А 0-28 1.13 5.85 3.81 2.23 0.185 12 16.00 22.5АВ 28-45 1.20 6.10 3.28 1.92 0.173 11 11.50 11.0С↓ 45-60 1.20 5.70 0.50 0.29 0.053 6 8.00 11.4

CF2А 0-22 0.81 5.30 3.95 2.31 0.155 15 19.25 19.4

АВ 22-46 1.14 5.75 1.88 1.10 0.107 10 10.50 15.2В (C)↓ 46-60 1.24 5.45 0.50 0.29 0.036 8 9.15 9.3

CF3А 0-19 0.92 5.80 9.11 5.33 0.333 16 17.50 13.8

АВ 19-43 1.13 6.00 8.40 4.91 0.295 17 8.00 12.5В↓ 43-60 1.20 6.00 0.41 0.24 0.068 4 6.50 10.1

Carbon stock in forest floor layers and mineral soil The estimated mean value of carbon stored in forest floor layers was 19.8 t C

ha-1 with variations from 12.5 to 26.1 t C ha-1. The tendency of an increase of C in forest floor samples in the pure chestnut experimental site (CF3) was observed (Ta-ble 5) and the determined stock was the highest among all three studied sites. For the litter layer formed in the chestnut ecosystems in 2010 the carbon input varied from 2.4 to 2.6 t C ha-1 y-1 without any significant difference between sampling plots.

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The highest carbon stock in soils was determined for the surface layer (0-10 cm depth) in all experimental sites (Table 5). In the mixed chestnut forests CF1 and CF2 the carbon stock in this soil layer was 20.1 t C ha-1 and 21.2 t C ha-1, respec-tively. These values were lower than the carbon stock in the same layer under the pure chestnut stand CF3 (34.1 t C ha-1). A decrease of carbon stock was observed with depth and for the 10-30 cm soil layers the values obtained were 23.8 t C ha-1 for CF3, 16.1 t C ha-1 for CF1 and 8.4 t C ha-1 for CF2. The variations in carbon stock were higher for the superficial layer in all studied sites compared with these in the lower depths. The lowest carbon stock was established in 30-50 cm of soils where the values varied from 2.3 t C ha-1 in CF3 to 4.3-4.5 t C ha-1 in the other two sites. The total carbon in mineral soil under studied chestnut ecosystems showed similar differences. The highest carbon stock was determined for soil in the pure chestnut stand (CF3) 60.2 t C ha-1, while for other mixed chestnut forests (CF1 and CF2) it was 41.6 t C ha-1 and 33.0 t C ha-1, respectively.

The carbon stock in both forest floor and mineral soil in studied sites was also higher in CF3 (84.04 t C ha-1) compared with CF1 (59.77 t C ha-1) and CF2 (50.5 t C ha-1). On the basis of data obtained about the total carbon stock in soil per site it could be also mentioned that studied forest are ordered as follows: CF1 (1453 t C) > CF3 (1176 t C) > CF2 (667 t C), mainly because of the differences in area covered by each forest.

Table 5. Total carbon stock (t C ha-1) in soils and forest floor in chestnut forests

Site

Carbon stock (t C ha-1)Soil system (forest floor and soil depths)

SOCplot(0-50 cm)

tSOC-plot

mean

tSOCsite

SOCff SOCplot(0-10 cm)

SOCplot(10-30 cm)

SOCplot(30-50 cm) (tC)

CF1n

meanrange

618.2 ± 1.912.5 – 23.4

621.2 ± 3.214.3 – 35.1

616.1 ± 2.211.3 – 26.6

64.3 ± 0,33.04 – 5.2

641,6 ± 3.034.6 – 55.0

59.8 1453

CF2n

meanrange

617.5 ± 1.3413.2 – 21.4

620.1 ± 1.922.4 – 26.0

68.4 ± 0.56.8 – 10.4

64.5 ± 0.33.6 – 5.4

633.0 ± 1.8

27.7 – 38.6750.5 667

CF3n

meanrange

623.8 ± 0.821.9 – 26.1

634.1 ± 5.120.4 – 48.3

623.8 ± 13.220.1 – 28.3

62.3 ± 0.31.5 – 3.0

660.2 ± 6.246.2 – 76.5

84.0 1176

n = Number of plots sampled. Mean = carbon stock average per site sampled. Range = minimum and maximum values of carbon stock by site sampled. ± = Standard error.

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Following the estimations of carbon stock including above ground biomass, the total stock in the studied chestnut forests could be ordered as follows: CF3 (105.8 t C ha-1) > CF1 (102.1 t C ha-1) > CF2 (76.3 t C ha-1). The pure chestnut forest CF3 characterized with the highest total carbon stock per hectare and only 20.6% from it is accumulated in the aboveground tree biomass. This confirms the high po-tential of carbon sequestration in soil system under pure and mature chestnut forests. At the same time the carbon in older mixed chestnut ecosystems dominated by beech in tree composition (CF1) was also high but 33.8% of carbon is accumulated in the aboveground chestnut biomass, while 21.0% of carbon is sequestered by chestnut trees in the other mixed stand CF2, dominated by chestnut. In CF3 the accumulation is mainly in forest floor and in mineral soil (especially in the superficial soil layer), which shows favourable conditions for the incorporation of organic substances in soil system under pure chestnut stands in mature growing phase.

Table 6. Total carbon stocked in the above-ground tree biomass, the forest floor and the 0-50 cm of soils in studied chestnut ecosystems

Cecosystem (t C ha-1)CF1 CF2 CF3

102.05 76.25 105.75Role of chestnut

biomass(% from total C stored)

33.8 % 21.0 % 20.6 %

Role of other tree species

(% from total carbon stored)

7.6 % 12.8 % 0 %

DISCUSSION

According to the Russian classification the SOM content between 2% and 4% is low, while from 4% to 10% is average (Grishina, 1986). The content of soil organic matter in soils under studied chestnut forests was higher in superficial min-eral horizons in all experimental sites with values from 3.8 to 9.1. Decreasing with depth was determined in all profiles but variations were higher (1.9 to 8.4%), while toward deepest layers SOM content was very low (< 0.5 % according Grishina, Or-lov 1978). There are significant differences in SOM content in the different layers between experimental sites. The higher content in superficial soil horizons is indica-tive for carbon accumulation especially in the superficial soil. The nitrogen content in soils depends on conditions of soil formation processes and the content of organic substances in litter inputs from plant, microbial biomass and soil fauna (Mitovska, Filcheva 1989). The total nitrogen content followed similar relative distribution as

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for the soil organic matter with well expressed decrease toward depth. Overall the soils were characterized by low nitrogen content with mean value of 0.224% for A-horizon. The lowest nitrogen content was determined for the deepest soil layers (mean = 0.053%).

The degree of decomposition of soil organic matter can be expressed by the ratio of carbon to nitrogen (C:N). The organic matter transformation rate is higher when the carbon content is lower and the nitrogen is higher (Penkov, 1991). The results obtained in present study showed that the C:N ratio for upper 0-10 cm soil layer varied between 12 and 16. The type of soil organic matter could be referred as Mull (C:N<14) to Moder (C:N=14-22), which indicates that transformation and decomposition of organic substrates is not complicated. In the data for general soil characteristics the content and type of soil organic matter as well as the C:N ratio is only slightly differentiated and the soils have similar organic matter levels. This similarity is an indication of relatively good decomposition rate of SOM.

The forest floor is often considered as a specific and relatively independent component of forest ecosystems, which could emphasis the need for independent assessment in the investigations on complex forest ecosystems and the dynamic of organic matter transformation (Zonn, 1964; Karpachewskij, 1981). Carbon storage and accumulation in forest floor layers depends on its quality, quantity and level of decomposition, which are influenced by the forest tree species, age, nitrogen deposi-tions, soil acidity, climatic factors and forest management. The level of decomposi-tion of organic substances is strongly related with the process of soil formation and consequently with the soil type. Principally, the organic substances are faster decom-posed in broadleaved forest ecosystems than in coniferous due to higher acidity of coniferous litter and the poorer microclimatic conditions under the stand (Schulp et al., 2008). The studies in mature broadleaved ecosystems from the region of Central Balkan Mountains showed that mixed broadleaved stands characterized with more lightly-textured forest floor compared with pure beech stands (Stoyanova, 1984). The author estimated the stock of forest floor in these mixed forests of 9 t ha-1, while for the pure oak and beech stands it varied between 4 and 14 t ha-1. Studies on the weight of forest floor layers in coniferous stands showed higher values compared with broadleaved stands and varied between 23 t ha-1 in young Scot’s pine planta-tions to 40 t ha-1 in Spruce stands (biosphere reserve Parangalitsa) (Zhiyanski, 2006). Our data presented show that the weight of forest floor in the mixed chestnut stands varied between 7.3 ± 0.6 t ha-1 in CF1 and 6.5 ± 0.3 t ha-1 in CF2 and these values are lower compared with the pure chestnut stand CF3 - 10.1 ± 0.4 t ha-1. The data for carbon content in litter input (L layer including leaves, branches, cones and flakes) could summarized that carbon varied between 2.7 and 7.1 t C ha-1 y-1 for oaks, 2.99 – 5.2 t C ha-1 y-1 for beech, 2.99 – 6.3 t C ha-1 y-1 for Scots pine and 2.7 и 5.7 t C ha-1 y-1 for Norway spruce (INFD 2010). As a result the organic matter decomposition in mixed forests is slower and incorporation of organic substances in organic-mineral soil horizon is also delayed. The layers of forest floor (L+H) formed on soils from

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the north part of temperate climatic zone, could store between 7.1 and 29.9 t C hа-

1(Zerva et al. 2005). For layers L + F a mean accumulation of 18 t C hа-1 is estimated (with variations between 4.1 and 40.8 t C hа-1 determined by tree species, soil and climate) (Berg et al., 2007). The values obtained in out study in chestnut mixed and pure stands are relatively lower compared with other reported although in mixed broadleaved forests, the composition of forest floor components is more diverse compared with these in the pure stands.

For broadleaved forests the average carbon content in the aboveground and below-ground biomass varied from 53 to 105 t C ha-1 (Cairns et al., 1995; Masera et al., 1997, 2001). Other studies report that in oak forests the variation of carbon content was between 76.7 – 181.7 t C ha-1 and the highest soil carbon stocks under oak forest was 34.5 t C ha-1 for 0-30 cm soil depth (Ordόñez et al., 2008). In present study the below-ground tree biomass was not included in the estimation. Compared with these data the carbon reported here was lower for the aboveground chestnut forest components, while the carbon stocked in soil system is relatively high, which confirms the important role of soils in long-term carbon storing.

Soils and belowground biomass store a significant part of carbon in ter-restrial ecosystems (Schlesinger, 1986; Janzen, 2004). Significant differences are reported for carbon content in soils under different types of forests. The main ef-fect of different tree species in the forest composition on soil organic carbon and C:N ratio are well defined for the forest litter component, which is directly related to litter fall quality and the characteristics of the organic substances (Aubert et al., 2010), while studies in mineral soils are relatively scarce (Skovsgaard et al., 2006; Vesterdal et al., 2008 ; Zhiyanski et al., 2008, Ordόñez et al., 2008). For the brown forest soils under natural beech stands relatively high organic carbon contents compared to soils under pine and spruce are reported (Filcheva et al., 2002; Sokolovska et al., 2005). Tree species influence the carbon content in soil, especially in the surface 0-10 cm of soils (Zhiyanski et al., 2008). The authors found higher values for soil carbon content under natural beech forests compared with spruce and pine plantations. Moreover, beech trees develop deep root system, which enhances the accumulation of carbon in mineral soil (Kreutzer et al., 1986) and is likely to be similar for chestnut forests.

Gallardo González-Hernández (2005) studied the variability and potential of C storage in soils under chestnut forests in Central-Western Spain, managed as cop-pices (timber production) or orchards (fruit production) and their results indicate that management has stronger effect on soil C contents compared with site location, where the soil C content at plot level showed variations between 4 and 93 mg C g-1. The authors reported the maximum soil C content 530 t C ha-1 and concluded that this value is considered to be the maximum C-storage potential of the chestnut soils. Orchard soils may have values as low as 44 t C ha-1, indicating limitation in soil organic matter inputs but having potential for enhanced C storage to at least 165 t C ha-1 under an appropriate management regime.

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According to data reported for Bulgarian soils the carbon content can vary be-tween 25 and 681 t C ha-1 in 0-100 cm soil depth (Filcheva et al. 2002). These varia-tions depend on the depth of soil included in the calculation of soil carbon stock, soil type and age of the studied forests, as well as on the content of coarse fractions in soils. Calculations of carbon stocks and energetic stocks in soil organic matter in Bulgarian main soil types are published in Boyadgiev et al. (1994), Raitchev, Geor-gieva (1989), Artinova et al. (2007). The authors reported that the ranges of varia-tions of organic carbon stocks for typical Chromic Luvisols in Bulgaria are 37-70 t C ha-1 for 0-20 cm depth. In cases with podzolization processes the carbon stock in this soil type decreases to 23-44 t C ha-1 for 0-20 cm depth. The results obtained for 0-50 cm soil depth in studied soils under chestnut forests are within the limits for the typical Chromic Luvisols.

CONCLUSIONS

The older matured mixed chestnut forest CF1 is characterized by relatively high total carbon stock (102.1 t C ha-1) and 41.4% from it is accumulated in the aboveground tree biomass. The fact that 33.8% from this stock is stored in chestnut tree biomass confirmed the high potential of the matured and old mixed chestnut ecosystems to sequester carbon by aboveground tree component. At the same time the pure chestnut ecosystems CF3 had the highest carbon storage 105.8 t C ha-1 , but the accumulation is mainly by forest floor and mineral soil (especially in the superfi-cial soil layer), which indicates the good conditions of incorporation of organic sub-stances in soil system under pure chestnut stands. Based on data obtained could be concluded that the pure chestnut forest ecosystems in Belasitsa Mountain typically have a high potential to sequester carbon in the components of their soil system.

Acknowledgements: This work was supported in part of the projects DO-02/265/2008 ‘Ge-netic characterisation and nut quality of chestnut (Castanea sativaMill.) populations’ – GENCAST and DMU 02/15/2009 ‘Soil carbon pools and fluxes in soil-to-plant system of urban forest parks’ funded by Bulgarian Science Fund at the Ministry of Education, Youth and Science of Republic of Bulgaria.

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E-mail: zhiyanski@abv.bg