The effects of broad-leaved tree species on litter quality and soil properties in a plain forest stand

The effects of broad-leaved tree species on litter quality and soil properties in a plain forest stand

Catena 150 (2017) 223–229 Contents lists available at ScienceDirect Catena journal homepage: www.elsevier.com/locate/catena The effects of broad-le...

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Catena 150 (2017) 223–229

Contents lists available at ScienceDirect

Catena journal homepage: www.elsevier.com/locate/catena

The effects of broad-leaved tree species on litter quality and soil properties in a plain forest stand Yahya Kooch ⁎, Behnaz Samadzadeh, Seyed Mohsen Hosseini Faculty of Natural Resources & Marine Sciences, Tarbiat Modares University, 46417-76489 Noor, Mazandaran, Iran

a r t i c l e

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Article history: Received 31 May 2016 Received in revised form 17 September 2016 Accepted 22 November 2016 Available online xxxx Keywords: Soil chemistry Earthworm Nematode Microbial respiration Temporal variability

a b s t r a c t The role of tree species on litter quality and soil characters is less known in mixed forest stand. For this reason, the effect of Carpinus betulus (CB), Acer velutinum (AV), Pterocarya fraxinifolia (PF), Quercus castaneifolia (QC) species on litter and topsoil physical, chemical and biological features was considered in northern Iran. Litter quality differs among species, with the highest total nitrogen (N) concentration (1.88%) and lowest organic carbon (C) (41.18%) under CB trees. Clay and water content did not differ among species, but soil bulk density and sand content were highest under CB (1.66 g cm−3 and 44.70%, respectively) with the least silt content (28%). Soil pH (7.10), EC (0.29 ds/m), total N (0.35%), available P (21.85 mg kg−1), K (316.66 mg kg−1), Ca (254.50 mg kg−1), Mg (58.50 mg kg−1), earthworm density/biomass (2.50 n m−2 and 29.59 mg m−2, respectively) with more share of epigeic, total nematode (443.90 in 100 g soil) and microbial respiration −1 ) were significantly higher beneath CB trees whereas a higher content of organic C (0.47 mg CO2-C g−1 soil day (3.15%) and C/N ratio (34.20) were found under QC. A greater quantity of fine root biomass was found under PF (92.80 g m−2) trees. In all the studied tree species, earthworms (epigeic, anecic and endogeic) and nematodes had the highest activities in autumn while the maximum of microbial respiration was recorded in summer season. The findings obtained in this study can be prioritized in the selection of appropriate species for the restoration of degraded areas. © 2016 Elsevier B.V. All rights reserved.

1. Introduction There is little doubt that trees play a major role in structuring terrestrial systems (Scheu et al., 2003). By creating their own habitat and regulating resource availability, trees can influence other organisms through several pathways (Ayres et al., 2004). Water relations (Anderson et al., 2001), availability of light (Thomsen et al., 2005), litter inputs (Sayer, 2006) and soil properties (Reich et al., 2005) may be altered by trees; even individual trees can control surrounding vegetation and soil chemistry (Brooker et al., 2008). In forest ecosystems, tree species have great impacts on physical, chemical and biological properties of soil (Guendehou et al., 2014; Gartzia-Bengoetxea et al., 2016) through their physical structure, especially their litter inputs (Schwarz et al., 2015). Litter quality characters, as the main driving force for soil processes, especially relative proportions of carbon (C) and nitrogen (N) contents vary between different tree species (Guendehou et al., 2014). Neirynck et al. (2000) found that C/N ratio in topsoil was variable under different tree species (i.e. Tilia platyphyllos, Fraxinus excelsior and Acer psedoplatanus) that was related to litter quality of these species. A ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Kooch), [email protected] (B. Samadzadeh), [email protected] (S.M. Hosseini).

http://dx.doi.org/10.1016/j.catena.2016.11.023 0341-8162/© 2016 Elsevier B.V. All rights reserved.

study carried out Lovett et al. (2002) revealed that tree species may affect the cycling of C, N and other nutrients in the soils under their canopies and this, in turn, can influence substantial processes at an ecosystem level. A study by Hagen-Thorn et al. (2004) revealed that tree species clearly have influence on different soil properties and this effect is more obvious at upper layer of soil. In a study, the soils under Fagus sylvatica demonstrated lower soil pH values, lower base saturation and higher C/N ratios when compared to forest soils under mixed deciduous forests comprising Fagus, Fraxinus, Tilia and other deciduous tree species (Thoms and Gleixner, 2013). Obviously, the impact of trees on the properties of ecosystem is likely to have important consequences for belowground communities (Ayres et al., 2004). Above and belowground inputs may be different between tree species in both quantity and quality. In particular, resource quality can be a strong determinant of soil community structure (Reich et al., 2005). Soil fauna are an important component in forest ecosystems, due to their functional role in accelerating the decomposition of organic matter and nutrient transformations. The positive influence of soil fauna on the decomposition of plant litter is widely known and well accepted for many ecosystems (Yang and Chen, 2009). The activity of soil invertebrates helps to enhance the physical, chemical and biological properties of soils (Sarlo, 2006). Among soil organisms, earthworms are the recognized ecosystem engineers. They are the major component of the

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(Talebi et al., 2014). Despite the importance of Hyrcanian forests, earlier study that evaluated the effects of dominated individual trees on litter quality and also spatio-temporal variability of soil characters at the stand level wasn't considered. This study aims to investigate: (i) if different tree species affect litter quality, soil physico-chemical properties and also earthworm density/biomass, nematode population and microbial activity, (ii) seasonal changes in soil organisms activity under different forest species within the forest and, (iii) which factors determine earthworm, nematode and microbial activity in these temperate forests. We hypothesized that (1) the influence of individual trees on soil properties is detectable even in mixed stands and the soil landscape may be considered a mosaic of profiles reflecting the litter chemical characters through individuals of the various tree species present, and (2) expected changes in climate will potentially have a stronger impact on soil biological activity than changes in forest diversity. 2. Materials and methods 2.1. Study area The study area is located at the Experimental Forest Station of Tarbiat Modares University, north of Iran (51° 46″ E, 36° 47″ N). The experimental plots were located at an altitude of 15 m above sea level. The area is on flat and uniform terrain (slope 0–3%). Mean annual rainfall is 803.4 mm and mean annual temperature is 17 °C at the Noushahr city metrological station, which is 1 km away from the study area (Anonymous, 2010). Based on the metrological station at study time, there is a dry season between May and August (Fig. 1). The parent material is dolomite limestone which belongs to upper Jurassic and lower Cretaceous period. The soil order name is Alfisols. Soil texture is silty clay loam. The natural forest vegetation is temperate deciduous forests containing broad-leaved species dominated by oak (Quercus castaneifolia C. A. M. macranthera F. & M.), hornbeam (Carpinus betulus L.), Caucasian wingnut (Pterocarya fraxinifolia), false walnut (Pterocarya carpinifolia Lam.), maple (Acer velutinum Boiss., Acer cappadocium Gled.) with some associated species such as ash (Fraxinus excelsior L.), alder (Alnus subcordata C. A. M., Alnus glutinosa Gaertn.), elm (Ulmus glabra Huds.), wild cherry (Prunus avium L), wild service tree (Sorbus torminalis Crantz), and lime tree (Tilia platyphyllus Scop.) (Mirzaei et al., 2007). These forests are uneven-aged mixed stands and the trees diameter at breast height (D.B.H.) varies from 10 to 110 cm (Anonymous, 2010). 2.2. Data collection

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Following a field trip, clumps of tree species were identified in the study area. Samplings were performed under individual tree of Carpinus betulus (CB), Acer velutinum (AV), Pterocarya fraxinifolia (PF), Quercus

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decomposer fauna in many forest ecosystems (Lavelle and Spain, 2001) and are categorized into three main ecological groups; epigeic, anecic and endogeic species (Edwards, 2004). Epigeic and anecic earthworms, the functional groups most affected by litter quality, are more likely to be directly influenced by the interspecific differences in plant residues. However, endogeic earthworms have also demonstrated preferences of some plant residues over others (Sarlo, 2006). Earthworm abundance/biomass is typically associated with soils that contain high organic matter content, which greatly affects soil pH, soil nutrients, waterholding capacity and aggregation (Yatso and Lilleskov, 2016) and the whole of these features can be affected by the type of tree species (Schwarz et al., 2015). Furthermore, soil nematodes occupy a central position in the detritus food web (Cesarz et al., 2013). They can be used as indicators of the structure and function of soil food webs and of general conditions of the ecosystem (Zhang et al., 2012). In forest ecosystems, the distribution and abundance of soil nematodes is determined by factors such as tree species, soil type, soil fertility, litter depth and forest management (Yeates, 2007). Although the effects of plant community composition on soil nematodes are inconclusive, Yeates (2007) suggested that soil nematode communities can provide important information about the role of forest species in structuring soil food webs. According to the report of Keith et al. (2009), the changes in the nematode population could be attributed to changes in the availability of food resource and environmental conditions. Plant species in forest ecosystems have important roles in determining the composition of the soil biota through both above- and belowground resource inputs and by altering abiotic conditions (Cesarz et al., 2013). Soil microorganisms are important drivers of soil processes, and they play a key role in the decomposition of recent plant material (Thoms and Gleixner, 2013). Soil respiration is the sum of the autotrophic component produced by roots and the heterotrophic component derived from soil microorganisms that decompose organic materials in litter. Tree species significantly contribute to soil respiration and deposition of C in the forest areas (Zifcakova et al., 2016). In forests dominated by different tree species, the forest floor may contain several different types of substrates for microbial growth, and variations in forest floor characteristics may produce differences in the composition and function of microbial communities (Jagadamma et al., 2014). The environmental conditions beneath the forest canopy (generated by trees and/or preexisting differences between the sites) influence soil microbial community and respiration. Microbial respiration from soils is regulated by environmental factors such as temperature and moisture and biotic factors (Makita and Fujii, 2015). The activity of soil biology in temperate broadleaf forests is strongly affected by the seasons through changes in biotic and abiotic factors (Kaiser et al., 2010). Unfortunately, studies considering the impact of tree species on soil organisms through seasonal cycles are rare (Collignon et al., 2011). In a study, for example, strong seasonality in the abundance of earthworm populations in mixed forest sites has also been demonstrated and has been attributed to differences in litter decomposition and fluctuations in the soil C pool over the years (Thoms and Gleixner, 2013). Tree fine roots (b 2 mm) are dynamic and short-lived, supplying considerable belowground litter inputs and accounting for up to 75% of the net primary production of forest ecosystems. After root death, the decomposition of fine roots will be an important heterotrophic source of CO2 (Makita and Fujii, 2015). Fine roots are very dynamic and play a key role in forest ecosystem C and nutrient cycling and accumulation. Generally, fine root biomass is affected by the type of tree species (Lei et al. 2012), availability of nutrient and other environmental conditions (Zhou et al., 2014). The Hyrcanian vegetation zone, also called Caspian forest, is one of the last remnants of natural deciduous forests in the world (Talebi et al., 2014). The natural forest vegetation is temperate deciduous forests containing broad-leaved species that are very similar to forests typical of central Europe, northern Turkey and the Caucasus (Adel et al., 2014). While the Caspian coastal areas included a milder climate, the inland plateau experiences extremes of hot summers and cold winters

Temperature (oC)

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Month Fig. 1. Mean monthly temperature and precipitation in study area based on Noushahr city metrological station.

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castaneifolia (QC) species in the same diameter class (≈50 cm). These species were surrounded by similar tree species. Ten replication of each species was done; litter and soil (30 × 30 × 15 cm) samples were taken under tree canopy cover in the summer (August) to characterize litter, soil physio-chemical and biological properties. Total C and N contents in litter samples were determined in quadruplicate using dry combustion with an elemental analyzer (Fisons EA1108, Milan, Italy) calibrated by the BBOT [2, 5-bis-(5-tert-butyl-benzoxazol-2-yl)thiophen] standard (Ther moQuest Italia s.p.a.). The obtained data was corrected for moisture content (Kooch et al., 2012). Soils were airdried and passed through 2-mm sieve (aggregates were broken to pass through a 2 mm sieve). Part of the soil samples were immediately transferred to a cooled, insulated container for transport to the laboratory and were stored at 4 °C until they were processed. Bulk density was measured by Plaster (1985) method (clod method). Soil texture was determined by the Bouyoucos hydrometer method (Bouyoucos, 1962). Soil water content was measured by drying soil samples at 105 °C for 24 h. Soil pH was determined using an Orion Ionalyzer Model 901 pH meter in a 1:2.5, soil: water solution. EC (electrical conductivity) was determined using an Orion Ionalyzer Model 901 EC meter in a 1:2.5 soil: water solution. Soil organic C was determined using the Walkey-Black technique (Allison, 1975). The N was measured using a semi Micro-Kjeldhal technique (Bremner and Mulvaney, 1982). Available P was determined with a spectrophotometer and the Olsen method (Homer and Pratt, 1961), and available K, Ca, and Mg (by ammonium acetate extraction at pH 9) were determined with an atomic absorption spectrophotometer (Bower et al., 1952). Fine roots (b2 mm diameter) were removed from each sample and dried at 70 °C to a constant mass (Neatrour et al., 2005). Seasonal (May, August, November and February) measurements were performed under studied tree species to find temporal pattern of the soil biological activities. The earthworms were collected simultaneously with the soil sampling by hand sorting, washed in water and weighed with mili gram precision. Species of earthworms were identified (epigeic, anecic and endogeic) by external characteristics (Kooch, 2012). Earthworms were counted in the field and brought to the laboratory and then they were oven dried at 60 °C for 24 h. Earthworm biomass was determined after drying (Kooch et al., 2014). Nematodes

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were extracted from 100 g soil sample (fresh weight) by a modified cotton–wool filter method (Liang et al., 2009). Soil microbial respiration was determined by measuring the CO2 evolved in 3 days incubation experiment at 25 °C (Alef, 1995). 2.3. Statistical analysis Normality of the variables was checked by Kolmogorov-Smirnov test and Levene's test was used to examine the equality of the variances. One-way analysis of variance (ANOVA) was used to compare litter and soil properties data among the tree sites. Two-way ANOVA and general linear models (GLM) were used to compare changes of soil biological data among trees species and seasons. Duncan's multiple comparison test was further employed to test for differences at the P = 0.05 level. All statistical analyses were conducted using the SPSS v. 20 statistical software packages. Factor analysis is a statistical tool for exploring complex relationships among variables. For this purpose, we used principal component analyses (PCA) to examine relationships in the multivariate data. Multivariate correlations were used to identify significant relationships among variables and principal components using PC-Ord version 5.0 (Mc Cune and Mefford, 1999). 3. Results 3.1. Litter quality, soil physico-chemical features Litter C was significantly higher under QC than in PF N AV ≈ CB tree species (Table 1). CB had significantly higher N concentrations than all other studied species (Table 1). Litter C/N ratio was significantly different among tree species and QC had the highest value of this character (Table 1). Soil bulk density was found in ranked order of CB N AV N PF N QC tree species (Table 1). CB species had significantly higher sand content than QC N AV ≈ PF species (Table 1). Soil silt content was found in ranked order of PF N AV N QC N CB (Table 1). Clay and water content were not significantly different among tree species (Table 1). Soil pH and EC were significantly higher under CB trees when compared with the other species (Table 1). A significantly higher content of organic C was found under QC when compared with

Table 1 Mean values and standard error (SE) (ten replications in all cases) of the litter and soil variable analyzed. The studied trees species were the Carpinus betulus (CB), Acer velutinum (AV), Pterocarya fraxinifolia (PF), Quercus castaneifolia (QC). Litter and soil features

Litter features

Physical features

Chemical features

Biological features

Litter C (%) Litter N (%) Litter C/N ratio Bulk density (g cm−3) Sand (%) Silt (%) Clay (%) Water content (%) pH (1:2.5 H2O) EC (ds/m) Organic C (%) Total N (%) C/N ratio Available P (mg kg−1) Available K (mg kg−1) Available Ca (mg kg−1) Available Mg (mg kg−1) Earthworm density (n m−2) Earthworm biomass (mg m−2) Total nematode (in 100 g soil) Soil microbial respiration (mg −1 ) CO2-C g−1 soil day Fine root biomass (g m−2)

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SE

41.18c 1.88a 21.92d 1.66a 44.70a 28.00c 27.30 35.32 7.10a 0.29a 1.13c 0.35a 3.65b 21.85a 316.66a 254.50a 58.50a 2.50a 29.59a 443.90a 0.47a

0.74 0.02 0.48 0.06 6.22 2.24 4.33 2.50 0.12 0.00 0.03 0.03 0.45 2.04 14.76 12.56 1.29 0.56 5.75 25.09 0.02

43.88c 1.53b 29.51c 1.57ab 27.70b 41.40ab 30.90 38.72 7.13a 0.22b 1.46c 0.28a 5.66b 32.61a 282.33ab 204.67b 52.16b 1.90ab 24.21a 354.60b 0.44a

0.84 0.08 1.38 0.04 4.04 2.84 1.68 1.41 0.14 0.01 0.08 0.02 0.66 1.22 23.02 3.91 1.58 0.45 5.78 19.22 0.00

54.11b 1.34c 41.23b 1.45bc 27.00b 44.30a 28.70 35.48 6.14b 0.23b 2.19b 0.19b 13.16b 15.70b 253.83b 181.17c 49.50b 1.90ab 24.66a 298.90c 0.32b

2.77 0.06 2.44 0.05 2.04 2.42 2.28 0.69 0.18 0.01 0.19 0.02 1.80 0.96 11.93 6.04 0.55 0.31 3.05 5.58 0.01

59.29a 0.97d 61.16a 1.30c 35.70ab 36.70b 27.60 34.41 5.90b 0.15c 3.15a 0.13b 34.20a 12.46b 175.67c 107.00d 39.50c 0.70b 8.61b 258.60c 0.27c

0.52 0.02 1.72 0.05 4.29 2.42 2.16 2.19 0.17 0.00 0.25 0.01 7.86 1.00 7.72 7.55 2.17 0.21 3.08 17.80 0.00

31.425 44.021 105.165 7.239 3.532 8.164 0.338 1.045 16.474 34.227 28.786 13.519 11.860 14.263 15.195 56.489 27.096 3.397 3.884 19.116 43.820

0.000 0.000 0.000 0.001 0.024 0.000 0.798 0.384 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.028 0.017 0.000 0.000

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Results from the ANOVAs are included (F test and P value). Different letters in each line indicate significant differences (P b 0.05 by Duncan test) between tree species.

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Fig. 2. Mean values of epigeic, anecic and endogeic density (a) and biomass (b) in different seasons under four tree species. The studied trees species were the Carpinus betulus (CB), Acer velutinum (AV), Pterocarya fraxinifolia (PF), Quercus castaneifolia (QC). For earthworm density, tree species (F = 24.239; P = 0.000), season (F = 79.850; P = 0.000) and Ts × S (F = 2.487; P = 0.011). For earthworm biomass, tree species (F = 27.638; P = 0.000), season (F = 102.534; P = 0.000) and Ts × S (F = 2.885; P = 0.004). Different capital and lowercase letters are indicating significant differences (P b 0.05 by Duncan test) between tree species and seasons, respectively.

PF and AV ≈ CB species (Table 1). Total N and available nutrients were significantly higher under CB when compared with the other studied tree species, while greater amounts of the C/N ratio were found under the QC species (Table 1). 3.2. Soil biological features

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explaining variations of earthworm density/biomass (50.04 and 57.26%, respectively), total nematode (69.98%) and microbial respiration (81.55%) in the study area (Fig. 5). The amount of fine root biomass varied among the study sites in the ranked order of PF N AV N CB N QC tree species (Table 1). From the PCA output, the first PC explained more than 90% of variance in litter and soil features under studied tree species (Fig. 6). The left PC1 shows the condition with good quality of litter, alkaline soil, accumulation of macro element nutrients and more biological activities and this can be attributed to CB and AV trees, while the right PC1 presented positions with low quality of litter, acidic soil, less macro elements nutrients and low biological activities imposed by QC species (Fig. 6). PF was located at the positive part of PC2 (explained variance less than 10%), and is associated with high values of fine root biomass and silt content (Fig. 6). In relation to soil organism's activity in the summer season, growth period, the litter and soil C, N and also available nutrients were more highlighted characters. On this regard, the earthworm's density/biomass were positively correlated with available P (r = 0.50** and 0.48**), available Mg (r = 0.48** and 0.47**), litter N (r = 0.46** and 0.48**), total N (r = 0.40** and 0.45**) but were negatively correlated with the litter C/N ratio (r = − 0.46** and − 0.48**), organic C (r = −0.43** and −0.45**), soil C/N ratio (r = −0.40* and −0.46**), respectively (Fig. 6). Soil nematode activity was positively correlated with available Ca (r = 0.71**), EC (r = 0.65**), litter N (r = 0.64**) and available Mg (r = 0.63**) but was negatively correlated with the litter C/N ratio (r = −0.65**), soil organic C (r = − 0.65**) and litter C (r = −0.63**) (Fig. 6). The soil microbial respiration was positively correlated with available P (r = 0.76**), available Mg (r = 0.75**), pH (r = 0.74**), litter N (r = 0.73**), total N (r = 0.71**) and available Ca (r = 0.70**) but was negatively correlated with the litter C/N ratio (r = − 0.81**), soil organic C (r = − 0.78**) and litter C (r = −0.73**) (Fig. 6).

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The greater amounts of soil earthworm density/biomass were detected under CB when compared with the other tree species, with an order of autumn N spring N summer N winter (Table 1; Fig. 2a, b). Epigeic had more share than in anecic and endogeic ecological groups (Fig. 2a, b). Soil nematode population was found to be significantly higher under CB than in AV N PF ≈ QC trees, especially in autumn season (Fig. 3). Soil microbial respiration was significantly higher in order of CB, AV, PF and QC species and also summer season (Fig. 4). Two-way ANOVA showed that the season was the more dominating factor for

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Fig. 4. Mean values (±SE) of soil microbial respiration in different seasons under four tree species. The studied trees species were the Carpinus betulus (CB), Acer velutinum (AV), Pterocarya fraxinifolia (PF), Quercus castaneifolia (QC). For microbial respiration, tree species (F = 103.445; P = 0.000), season (F = 826.378; P = 0.000) and Ts × S (F = 11.803; P = 0.000). Different capital and lowercase letters are indicating significant differences (P b 0.05 by Duncan test) between tree species and seasons, respectively.

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Fig. 3. Mean values (±SE) of nematode population in different seasons under four tree species. The studied trees species were the Carpinus betulus (CB), Acer velutinum (AV), Pterocarya fraxinifolia (PF), Quercus castaneifolia (QC). For total nematode, tree species (F = 198.322; P = 0.000), season (F = 702.943; P = 0.000) and Ts × S (F = 18.384; P = 0.000). Different capital and lowercase letters are indicating significant differences (P b 0.05 by Duncan test) between tree species and seasons, respectively.

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Fig. 5. Contribution (%) of independent single factors “tree species”, “season” and the combination of both “tree species + season” to variation of soil organisms by application of two-way ANOVA test.

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Fig. 6. PCA based on the correlation matrix of the tree species, litter, soil physico-chemical and biological features. The studied trees species were the Carpinus betulus (CB), Acer velutinum (AV), Pterocarya fraxinifolia (PF), Quercus castaneifolia (QC).

4. Discussion 4.1. Litter quality Based on our results, litter characters were affected by different tree species. The whole of the studied species were in deciduous form, but also clearly differ in litter quality. Good qualities (higher concentration of N with lower levels of C and C/N ratio) of litters were found under CB trees, whereas QC species presented low quality of litters (lower concentration of N with higher levels of C and C/N ratio). According to the report of Kooch (2012), it is known that CB species produce litter poor in C and rich in N compounds which confirms our findings. Aubert et al. (2003) pointed that CB trees generally have low lignin-N ratios, high tannins and fast decomposition rates. Low-quality litter under QC species will release less N than the high-quality litter under the other tree species; this is because those available nutrients are immobilized more rapidly by microbes decomposing low quality, nutrient-poor litter (Giardina et al., 2001). Narrow C/N ratio in the litter of CB individual trees caused the greatest part of C to be converted into CO2 by oxidative processes as the end product of organic matter decomposition (Kooch, 2012). 4.2. Soil physico-chemical features Our results showed an increase in soil bulk density under CB trees, due to the presence of low organic matter (Xie et al., 2007; Schulp et al., 2008). The changes of soil texture fractions, sand and silt, among studied tree species indicates a different evolution of the soil profile when covered by forest, especially in terms of erosion (Kooch et al., 2012). It is well known that QC litter is more acidic than the other studied tree species; thereby validating the theory that litter pH affects soil pH (Marcos et al., 2010). Actually, the accumulation of organic matter is a mechanism that can lead to soil acidification in forest stands (Smal and Olszewsk, 2008). In this study, QC trees litter has low speed of decomposition and construct thicker organic layer beneath these trees, which results in lower pH when compared to other trees. According to the findings of Haghdoost et al. (2011), the soil EC variability under different species may be caused by different foliage properties and the litter quality. Based on our results, the highest and the lowest soil organic C contents belonged to QC and CB, respectively. In general, different tree species play an important role in biogeochemical regulation of ecosystems through the stabilization of soil organic C (Chen et al., 2003). Lower content of organic C in soils under CB trees is due to

the rapid mineralization of organic matter following the high pH (Chase and Singh, 2014). The high contents of N in CB litters, following more rapid decomposition (Kooch, 2012), improved the soil N when compared to the other tree species. The large ratios of soil C/N under QC trees likely resulted from low mineralization rates and, consequently, their levels of total N and available nutrients were low (Sayyad, 2009). A higher pH is typically associated with improved nutrient cycling, especially of elements that are critical for plant growth (Sayyad, 2009). Based on this, the CB and AV had the lower acidity, alkaline soil, than QC and PF trees with higher soil fertility due to the accumulation of macro elements. In fact, soil acidity increases Al and Fe solubility, which eliminates base cations from the exchange complex (Scharenbroch and Bockheim, 2007). Therefore, under QC, base cations are likely to be displaced from the CEC to the soil solution where they are leached from the upper soil. On the other hand, the level of soil macro elements content are directly linked to nutrients released by trees and nutrient cycling through litter (Dijkstra et al., 2001; Dijkstra and Smits, 2002), therefore, CB trees provided fertile soils considering the good quality of litter. 4.3. Soil biological features The results obtained from this study show that the earthworm density/biomass presented greater amounts under CB, with more share of epigeic, especially in the autumn season. Actually, it is well-known that earthworm distribution and biomass are affected by changes in vegetation (e.g., differences in litter quality) and/or soil features (e.g., water content, pH, nutrient availability; Kooch et al., 2015a). The ambient temperature and soil moisture regime plays an important role in earthworm dispersal in soil (Suthar, 2012). In line with the findings of Suthar (2012) and Ewing et al. (2015), our findings revealed that the earthworm density/biomass demonstrated higher amounts in autumn and spring, when soil moisture and air temperature was more favorable than in summer and winter seasons (see Fig. 1) under whole tree species. In fact, more assemblage (larger number and greater biomass) of earthworms in the autumn can be related to higher amounts of soil moisture and also lower temperature (Ewing et al., 2015). Epigeic and anecic earthworms, the functional groups most affected by litter quality, are more likely to be directly influenced by the interspecific differences in plant residues. However, endogeic earthworms have also demonstrated preferences of some plant residues over others (Sarlo, 2006). In summer, under CB species, lower C (Sackett et al., 2013), higher N (Sigurdsson and Gudleifsson, 2013), lower C/N ratios (Watmough and

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Meadows, 2014) and more available nutrients (Sigurdsson and Gudleifsson, 2013; Sackett et al., 2013) drive increased earthworm density/biomass. Soil nematodes are the important bio-indicator of soil environmental change, and their responses play a critical role in the feedbacks of terrestrial ecosystems to climate change (Saul-Tcherkas et al., 2013). In the forest ecosystems, precipitation is a key factor that influences soil biota communities. Changes in precipitation will have a more direct impact on soil moisture status and will profoundly impact soil biochemical properties and soil organisms (Wall et al., 2010; Saul-Tcherkas et al., 2013). Our results also showed an increase in the population of nematode based on increase precipitation in the autumn (see Fig. 1). In addition, soil nematode population demonstrated considerable variation among studied trees species linked to the variability of litter quality and soil properties. The nematode population was increased under CB trees that are related to higher N of litter (Salamon et al., 2004), lower C and C/N ratio in litter and soil (Sun et al., 2013) and more nutrient elements (Salamon et al., 2004). The average soil respiration rate in the summer season was higher compared to the winter period. This is in line with higher summer CO2 emission (Li et al., 2008; Liu et al., 2012; Luo et al., 2014) and with a reduction in CO2 emission observed in the other seasons (Evans and Wallenstein, 2012). In our study area, temperature increases during summer season (see Fig. 1) and it speeds up microbial-mediated processes in soil (Kooch et al., 2015b). Furthermore, different values of soil microbial respiration in this present study area indicate that the substrate quality (humic substances) and quantity (mass) are very variable under different tree species (Singh et al., 2012). Based on this, the soil under QC and CB-AV tree species presented the minimum and maximum microbial respiration, respectively. In line with the data obtained in this study, Brumme and Khanna (2009) who found the negative correlation between C/N ratio and soil respiration indicated that the quality of substrate affects heterotrophic respiration. Furthermore, according to the findings of Martin et al. (2009), N could be limiting which reduces microbial respiration at a C/N ratio N25–30 which is found under QC tree species. The influence of the soil pH in this study agrees with the established concept that soil pH strongly determines microbial respiration, soil pH close to 7.0 is most suitable for microbial respiration (Kooch, 2012) beneath CB and AV canopy covers. In addition, nutrient availability, as a major factor controlling the variability in soil microbial respiration (Tardy et al., 2014), improved the microbial respiration under CB and AV trees. Following the variability of fine root biomass under different tree species, the correlations between this character and site fertility seemed to be species specific. According to the report of Lukac (2012), the fine root production is very much in boreal forests due to low fertility of soil. In support of this claim, we suspect that higher fine root biomass under PF trees can be related to lower fertility of soil compared to AV and CB species. About QC trees, the very high level of C/N ratio, greater than 30, limits the growth of fine root (Helmisaari et al., 2007; Sariyildiz, 2015). Given that fine roots could be easily affected by soil environmental factors (Xu et al., 2013), the study of other soil parameters and also the root structure of the studied tree species are proposed. 5. Conclusion The results demonstrated that the presence of trees have different significant effects on litter quality and soil properties even in a mixed stand. The C. betulus trees enhanced the quality of litter (higher N concentration with lower levels of C and C/N ratio), soil pH, fertility (higher amounts of total N and available nutrient including P, K, Ca and Mg) and biological activities (greater amounts of earthworm density/biomass, total nematode, microbial respiration) than in A. velutinum, P. fraxinifolia and Q. castaneifolia species. The P. fraxinifolia increased soil fine root biomass when compared to the other species. Under studied tree species, ecological groups of earthworms (epigeic, anecic and endogeic) and nematodes had more activity in autumn whereas the most values of

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