Introduction

The Caspian forest (Hyrcanian region) is a natural green belt that covers the northern slopes of the Alborz Mountains and extends to the southern slopes of the Caspian Sea. The Hyrcanian region is approximately 110 km wide, 800 km long and has a total area of 2.1 million ha, equivalent to 15% of Iran’s total forest area and 1.1% of Iran’s entire area. Hyrcanian region or Caspian forests is one of the last remnants of natural temperate deciduous forests in the world (Sagheb Talebi et al. 2014), which are very valuable in terms of ecological, social, economic, and cultural terms and recently were included in the UNESCO World Heritage List (Hosseini 2019). Forest areas located at high altitudes are often exposed to strong wind events. In this regard, some tree species are more affected by the wind and uprooted due to their special crown and root structures. Deadwoods that appear due to the uprooting of trees in the forest will have many ecological effects on the habitat and affect the soil functions over time. Deadwood plays a vital role in protecting biodiversity, forest ecosystem dynamics, soil fertility, and C stocks (Shannon et al. 2022; Uhl et al. 2022; Sacher et al. 2022). The type and amount of deadwood are the results of several factors occurring in forest ecosystems, including climatic factors such as humidity and temperature conditions (Herrmann and Bauhus 2013), tree species (Kahl et al. 2017; Minnich et al. 2021), decay age (Bond-Lamberty et al. 2002; Jomura et al. 2022), chemical properties such as nitrogen (N) and lignin content (Berg and McClaugherty 2008; Błońska et al. 2017), fungal community structure (Fukami et al. 2010; Odriozola et al. 2021), enzyme activities, and decomposer biomass (Bradford et al. 2014; Jiang et al. 2022). Since felled deadwood is in direct contact with the forest floor, the soil microbial community penetrates the wood and accelerates the wood degradation process (Tláskal et al. 2021). The main processes involved in deadwood decomposition include fragmentation, respiration, transformation, and leaching (Magnússon et al. 2016).

Over time, as dead wood decomposes, residues enter the soil and alter the fertility (Błońska et al. 2019; Jiang et al. 2022) and several characteristics of the soil (Błonska et al. 2017; Piaszczyk et al. 2019; Shannon et al. 2021; Hollands et al. 2022). Carbon includes almost 50% of the woody dry mass (Weiss et al. 2000). During the decomposition process, C is partially released into the atmosphere and at certain shares stored in the soil, thereupon increasing C resources in the forest soil (Błońska et al. 2019). Biologically, deadwood is an important component of resilience and belongs to the above-ground litter fraction, and its accumulation leads to significant interactions with below-ground soil components. Deadwoods are major contributors to soil organic matter (SOM), providing input to long-term nutrient cycling, ensuring nutrients for beneficial soil organisms, and forming ectomycorrhizal root tips. Despite the global significance of forest sustainability, the N and C cycle in forest ecosystems, and the effects of dead wood in nutrient cycling and C dynamics (Persoh 2015), very little information is available concerning nutrient dynamics, soil functional indicators and providing of microclimates in temperate forests (Woodall et al. 2020; Shannon et al. 2022). Deadwood is a central structural element influencing various essential ecological and biogeochemical functions by generating heterogeneous microsites in old mixed forests (Perreault et al. 2021). Natural old mixed forests are trees of different ages with a high amount of CWD in different sizes and species in all stages of decay in the forest ground (Lassauce et al. 2011). Deadwood is not only an essential reservoir of terrestrial biomass, soil carbon (C), and nutrients but also provides a habitat and food source for a variety of forest species, including microbes and fungi (Norden et al. 2004; Mayer et al. 2022), fauna communities (Zuo et al. 2021), vertebrate species (Spiering and Knight 2005), bryophytes and lichens (Paltto et al. 2008), and also vascular plants (Scheller and Mladenoff 2002).

Assessing the sustainability of forest management requires indicators, and in this regard, forest soils are of great importance due to their ability to supply the nutrients necessary to sustain forest productivity and sustainability (Dale et al. 2008; Jamil et al. 2016). Soils provide a wide range of vital ecosystem services that maintain global sustainability and ecological balance (Drobnik et al. 2018). Sustainable forest management includes soil quality as the criterion for sustainability. In the past decades, several attempts were made to introduce terms such as soil functions, soil quality, soil health, and soil resilience to support politicians and managers related to soil conservation and rehabilitation (Álvarez et al. 2003; Wienhold et al. 2004). However, in recent years the concept of soil functions has become synonymous with soil quality. (Kooch et al. 2022). Soil quality can be determined by appropriate indicators, which are the results of the physical, chemical and biological processes of the soil. Soil science has embraced the concept of soil functions, which brings various capabilities of soils to the fore and has emphasized the multi-functionality of soils and their very different properties (Greiner et al. 2017). Soil functions cannot be measured independently, but soil functions can be evaluated by measuring the physical, chemical, and biological soil characteristics (Doran and Parkin 1994; Larson and Pierce 1994). Therefore, new studies are needed to explore ecological processes that can explain changes in various soil functions when various indicators are taken into account (Teixeira et al. 2020). In forest ecosystems, various factors can affect soil functions and quality; deadwood, or coarse woody debris (CWD), which contributes to forest ecosystem functioning and carbon, nutrient and gas fluxes, improving microclimate, increasing nutrient availability, and habitat quality (Magnússon et al. 2016; Błonska et al. 2017).

Although deadwood is increasingly being included in ecological studies and environmental assessments, even in the Hyrcanian forest (Kooch et al. 2012; Moghimian et al. 2020), here we tried to monitor the long-term effects (15 years) of deadwood, in a mixed beech forest in northern Iran, on a wide range of functional characteristics of the soil (various parameters of soil physical, chemical, and biological) in order to achieve its effects on sustainable forest management. We hypothesized that the tree species with a lower C/N ratio and higher nutrient content would be degraded faster and, therefore, would contribute to soil fertility more instantly than other, more recalcitrant wood species. We also expected that during the monitoring time and further decomposition of deadwood, the biological activities of the soil are changed and differ depending on the deadwood of different tree species.

Materials and methods

Study area

Khanikan forest, covering an area of 2807 hectares, is located near the city of Chalus in Mazandaran Province, northern Iran (Fig. 1a). The study area has an elevation range of 100–1600 m above sea level with a slope range of 0–30%. The average annual temperature is 16.05 °C, the average annual rainfall is 1313 mm, and the dry season runs from May to August (Fig. 1b). From late December to early April, the land is usually covered with snow in higher altitudes. According to the USDA Soil Classification, the parent material is dolomitic limestone from the late Jurassic to the early Cretaceous. The soil is in Alfisol composition with a loamy texture from silty clay. The area consists of a mixed beech forest dominated by oak (Quercus castaneifolia C. A. Mey.), besides oriental beech (Fagus orientalis Lipsky), zelkova (Zelkova carpinifolia Dippe), maple tree (Acer velutinum Boiss.), wingnut (Pterocarya fraxinifolia Lam.), lime tree (Tilia begonifolia Stev.), Caucasian hornbeam (Carpinus betulus L.), ash (Fraxinus excelsior L.), Caucasian alder (Alnus subcordata C.A. Mey.), and Cappadocian maple (Acer cappadocicum Gled). The average height and diameter of the trees were 20–25 m and 50–60 cm, respectively. Tree density was 250–300 Nha−1 in the Khanikan forest. Other less common species (< 10%) are the wild service tree (Sorbus torminalis Crantz), elm (Ulmus glabra Huds.), and the wild cherry (Prunus avium L.). Herbaceous plants such as Hypericum androsaemum L., Euphorbia amygdaloides L., Asperula odorata L., and Polystichum sp. covered more than 85% of the forest floor.

Fig. 1
figure 1

Study area located in Mazandaran province, north of Iran (a). Climate diagram of monthly temperatures and precipitation for the years 1991–2021 (b). Schematic representation of the spatially separated sampling sites (figure not to scale) (c) and state at the time of sampling (d)

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Sampling and laboratory analysis

In January and February 2006, several strong wind events occurred in the forest of Khanikan, which led to the windthrow of some trees (Kooch 2008). All of the fallen trees remained in the protected parcel (an area of about 60 ha−1 of the forest where there is no human intervention) and were not exited of the forest. This study was carried out in the protected parcel (control), where harvesting and any human activity that leads to the cutting, logging, and removal of trees from the protected parcel (control) is prohibited (in all the years). The windthrow event caused the falling of individual trees across the protected parcel. Immediately after tree felling in that field, five common tree species of the Hyrcanian forest [Fagus orientalis (Fagus; with 17.6% of the volume and 30% of the total number), Acer velutinum (Acer; with 5.8% of the volume and 2.7% of total number), Carpinus betulus (Carpinus; with 30.5% of the volume and 30% of the total number), Alnus subcordata (Alnus; with 9.1% of volume and 5% of the total number), and Tilia begonifolia (Tilia; with 2.7% of the volume and 1% of the total number)] (Sagheb Talebi et al. 2014) with the highest abundance of deadwood (with 14 individual trees) were marked as permanent plots to be monitored in order to study their effect on soil functions. All selected deadwoods (with a range of 40–50 cm diameter at breast height and height 20–25 m) are in close proximity to each other with a distance of less than 1000–1500 m between them and were considered if physiographic characteristics (i.e., elevation between 1500 and 1600 m above sea level, the slope between 10 and 15% and north aspect) of bedrock and climate had similar conditions (Fig. 1c). Wood samples were collected by using a core driller (12-mm drill bit) taking samples from the trunks at the lower part of deadwood (0–10 cm depth) in August of 2011, 2016, and 2021 (Fig. 1d). Three soil samples (0–10 cm depth) were taken from the right and the left side along the trunks using iron frames (30 × 30 cm area) and mixed. In addition, 14 soil (0–10 cm depth) samples were taken from areas that did not have deadwood (at a distance of about 50 m from the deadwood) as control areas using iron frames (30 × 30 cm area). For the study of soil biological parameters, samples were collected in August and November (2011, 2016, and 2021). However, soil samples for monitoring N transformation were taken in August, September, November, and December (2011, 2016, and 2021).

Wood and soil samples were immediately transferred to cooled and insulated containers, transported to the laboratory, and stored at 4 °C until further analysis. Several properties of deadwood and soil were measured at the laboratory based on standard protocols (see Table 1). In detail, deadwood properties, including carbon (DWC), phosphorous (DWP), potassium (DWK), calcium (DWCa), and magnesium (DWMg) 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 (ThermoQuest Italia s.p.a.) (Kooch et al. 2017). The pycnometer method (Blake and Hartge 1986) and the cold method (Plaster 1985) were applied to measure the particle and bulk density, respectively. We used the formula [1—(bulk density/particle density)] for soil porosity calculated that proposed by Pires et al. (2014). Yoder’s method was used to determine soil stability (Kemper and Rosenau 1986). The Bouyoucos hydrometer method proposed by (Bouyoucos 1962) was used for measuring soil texture. We used a wet sieve with a sieve size of 0.25–0.50 mm to determine the particle size distribution of the aggregates. The size of micro- and macrograins was calculated between 0.053–0.25 mm and 0.25–0.50 mm, respectively (Cambardella and Elliott 1992). The Walkley–Black method was used for measuring soil organic carbon (Allison 1975), and then the formula (OM % = Organic C % × 1,724) was applied for calculated organic matter content (OM) (Ezeigbo et al. 2013). The stratification of soil organic matter was calculated based on the OM (0-10 cm)/OM (10–20 cm) depths (Bongiorno et al. 2019). The semi-micro-Kjeldahl method was used to measure total N (Bremner and Mulvaney 1982). Soil C and N stocks were calculated using the bulk density data and C and N concentrations at the studied depth (e.g., 0–10 cm) (Sariyildiz et al. 2015). The carbon pool index (CPI) was calculated from the change in total carbon, and the lability index (LI) was determined based on changes in the proportion of labile C in the soil. These two indices were applied to calculate the carbon management index (CMI), with CMI = C pool index (CPI) × lability index (LI) × 100 (Blair, et al. 1995). The soil/water solution was used to measure soil pH and EC of Orion Ionalyzer Model 901 pH and EC meter in a 1:2.5. Available phosphorous (P) was measured spectrophotometrically using Olsen’s method (Homer and Pratt 1961), and available potassium (K), calcium (Ca), and magnesium (Mg) were determined using atomic absorption spectrophotometry (Bower et al. 1952). Determination of particulate organic carbon (POC) and particulate organic nitrogen (PON) was done by physical fractionation. Determination of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) was performed using a total organic carbon (TOC) analyzer (Shimadzu TOC-550A) (Cambardella and Elliot, 1992). The buried-bag technique was used to estimate soil N mineralization, nitrification, and ammonification rates over 30 days. NH4 + and NO3- extraction was done with 2 M KCl solution (soil/solution, 1:5) and filtered using a 0.45 µm filter. The NH4 + and NO3- concentrations in the extraction solution were determined colorimetrically at 645 and 420 nm, respectively (Li et al. 2014).

Table 1 Methods for measuring deadwood and soil properties and associated calculation procedures
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Earthworms were collected by manual identification at the same time as soil samples, which were divided into ecological groups (i.e., epigeic, anecic, and endogeic) based on external characteristics and weighed (Jeffery et al. 2010). Soil Acarina and collembola were extracted using a modified Tullgren funnel (Hutson and Veitch 1987). Soil nematodes were extracted from a 100 g soil sample (fresh weight) using a modified filter method as described by Hutson and Veitch (1987). Soil protozoa population density was calculated microscopically according to the extraction method (Mayzlish and Steinberger 2004). Potato dextrose agar (PDA) and nutrient agar (NA) were used for bacterial and fungal culture, respectively (Wollum, 1982; Asadu et al. 2015). The evolved CO2 was determined during a 3 day incubation at 25 °C to measure soil basic respiration (BR) (Alef 1995). Glucose 1% was used as a substrate to measure substrate-induced respiration (SIR) and, after 72 h of incubation, the released CO2 was measured. CO2 evolved was absorbed in NaOH and then measured by titration with HCl (Anderson and Domsch 1990). The chloroform fumigation method was used for extraction of soil microbial biomass C (MBC) and microbial biomass N (MBN) (Brookes et al. 1985). Activities of urease (EC 3.5.1.5), acid phosphatase (EC 3.1.3.2), arylsulfatase (EC 3.1.6.1), and invertase (EC 3.2.1.26) were determined according to the method described by Schinner and von Mersi (1990).

Statistical analysis

The normality of the variables (p-value > 0.05) was checked by the Kolmogorov–Smirnov test, and the homogeneity of the variances was assessed using Levene’s test. Two-way repeated measured analysis of variance (ANOVA) was employed for deadwood and soil physical and chemical properties concerning different tree species and years. Three-way repeated measured analysis of variance (ANOVA) was used for N transformation and soil biological properties in relation to deadwood trees, years, and seasons. A post hoc test (using Duncan’s test) was used to make multiple comparisons of features with significant differences (p-value < 0.05). All analyzes were performed in SPSS ver. 22.0 statistical software. Principal component analysis (PCA) was applied to analyze the relationships between deadwood and soil properties using PC-ORD ver. 5.0. Correlations between soils and PCA components (i.e., PC1 and PC2) were analyzed using the Pearson correlation coefficient. The heat plots for soil function indicators were identified as part of the PCA output.

Results

Deadwood properties

The results of repeated measured analysis of variance (ANOVA) indicated that there were significant differences (p-value < 0.05) in the deadwood properties among different deadwood tree species (Table S1). Among the deadwood properties studied, only N and Ca had significant differences (p-value < 0.01) comparing different years of sampling (2011, 2016, and 2021). During our study period, the highest amount of OC was in Fagus deadwood, whereas the lowest amount was detected in Alnus (by 24–32%; Alnus < Carpinus ≈ Acer ≈ Tilia < Fagus) (Fig. S1a). However, Alnus had 1.5–2 times higher deadwood N at three sampling years than in the other deadwood trees (Alnus > Carpinus > Acer ≈ Tilia ≈ Fagus) (Fig. S1b). The C/N ratio of Fagus deadwood increased by 30 units in Fagus ≈ Tilia > Acer > Carpinus ≈ Alnus (Fig. S1c). The total P content in deadwood of Alnus was 45% higher than under Fagus and Tilia (Alnus > Carpinus ≈ Acer > Tilia ≈ Fagus) (Fig. S1d). Total amounts of deadwood K and deadwood Ca in Alnus were nearly half of those found in the other deadwood tree species (Alnus > Carpinus ≈ Acer ≈ Tilia > Fagus) (Fig. S1e and f). Deadwood of different tree species did not significantly differ (p-value < 0.01) concerning the amount of total Mg (Fig. S1g).

Soil properties

ANOVA results indicated that there was a significant difference (p-value < 0.05) in the soil’s physical and chemical properties in the vicinity of the deadwood of several tree species (Table S2). The bulk density was significantly higher in the control plot than for soil around deadwood of the other studied tree species with the lowest values for Alnus and Carpinus (18 and 25%) (Control > Fagus ≈ Tilia ≈ Acer < Carpinus < Alnus), respectively. The soil around deadwood from five tree species had no significant differences in soil particle density (Table S2). Different years (2011, 2016, and 2021) did not affect bulk density and particles among tree species. However, there were significant differences in the porosity of soil around deadwood, with the highest values under Alnus and Carpinus and lowest values for the control (Alnus ≈ Acer ≈ Carpinus ≈ Tilia > Fagus > control), respectively (Table S2). The soil under the studied tree species had the same texture, but sand decreased in the vicinity of deadwood under Alnus and Carpinus species (by 40–50%) compared to the control (control > Fagus > Tilia > Acer > Carpinus > Alnus), whereas the clay content increased (by 40–50%) under deadwood from Alnus and Carpinus compared to the control (Alnus > Carpinus > Acer > Tilia > Fagus > control). Soils under deadwood from the trees under study had no significant differences (p-value > 0.05) in silt content (Table S2). The amount of soil clay increased significantly in 2021 compared to other years, but this amount of silt decreased in 2021 in soils under all deadwood trees. Soil pH and EC under Alnus (pH = 7.04; EC = 0.32) were higher compared to other deadwood in ranked order of (Alnus > Acer ≈ Carpinus ≈ Tilia ≈ Fagus ≈ control) (Table S2).

Soil organic C and C stock did not vary significantly in deadwood trees, except in 2021, when Acer, Carpinus, and Fagus had significantly higher values than other species and lower values for the control (Acer ≈ Carpinus ≈ Fagus > Alnus > Tilia > control). The amount of soil C in 2021 was significantly higher than in other years (p-value < 0.05). The total soil N varied among deadwood tree species under Alnus ≈ Acer ≈ Carpinus, being higher than Fagus and the control (Table S2). The C/N ratio in soils under Alnus deadwood was almost half of those in the control and Fagus, whereas available forms of P, K, Ca, and Mg in Alnus and Carpinus were nearly triple those in the control and Fagus (Alnus > Carpinus > Acer > Tilia > Fagus > control). Annual changes from 2011 to 2021 showed higher effects on Alnus for available forms of P, K, Ca, and Mg, with higher values in 2021 and 15 years after the wind throw (Table S2). Soil DOC, POC, and contents of N in macro- and microaggregates, the N stock as well as N mineralization, the contents of NH4+and NO3−, nitrification, ammonification, PON, and DON indicated a significant decrease of almost 50–80% from soils under Alnus deadwood to the control (Alnus > Acer > Carpinus > Tilia > Fagus ≈ control), respectively (Table S2 and Fig. 2a–e). Based on the data from the three studied sampling periods, lower values for soil C in macro- and microaggregates were found under control compared to other deadwood tree species. The C in macro- and microaggregates in 2016 and 2021 were significantly higher than in 2011 (p-value < 0.05). The stratification ratio (OMstr) was higher in soils around Alnus and Carpinus deadwood (almost 25%) compared to Tilia, Acer, Fagus, and the control (Fig. S2a). The carbon management index (CMI) showed differences in the deadwood of different tree species, with the highest value detected under Tilia (Fig. S2b).

Fig. 2
figure 2

Mean (± SE) of NO3− − N (a) and NH4 +  − N (b) concentration, nitrification rate (c), net ammonification rate (d) and N mineralization in the top mineral soil. Different lower case letters indicate significant difference (P < 0.05) among sampling times (i.e., 2011, 2016, and 2021), and different capital letters indicate significant differences (P < 0.05) among deadwood from different tree species and control plots

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All studied soil enzymatic activities (i.e., urease, arylsulfatase, invertase, and acid phosphatase) around deadwood were significantly affected by deadwood tree species (Alnus > Carpinus > Acer > Tilia > Fagus > control), i.e., by a decrease of approximately 60% from Alnus to the control. The annual variation from 2011 to 2021 was most effective in soils around deadwood under Alnus (Table S3). Deadwood tree species significantly affected all studied soil biological characteristics (Tables S3 and S4; Table 2). Except for the total earthworms, the anecic and endogenic biomasses and the seasons significantly affected soil biological characteristics (Tables S3 and S4). Alnus and Carpinus deadwood increased soil density and biomass (Alnus > Carpinus > Acer ≈ Tilia ≈ Fagus ≈ control) of ecological earthworm groups (epigeic, anecic, and endogeic). The populations of collembola, Acarina, protozoa, nematodes, as well as soil fungi and bacteria around deadwood were significantly affected by tree species, seasons, and years (Tables S3 and S4); and declined along the order of Alnus > Carpinus > Acer > Tilia > Fagus > control, respectively (Table S4). Soil microbial indicators (i.e., BR, SIR, MBN, and MBC) had the highest values under Alnus and Carpinus deadwood and lowest values in the control and under Fagus (Table S4).

Table 2 Earthworm species found in the studied forest sites
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Relationship among deadwood and soil properties

The PCA output clustered properties of soil and deadwood (Fig. 3a–i; Table 3). Significant differences were found in the soil surrounding the deadwood of the five species. Furthermore, the studied tree species, deadwood quality, and soil biochemical characteristics in the years 2011–2021 could be separated by PCA output. In general, 59.76 and 11.34% (in 2011), 57.74 and 13.12% (in 2016), and 57.06 and 12.42% (in 2021) of the variance were explained by axis 1 and axis 2, respectively. The deadwood quality, soil functional, and soil biological properties had the highest values under Alnus and Carpinus. In contrast, the lowest values of soil biochemical characteristics were observed under control and Fagus. In this regard, Alnus and Carpinus had more deadwood nutrients (i.e., N, K, P, and Ca). The soil under these deadwood species had higher aggregate stability and soil porosity, higher EC, pH values and nutrient contents, as well as increased organic matter fractions, soil fauna and microbial populations. The lower values of deadwood C, deadwood C/N ratio, soil bulk density, sand, organic C, and C stock were assigned to Alnus and Carpinus deadwoods. Soils around deadwood having a higher fertility and increased biota abundance can be attributed to Alnus in the disintegration development stage (regarding the heat map plots; see Fig. 4), while the controls had a lower fertility and biota abundance.

Fig. 3
figure 3

Grouping of deadwood species and soil properties based on principal component analyses (PCA) at different sampling times. Abbreviations are explained in Table 1

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Table 3 Correlations of deadwood and soil properties with PCA components
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Fig. 4
figure 4figure 4

Ranking of the tree species, deadwood species and soil properties based on axis 1 (a) and 2 (b) of principal component analyses (PCA). Abbreviations are explained in Table 1. The intensity in red color (from weak to strong) indicates the increase of values for deadwood species and soil properties during the years 2011–2021

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Discussion

Deadwood properties

During our study period, C concentration in deadwood of Fagus and Tilia was significantly higher than that in Alnus, reflecting the higher C concentration in undecomposed wood of Fagus compared to Alnus (Thomas and Martin 2012; Kahl et al. 2017; Herrmann et al. 2018). With advancing decomposition from 2011 to 2021, a slight increase in C concentration was observed for all wood species, but this difference was not significant. Lignin (with C content of 63–66%) has a higher C concentration than cellulose (with C content about 44%), so this change is probably due to an increase in lignin and a decrease in cellulose (Herrmann et al. 2015). Accordingly, the results of Christensen and Vestural (2003) that followed C concentrations in beech did not observe a significant difference in the C concentration of deadwood with different decomposition stages. Beech (Fagus) stores more C than alder (Alnus) because of different wood densities among these species (Herrmann et al. 2018). Generally, we obtained the most significant increase in nutrient concentrations in dead Alnus trees. In our study, all species showed an increase in N concentration with a progressive decomposition stage. The N content increased after 15 years of decay, probably due to N fixation by bacteria, accumulation of fungal biomass and the effects of rainfall input (Garrett et al. 2008). The increase in N content resulted in a declining C/N ratio with the decay process in 2021; thus, the C/N ratio could be a potential indicator of the decay rate (Yang et al. 2010). With the ongoing decomposition from 2011 to 2021, a slight increase in P and K concentration was observed for all wood species, but this difference was not significant. Generally, we observed the most significant increase in nutrient concentrations in CWD of Alnus and Carpinus compared to Fagus. The variation in wood properties between the tree species (e.g., wood density) may cause these differences in deadwood nutrient concentration during decomposition (Harmon et al. 2013). This has been attributed to lower absolute lignin concentrations and higher nutrient contents (Stutz et al. 2017) in Alnus compared to Fagus. According to Laiho and Prescott (2004), the contents of P, K, Ca, and N in CWD depend mainly on the wood’s initial N, P, K, and Ca concentrations, which are determined by tree species and the nutrient status of the site, besides the characteristics of the wood (e.g., wood structure and type, C/N ratio). We tested the effect of each nutrient on the degradation of CWD and found that most nutrients were positively related to the duration of deadwood decay after the windthrow event. The content of most nutrients increases during the study period, suggesting that nutrient content changes with mass loss in CWD (Krankina et al. 1999; Yang et al. 2010).

Soil properties

Soil physical properties

In the present research, a wide variety of soil properties were assessed to inform how soil functions change and interact during the decay of deadwood from different tree species. Our study demonstrated the effects of deadwood decomposition on several physical properties of forest soils. Differences in the bulk density between soil around deadwood and control plots were found for five species, especially for Alnus and Carpinus deadwood. As organic matter increased due to the ongoing decomposition of deadwood, soil bulk density decreased. According to Yue et al. (2017), organic matter content effectively affects soil bulk density, thus, organic matter concentration predicts soil bulk density (Prévost et al. 2004; Perie and Ouimet 2008). In general, wood quality influences soil properties that provide sources for SOM formation (Wambsganss et al. 2017). We observed a positive relationship between soil structural stability, clay content, porosity, and macro- and microaggregation around dead trees across tree species. Soil properties such as soil type, texture, aggregate stability, and moisture influence the soil’s organic matter content (Matus 2021) and affect the decomposition process. Organic matter derived from dead trees enriches soil and determines soil particles’ structure and cohesion. Plant residues such as decaying wood stimulate microbial activity and contribute to stabilizing soil structure (Piaszczyk et al. 2017; Piaszczyk et al. 2019). Releasing organic matter from deadwood can stimulate the formation of soil aggregates, improve soil porosity, significantly increase the number of micropores, and increase soil water-holding capacity (Villada 2013; Matus 2021; Jiang et al. 2022).

Soil chemical properties

The soil pH was lower in control compared to the soil around deadwood species. Jiang et al. (2022) pointed to the leaching of nutrients from deadwood decomposition into the soil. In addition, Kooch et al. (2021) demonstrated that increasing urease activity contributes to increasing soil pH. In our study, urease enzyme activity increased in the soil around deadwood for all species compared to the control. We observed an increase in several soil biochemical characteristics located directly under decomposing deadwood compared to control soils. Thus, the wood decomposition process determines the activity of enzymes and nutrient contents in the soil under the deadwood. Available forms of P, K, Ca, and Mg in Alnus and Carpinus were nearly triple those in the control areas. Deadwoods are rich in macroelements such as N, P, K, and Ca, which ultimately end up in the soil and significantly improve soil quality (Piaszczyk et al. 2019). In our study, examining the characteristics of wood showed that N, K, P, and Ca concentrations in Alnus were higher compared to other species. Furthermore, our results claimed that wood with the highest stock of microelements (such as Alnus and Carpinus) released more ions to the soil surface than wood with lower concentrations of microelements (Lasota et al. 2018). In general, the concentration of nutrients in the topsoil layer released by dead trees depends on the type of wood and its decay stage (Jomura et al. 2022). Similar results were reported by Błońska et al. (2019) for the dead trees of seven tree species in the Czarnalozga reserve in central Poland.

Total N, contents of NH4+, NO3, nitrification, ammonification, N mineralization, DON, as well as N in macro- and microaggregates and PON concentrations are significantly higher in soils under degrading deadwood. Therefore, we hypothesize that the increased N concentrations are due to fungal N translocation (Hart 1999) and bacterial N fixation (Hicks et al. 2003) contributing to the higher concentration of N in wood and its transfer to the soil during the decomposition process, especially at the late decay process in Alnus. Ammonification rates are affected by C/N ratio, soil pH, and soil nutrient contents (Lee et al. 2009). Residues with a high C/N ratio cause strong competition for available soil N (Vallejo et al. 2011). Compared to other species, Alnus and Carpinus had higher values of NH4+-N as a result of stronger abiotic fixation or microbial competition (Giardina et al. 2001). Decreased nitrification rates in Fagus species were associated with higher C/N ratios in soil and deadwood (Burton et al. 2007). Also, soil pH is crucial in the nitrification reaction, as the nitrification rate decreases rapidly below pH 7.0 (Ahn 2006; Lee et al. 2009). It is generally assumed that acidic forest soils have negative nitrification levels because autotrophic nitrification is inhibited (Li et al. 2001). Soil N mineralization rate is impacted by forest conversion and site management (Yan et al. 2007; Scott et al. 2014). Correspondingly, Alnus had a lower C/N ratio than Fagus in the dataset of this study. Forest areas containing N-fixing species can increase soil N mineralization (Rothe et al. 2002). Differences in soil N mineralization may be due to differences in the availability of labile N substrates (Mc Kinley et al. 2008). High-quality deadwood reduces microbial immobilization of N, leading to N mineralization and improved plant N availability (Mc Kinley et al. 2008). The contents of soil organic C, DOC, C in macro- and microaggregates, and POC are significantly higher in the soil around deadwood than those in the control, corresponding to the conclusion by Błonska et al. (2019) and Jiang et al. (2022) in other forest ecosystems. These findings show that C input by deadwood contributes to decomposition processes that impact the composition of soil organic C, but this impact is a long-term process only to be observed in the long-term because most of the C characteristics were not yet significant in 2011 and 2016. The content of DOC in the soil below fallen wood at the Alnus site is higher than that of the control and for Fagus, which may be caused by a lower decomposition rate of the fallen Fagus wood compared to Alnus. Furthermore, the high concentration of DOC in leachates from deadwood in Alnus can be explained by wood structure and state of fragmentation. The Alnus wood features a soft and loose structure, resulting in greater water-holding capacity and hydrophilicity (Błońska et al. 2018).

Soil enzymes properties

The present study demonstrates the prominent role of the tree species and decay processes in promoting soil enzyme activity in the immediate vicinity of deadwood. Activities of all enzymes studied (i.e., urease, arylsulfatase, acid phosphatase, and invertase) showed a trend of increase for all tree species compared to control plots. This finding is most likely due to the increased N and P content of soils under deadwood. Moreover, the increased SOM accumulation contributes to enhanced extracellular enzymatic activity by providing a more comprehensive range of C and N substrates that can be accessed and utilized by different groups of soil microbes (Cenini et al. 2016). Urease is an enzyme widely considered to be a proxy of nitrogen (N) mineralization; therefore, its activity strongly depends on the amount of N (Keeler et al. 2009). Urease plays an influential role in the hydrolysis of urea to carbon dioxide and ammonia and may contribute to increasing soil pH. The results of Ling et al. (2014) claimed that increasing amounts of clay increased the activity of arylsulfatase and invertase (Zvyagintsev and Velikanov 1968; Mahmoud and Attia 1970). It can be concluded that increased clay accumulation in the soil around dead trees of various tree species plays an essential role in the increased activity of this enzyme (Kooch et al. 2021). P is an essential element for life, which is often limiting in terrestrial ecosystems, with organic P being the most important P fraction for soil microorganisms and acid phosphatase activity (Cleveland and Townsend 2006; Zhao et al. 2017). Other studies also determined an increased soil enzymatic activity directly impacted by decomposing deadwood (Błońska et al. 2017). The increased enzymatic activity may be related to the translocation of deadwood degradation products or the leaching of soluble compounds to the soil directly beneath. Accordingly, changes in soil microbial activity and changes in the structure of soils directly exposed to deadwood have been reported by Angst et al. (2018) and Bani et al. (2018).

Soil microbial properties

Throughout the present study, BR, SIR, MBC, and MBN showed a clear seasonality with a peak in summer. Statistically, significantly higher values for BR, SIR, MBC, and MBN were observed in soils under the direct impact of decomposing deadwood, with a maximum in the Alnus site and a minimum in the control. The high BR for Alnus may result from conditions favorable to microbial growth and enzymatic activities, due to increased input of labile organic substrates, especially in the upper soil layers (Kooch et al. 2020). In the current study, the lowest respiration activities were observed in the controls, which is probably due to the low organic matter contents and microbial activities. Deadwood had a strong effect on MBC for all tree species. Even deadwood in Fagus led to an increase in MBC with slow decomposition conditions in 2021. Microbial biomass is directly related to soil C and N contents. According to Peacock et al. (2001), the amount and quality of detritus input impact the amount of soil microbial biomass, which can be easily measured, indicating changes in soil organic C content at an early stage (Wiesmeier et al. 2019). Furthermore, microbial biomass C and N reflect the size and activity status of the soil microflora and provide a biologically bonded pool of nutrients in the soil (Nair et al. 2012). The results of this study support the view that soil microbial biomass can be efficiently used to readily detect long-term changes in soil functions, especially organic C transformation.

Soil fauna and flora properties

Earthworms are known as ecosystem engineers. It has been mentioned in many studies that earthworms have a specific food preference (Kleeman et al. 2017; Capowiez et al. 2020). Accordingly, populations around deadwood are very different from species in other environments. The results of the present study indicate that the abundance of earthworms from the Lumbricidae family and the epigeic ecological group was increased around deadwoods. According to several literature reviews (e.g., Nahidan and Ghasemzadeh 2022; Xiao et al. 2022; Zhao et al. 2022), Lumbricus terrestris is one of the most active species of earthworms in forest ecosystems of the temperate regions. Accordingly, in our study, among the different species of earthworms, Lumbricus terrestris was observed in all the treatments and years throughout the study area (see Table 2). Furthermore, the populations of collembola, Acarina, protozoa, nematodes, as well as fungi and bacteria in the soil around deadwood were significantly affected by tree species, seasons, and years; and were declining along the order of Alnus > Carpinus > Acer > Tilia > Fagus > control, respectively. Various studies have shown that in fresh wood, the density of bacteria, fungi, or microbial communities is generally minimal and increases with the progress of the decomposition process (Johnston et al. 2016; Wilhelm et al. 2019). Consistent with Gómez-Brandón et al. (2020), more ecological niches become available for microbial colonization as decomposition progresses, increasing the wood surface area and the availability of nutrients (e.g., N and P). Moreover, our results suggest increased metabolic diversity is required within bacterial and fungal communities to take advantage of the gradually available substrates as wood decay proceeds. Total N and soil pH were the main factors driving microbial communities, which is confirmed by the conclusions of Dong (2021). Particularly, increasing the N content provides more N sources for soil microbial growth and improves microbial community richness.

Relationships among deadwood species and soil properties

Generally, it can be pointed out that tree species and processes of deadwood decay have various effects on soil properties and functions (Gómez-Brandón et al. 2020; Pastorelli et al. 2021; Jomura et al. 2022). The output of PCA showed apparent differences in soil properties between plots with different tree species, showing that the decomposition period and tree species are essential factors determining soil functions. The current findings indicate that soil biological and chemical properties are decreased following tree species in the rank order of Alnus > Carpinus > Acer > Tilia > Fagus > control. Retaining deadwood trees or biological legacies can prepare a hotspot of soil functions, such as biological activities, abundance and diversity of microbial communities, and nutrient transformation in old mixed beech forests. While removing dead trees in the forest ecosystems, especially in case of Alnus and Carpinus, can threaten soil functional indicators and disrupt ecosystem services in the forests. The beneficial effects of deadwood on soil functions can suggest various strategies to improve forest sustainability and ecosystem conditions for forest management. A first step should be to prevent the removal of deadwood from forests, especially in areas of high disturbance. The next step is to determine the amount and type of deadwood that should be left in the forest to improve soil functional indicators. However, more research on deadwood is needed to plan for protection of soil function and health in sustainable forest ecosystems.

Conclusions

The deadwood of five tree species (i.e., Alnus, Tilia, Carpinus, Acer, and Fagus) strongly controls the dynamics of soil functional indicators during chemical modifications of the wood in an old mixed beech forest of northern Iran. Most of the deadwood properties differed significantly among the tree species of measured parameters for Alnus. Consequently, deadwood nutrients distinctly change with degradation and differ between tree species. In the case of Fagus deadwood, the C/N ratio was higher, and macroelement contents were lower values than for other tree species. This issue has also caused a reduction of soil nutrients around Fagus deadwood compared to other dead trees. Thus, in support of the first hypothesis of our study, we can claim that the tree species with lower C/N ratio and higher nutrient content would contribute to soil fertility more rapidly than species with more recalcitrant wood quality. Although not all features showed exactly the same pattern, our data strongly confirm the effects of deadwood on soil biological activities during the stages of wood decomposition, confirming our second hypothesis. In general, soil functions increased ranked in the order of control < Fagus < Tilia < Acer < Carpinus < Alnus, which can be assigned to the higher quality of deadwood in the different trees. It can be concluded that tree deadwood is essential in promoting soil functions and sustainability in old-growth beech forest ecosystems. Moreover, the present investigation provides a theoretical basis for the importance and applications of deadwood for sustainable forest management that can be useful in a broader range of forest ecosystems in determining the mechanisms that explain these phenomena.