Introduction

Tree cover, tree species diversity and identity belong to the main factors influencing overstory flora, aboveground and belowground fauna in forest soils, due to their influence on chemical and physical soil properties such as pH, organic matter content, soil structure and microclimate such as light or water supply (Cesarz et al., 2013; Kara et al., 2008; Tinya & Ódor, 2016). Consequently, forest management is therefore widely considered to have an impact on the understory plant diversity, essential for sustainable forestry and nature conservation (Ujházy et al., 2017). Databases contain a growing literature number describing the consequences of deforestation (Boros et al., 2019; Carvalho et al., 2019; Elek et al., 2018; Runyan et al., 2012) or thinning (Willms et al., 2017; Xu et al., 2020) on vegetation dynamics and faunal groups. On the other hand, restoration impacts after massive deforestations on forests biota has been also well documented (Damptey et al., 2023; Moir et al., 2005).

In the Slovak Republic, a European country where forests cover more than 40% of the territory (FAO, 2010), Scots Pine (Pinus sylvestris L.) belongs to the second most abundant conifer, constituting 7% of tree vegetation (Leontovyč et al., 2018). While pine naturally thrives in extreme habitats such as skeleton soils of limestone or sandstone mountains, the most extensive forests are found in the Záhorská lowland on sandy soils where grown in man-made monocultural plantations. Sandy soils belong to the substrates that have the poorest nutrient content and also have the lowest mineral strength, leading to slow vegetation development and growth. Moreover, they pose extreme conditions not only for plants but also for soil organisms.

Since the seventeenth century, pine forests on those aeolian sands have been managed by standard forest management (SFM) based on artificial regeneration using seedlings. Pine seedlings are planted, grown and the forests are shaped through standard management practices such as weed control, multiple loggings and the removal of wild shrubs and trees over several decades. Finally, all productive trees from the plantations are harvested and soils are subsequently ploughed, cleaned and stripped of organic matter, roots and stumps (Saniga, 2019). However, currently, efforts are underway to manage plantations in a natural way, through close-to-nature forest management (CNFM) avoiding complete clear-cutting of mature forests. CNFM aims to minimize human intervention and instead relies on natural processes (Bauhus et al., 2013). Nevertheless, the specific habitat and stand condition play a crucial role in spontaneous natural regeneration, which has been observed in studied forests on aeolian sands across several stands following pine seed years. Over a past decade, selective thinning and removing of mature trees has been implemented in these stands to promote growth of young pine seedling.

Research indicates that CNFM enhances ecosystem carbon sequestration by increasing tree growth, by altering litter decomposition rates resulting from shifts in species composition and stand density (Jandl et al., 2007; Ming et al., 2020). CNFM has also been shown to alter the composition of microbial fungal community (Wan et al., 2019); increase the abundance and species richness of beetles (Błońska et al., 2020; Lange et al., 2014) and support diverse birds of high conservation value (Gresh & Courter, 2022). However, despite these findings, there is currently no literature available on the impact of CNFM on soil microfauna.

Soil nematodes are small, vermiform, hydrobionts that are strongly dependent on the availability of free water for their activity (Yeates et al., 1993). As a species-rich and abundant component of the belowground system, soil nematodes occupy a wide range of trophic positions and contribute significantly to soil food web interactions (Ferris et al., 2001). Soil nematodes have been shown to be influenced by factors as plant species and diversity (Cesarz et al., 2013; De Deyn et al., 2004); soil properties (Wang et al., 2023); forest management practices (Čerevková et al., 2021) and various ecological and anthropogenic disturbances (Renčo & Čerevková, 2017; Renčo et al., 2019, 2022a). On the other hand, soil nematode communities have been confirmed as valuable indicators of ecosystem recovery after disturbance (Bobuľská et al., 2020; Gruzdeva & Sushchuk, 2010; Li et al., 2015; Renčo et al., 2022b). To evaluate soil nematode communities, several community indices have been developed in the recent decades. These indices help to evaluate and compare the state of the ecosystems, to characterize the structure and functioning of soil food webs as well as the extent of their disturbance (Du Preez et al., 2022; Ferris & Bongers, 2009). These indices are based on classification of nematodes into trophic groups according to their feeding preferences (Yeates et al., 1993), “colonizer–persister (c–p) scale” (1–5) based on their r and k characteristics following Bongers (1990) and Bongers and Bongers (1998) as well as functional groups following Ferris et al. (2001).

Besides nematodes, soil microbes have been successfully used as indicators of soil quality, natural or anthropogenic disturbances, environmental pollution or climate changes (Schloter et al., 2003). Various microbial and biochemical attributes such as respiration, N-mineralization, or enzymatic activities can be reliably measured and are frequently used for these purposes (Blagodatskii et al., 2008; Bobuľská et al., 2015; Gömöryová et al., 2011). Research indicates that microbial biomass, microbial quotient, mineralizable carbon, basal respiration and metabolic quotient were significantly altered by forest type within the study of combined natural and artificial regeneration after clearcutting (Fang et al., 2016). According to Kuuluvainen et al. (1996), forest management substantially affects stand structure as a consequence of differences in microclimate due to management regime, therefore changes in some soil properties, including microbial community biomass, activity or composition can be expected.

Based on previous research demonstrating the positive impact of CNFM on tree growth, litter decomposition rates, and overall forest ecosystem dynamics, we predict that the implementation of CNFM will alter the composition and abundance of soil nematodes. Additionally, we hypothesize that changes in forest management practices will affect microbial biomass, enzymatic activities, and other biochemical attributes, reflecting shifts in soil health and quality. We address the following research questions: are soil physicochemical parameters (organic carbon content, N content, pH value, water content) different between close-to-nature managed and standard managed pine forests? How do management and soil physicochemical parameters affect soil nematode communities, nematode species and the richness and activity of soil microbial communities?

Material and methods

Study forest

Our investigation was conducted in pine tree plantations of Pinus sylvestris L. located in the Záhorská lowland (48°37′N, 17°06′E), characterized by a moderately warm and humid climate. The selected study sites represent typical and widely spread pine tree plantations within the Záhorská lowland, where the average annual precipitation is approximately 630 mm per year. The average annual temperature is around 9.5 °C, with and average growing season temperature of 15 °C.

Fifteen CFNM stands were selected in 110-years-old pine forests. The CNFM implementation was established ten years ago, so the tree layer consists of 110-year-old and young10-year-old pines (natural growth), but oaks have also begun to appear. The understory vegetation was well developed, comprised mainly of grasses such as Calamagrostis epigejos, Festuca ovina agg. and Festuca dominii, fern Dryopteris filix-mas or bryophytes such as Pleurozium schreberi. Calluna vulgaris was less common. Most of the soil was covered with a thick layer of organic matter.

For comparison, fifteen stands that were under standard management (SFM) were selected for the study. The stands were 120 years old, falling with the 81+ years age class. The dominant tree species is P. sylvestris, covering 100% of the stand area. Understory vegetation was very pure, dominated by bryophytes and very sparse grasses such as Festuca vaginata or Festuca dominii. In total, thirty stands were selected, forming 15 stand pairs (N = 15 CNFM, N = 15 SFM).

Soil sampling

At each of the thirty stands, one plot (25 × 25 m) was randomly selected. Along two diagonal transects from west to east and north to south, ten soil samples were collected at each plot. The samples included bulk soil, rhizosphere soil and plant roots, using a soil corer (diameter 10 cm) to a depth 10 cm. The ten samples collected from each plot were carefully mixed by hand to gain one composite management-specific sample per plot and transferred into plastic bags. Sampling was performed during September 2021. The samples were transferred to the laboratory in cooling boxes and stored at 5 °C until processing for the nematode analysis (within less than one month).

Soil properties

Total soil carbon and nitrogen contents, soil moisture (SM) content, and pH were measured in all samples. The organic C (Cox) and total N (Ntot) contents were determined using a Vario MACRO Elemental Analyzer (CNS Version; Elementar, Hanau, Germany). SM content was estimated gravimetrically by oven-drying fresh soil at 105 °C overnight, and pH was measured potentiometrically in 1 M CaCl2 suspension using a digital pH meter (WTW InoLab pH 720 Laboratory Meter) separately for each representative sample.

Microorganisms

A portion of each sample for the microbial analysis was frozen the same day of material was transferred to laboratory and stored at − 20 °C until analysis. To estimate the soil microbial activity, several microbiological parameters were measured. The microbial biomass C (Cmic) was determined following the procedure described by Islam and Weil (1998). Ten grams of oven-dried equivalent (ODE) field-moist soil adjusted to 80% water-filled porosity was irradiated twice by microwaves at 400 J/g ODE soil to kill the microorganisms. Cooled samples were extracted with 0.5 M K2SO4, and the C content of the extract was quantified by oxidation with K2Cr2O7/H2SO4. The same procedure was performed with a non-irradiated sample. Cmic content was determined as (Cirradiated content—Cnon-irradiated content)/KME, where KME represents the extraction efficiency (0.213) (Islam & Weil, 1998). Basal soil respiration (BR) was measured by estimating the amount of CO2 released from 50 g of fresh soil after a 24 h incubation at 22 °C and absorbed in 25 ml 0.05 M NaOH. The amount of carbonate was determined by the titration with 0.05 N HCl after the precipitation of carbonates by 5 ml BaCl2. N-mineralization (Nmin) was determined using the laboratory anaerobic incubation procedure described by Schinner et al. (1991). Soil samples (5 g) under waterlogged conditions were incubated at 40 °C for 7 days to prevent nitrification, and NH4–N was measured by a colorimetric procedure. Catalase activity (Catal) was measured 10 min after 20 ml 3% H2O2 was added to 10 g fresh soil sample based on the volume of discharged oxygen according to the method of Khaziev (1976).

Nematodes

Prior the nematode extraction, a portion of composite sample from each plot was homogenized by gentle hand mixing, and stones were manually removed. The nematodes were extracted from 100 g of fresh soil by a combination of Cobb sieving and decanting (Cobb, 1918) and a modified Baermann technique (van Bezooijen, 2006). A soil sample (100 g) was soaked in l L of tap water for 60 min to disrupt soil aggregates and promote nematode movement. The soaked sample was carefully passed through a 1-mm sieve (16 mesh) to remove plant parts and debris, and this suspension was passed through a 50-μm sieve (300 mesh) 2 min later to remove coarser soil particles. Finally, the water suspension containing nematodes and fine soil particles was placed to Baermann funnels to set of two cotton-propylene filters. Suspensions containing the nematodes were collected after 24 h of extraction at room temperature (approx. 22 °C). Nematodes were killed in water bath (approx. 65 °C), fixed in a 4% formaldehyde and pure glycerol solution and the total number of individuals per sample was determined. Nematode abundance was expressed as the number of individuals per 100 g of dry soil.

Of the counted nematodes 10%, but not less than 100 individuals per sample, were microscopically (100, 200, 400, 600, and 1000 × magnification) identified to the species level (juveniles to the genus level) from temporary slides using an Eclipse 90i light microscope (Nikon Instruments Europe BV, Netherlands). Nematode species were assigned to feeding groups as suggested by Yeates et al. (1993) and Wasilewska (1997), but adjusted and supplemented as Sieriebriennikov et al. (2014) recommended: bacterivores (B), fungivores (F), plant parasites (PP), predators (P) and omnivores (O). Species (not trophic groups) were characterized as eudominant at D > 10%, dominant at D = 5–10%, subdominant at D = 2–5%, recendent < 2% (Losos et al., 1984).

To evaluation of condition in CNFM and SFM stands based on the composition of the nematode community, several community indices were computed. To this, nematodes were assigned to colonizer–persister (c–p) groups according to Bongers (1990), which divided nematode families into five c–p groups (from c–p1 to c–p5). C–p1 consists of ‘r-strategists’ with short generation times, small eggs and high fecundity, high colonization ability and high tolerances to disturbance. Colonizers generally live in ephemeral habitats. In contrast, c–p5 represent ‘K-strategists’ with longest generation times, largest body sizes, lowest fecundity and high sensitivity to disturbance (Ferris et al., 2001). Persisters are never dominant in soil and generally are more abundant in stable habitats (Bongers, 1990). C–p scaling allows the calculation of the basal maturity index (MI) for non-parasitic nematodes, the plant-parasitic index (PPI) for plant parasites only (Bongers, 1990) and the MI2-5 maturity index (Yeates, 1994) for c–p2-5 nematode taxa. All maturity indices are weighted means computed as ∑[cp- value (i) × f(i)]/[total numbers of nematodes] where (i) is the individual taxon and f(i) is the frequency of taxa in the sample (Bongers, 1990).

Linking nematode c–p groups with nematode life strategy, resulted in the concept of nematode “functional guilds” sharing the same feeding type and c–p value (Bongers & Bongers, 1998). As they noted, the assessment of soil quality based on the presence of all trophic groups and of all c–p groups alone are not sufficient to predict whether the soil ecosystem functions or not. Grouping nematodes to functional guilds played considerable tools in the concept of Ferris et al. (2001) where food web conditions were described in “weighted faunal analysis”. The basal, structured and enriched condition of the soil food web were thus described.

Additionally, functional guilds allow the calculation of the enrichment index (EI), an indicator of conditions supporting fast-growing bacterivores; the structure index (SI), an indicator of more highly structured soil food webs and enhanced ecological functions due to higher trophic links; the basal index (BI), an indicator of a food web diminished by stress or limited nutrient resources; and the channel index (CI), an indicator of fungal-mediated dominance of organic matter decomposition proposed by Ferris et al. (2001). The enrichment index (EI) and structure index (SI) are used to assess food web location along enrichment and structure trajectories which can be depicted graphically (de Goede et al., 1993).

The diversity of the species in the nematode communities in CNFM and SFM stands was determined by the Shannon diversity index H′spp (Shannon & Weaver, 1949). This index follows equation: H′spp = − Σ (pi × ln pi), where pi is the proportion of the i-th family in a sample. All indices (except H′spp) and total biomass were calculated using the NINJA online program (Sieriebriennikov et al. 2014; https://shiny.wur.nl/ninja/)

Statistical analyses

Statistical analyses were done using STATISTICA version 14.0 (TIBCO, 2020) and CANOCO 5 for Windows version 5 (Ter Braak & Šmilauer, 2012). All response variables were subjected to a one-way ANOVA to determine the overall effect of CNFM on the nematode communities and microbial activity, after checking the homogeneity of variance using Levene’s test. When necessary, data were log(x + 1) transformed. Tukey’s honestly significant difference (HSD) was applied to identify significant differences in the variables between stands at p < 0.05. Nonparametric Spearman’s correlation coefficient was calculated to identify the relationships between the nematode abundance, functional guilds abundance, community indices, microbial parameters and the measured soil properties at the study stands. Correlations at p < 0.05 and p < 0.01 were considered significant.

Redundancy analysis (RDA) was used on the main nematode genera and microbiological data associated with the investigated management specific stands, with the soil properties as explanatory variables, to identify the relationships between the nematode genera abundance and microbial parameters and soil properties. All data were log-transformed before use. The effects of the explanatory variables were quantified by automatic forward selection.

Results

Soil properties and microbial activity

The variation of the soil properties were observed between the CNF and SFM stands (Table 1). Specifically, soil moisture, C and P contents showed a significant increase after CNFM (p < 0.05), while the levels of N, S and C/N ratio did not differ significantly between the differently managed stands (Table 1). The sandy soils at both CNFM and SFM pine plantations were highly acidic (mean pH range 3.22–3.92, respectively), and no significant differences were observed between management practices (p < 0.05). An analysis of the soil microbial characteristics as management indicators showed significant increase in N-mineralization, microbial biomass carbon (Cmic) and basal respiration following the application of CNFM (p < 0.05). However, activity of catalase was not affected by CNFM.

Table 1 Values (mean ± S.D) and range of soil physical–chemical properties and microbial parameters of study plots
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Nematode species, diversity and abundance

A total of 101 nematode species, belonging to 69 genera, were identified (Table S1). Out of these, 38 were bacterivores, 20 were fungivores, 13 were omnivores, six were predators, and 22 were plant parasites. Three species, namely Bunonema ditvelseni, Plectus infundibulifer and Metaporcelaimus labiatus, are new for the dataset of Slovak nematode fauna (Lišková & Čerevková, 2011). The species Acrobeloides nanus was the most abundant/eudominant bacterivores in both CNFM and SFM stands. Among the fungivores, the species Aphelenchoides composticola and A. minimus prevailed. Crassolabium etterbergense and Eudorylaimus carteri as omnivores dominated in CNFM stands, while Eudorylaimus juveniles in SFM stands. Among the predators, Mononchus parvus was the most abundant species in CNFM stands, and Trypila monohystera in SFM stands. The species Malenchus bryophilus and Helicotylenchus digonicus were the most abundant plant parasites in both, CNFM and SFM stands.

CNFM had no significant effect on nematode species diversity, as well as on the number of species (Table 2). The average nematode abundance ranged from 1370 to 2187 individuals per 100 g of dry soil. The nematode abundance was significantly increased after CNFM (p < 0.05). Spearman’s correlation analysis identified significant positive correlations between total nematode abundance and soil moisture content (p < 0.01) (Table 3).

Table 2 Total nematode abundance, number of genera, nematode community indices associated with close to nature (CNFM) and standard managed (SFM) pine plantations (mean ± S.D.)
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Table 3 Spearman´s rank correlation coefficients between nematode abundance, species number, functional guilds, nematode community and ecological indices; microbial parameters and soil properties (SM, soil moisture content; pH/ CaCl2; acidity; Ntot, total nitrogen content; Cox, organic carbon content; C/N, carbon-to-nitrogen ratio), P, phosphorous
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Nematode community structure and biomass

Numerically, bacterivorous nematodes prevailed in both stands, followed by plant parasites, omnivores, fungivores and predators in CNFM; and by plant parasites, fungivores, omnivores and predators in SFM stands (Table 4). Nevertheless, only omnivores and predators represented by community stabilizers (c–p 3–5) were significantly higher in CNFM stands, compared to SFM stands (p < 0.05) (Table 4 and Fig. 1). Among them, the functional guilds Om4, P3 and P4 prevailed in both stands, showing a significant increase after CNFM application (p < 0.05) (Table 4). Additionally, Spearman’s correlation analysis pointed to significant negative relationships between SM and abundance of Om4 nematodes (p < 0.01). In contrast, a positive interaction for P2 and P3 nematodes with SM content was recorded (p < 0.05; p < 0.01) (Table 3).

Table 4 Mean abundance of nematode trophic groups and nematode functional guild associated with close to nature (CNFM) and standard managed (SFM) pine plantations (± S.D.)
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Fig. 1
figure 1

C–p triangles of unweighted proportions of nematode c–p classes (colonizer–persister; c–p 1–5) in pine plantations 10 years after close-to-nature management establishment (CNFM, circles) and in the standard managed stands (SFM, triangles). The red line represents enrichment opportunists (c–p 1), the blue line represents stress tolerators (c–p 2), and the green line represents community stabilizers (c–p 3–5)

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The numbers of stress tolerant groups Ba2, Fu2 and enrichment opportunists (Ba1) also increased after CNFM, however no significant differences were recorded compared to SFM stands (Table 4 and Fig. 1). However, several interesting correlations with soil properties were there recorded. Ba2 nematodes correlated positively with SM and Ntot, but negatively with soil pH and Corg content (p < 0.05; p < 0.01). A positive relationship was found between SM, Corg and C/N ratio, but negative with soil pH and abundance of Fu2 nematodes (p < 0.05; p < 0.01) (Table 3). Among plant parasites functional guilds Pp2 and Pp3 prevailed in both management-specific stands, their abundance was slightly higher after CNFM (Table 4) but not statistically significant (p < 0.05). Both functional guilds positively correlated with soil moisture content, Pp3 moreover also with pH and Corg (p < 0.05; p < 0.01) (Table 3).

Both Maturity indices (MI, MI2-5), structure index (SI) as well as total nematode biomass were significantly higher in the CNFM stands (p < 0.05), while channel index (CI) reached higher values in SFM stands (p < 0.05) (Table 2). Spearman’s correlation analysis identified significant positive correlations between nematode biomass, soil moisture and carbon content (p < 0.05; p < 0.01). In contrast, nematode biomass, negatively correlated with soil pH (p < 0.05).

Moreover, the RDA analysis indicated, that the abundance of most nematode genera and the microbial parameters were positively correlated with soil pH, soil moisture, C and N contents and tended to be higher for CNFM (Fig. 2).

Fig. 2
figure 2

Redundancy analysis (RDA) of nematode genera, microbial parameters and soil properties in pine plantations ten years after close-to-nature management establishment (CNFM) and in the standard managed stands (SFM). The 37 most abundant and frequent genera are shown. Acrobels–Acrobeles; Acrobelo–Acrobeloides; Aphelench–Aphelenchoides; Aphelenhu–Aphelenchus; Aporcela–Aporcelaimellus; Cephalob–Cephalobus; Ceratopl–Ceratoplectus; Cervidel–Cervidelus; Coslench–Coslenchus; Crassolb–Crassolabium; Deladens–Deladenus; Diphterp–Dipthrepthora; Ditylenc–Ditylenchus; Eucephal–Eucephalobus; Eudoryla–Eudorylaimus; Eumonhys–Eumonhystera; Ereptonm–Ereptonema; Helicotl–Helicotylenchus; Heterocp–Heterocephalobus; Laimaphl–Laimaphelenchus; Malenchs–Malenchus; Mesocric–Mesocriconema; Metatert–Metateratocephalus; Mononchs–Mononchus; Panagrol–Panagrolaimus; Paraphel–Paraphelenchus; Prismatl–Prismatolaimus; Prodesmd–Prodesmodora; Rhabidit–Rhabditis; Teratopc–Teratocephalus; Tylenchul–Tylencholaimus; Wilsonem–Wilsonema

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Discussion

As noted by Larsen et al. (2022), close-to-nature forest management holds the potential to support biodiversity, adapt forests to climate change and provide ecosystem services at a higher level than conventional forest management. However, while the general principles CNFM should be consistent across all regions, different yet related management approaches should be employed in various regions of Europe. These variations reflect the differences in forest types across the continent, differences in the intensity and extent of natural disturbance regimes, and the ways in which forests have been used in the past and will need to be managed in the future (Larsen et al., 2022). According to Konôpka et al. (2012), on the sandy soil of the Záhorská lowland, the successful establishment close-to-nature forest management, measured based on sufficient natural pine regeneration, was found only in 15% of the area left to this approach.

Nevertheless, our results demonstrate that, in comparison with standard-managed pine plantations, CNFM has considerable positive effect on several soil properties, microbial activity and nematode community parameters. While soil pH, values of N, S and C/N ratio remained similar in both differently managed plantations, CNFM notably increased soil moisture, and contents of soil carbon and phosphorous. This can be attributed to changed microclimate which enhanced the growth of understory vegetation and lead to the accumulation of a thick layer of organic matter. Previous records indicated, that close-to-nature techniques, which produce a more complex stand structure, had a positive effect on soil properties, soil moisture, tree growth and understory diversity (Chaudhary et al., 2016; Collado et al., 2023; Rothe et al., 2002). In contrast, considerably changed overstory structure by thinning, burning or clear-cut managements altered soil properties and resource availability for the understory plants and finally, significantly affecting the understory (Barbier et al., 2008; Moghaddas & Stephens, 2007). A study by Ujházy et al. (2017) clearly demonstrated that species composition of understory in managed beech and spruce stands differed significantly from the unmanaged fir-beech old-growth forests on the same site type. Plant diversity was also strongly related to the overstory age and tree density, and differed depending on the dominant tree species, suggesting that overstory structure largely determines light conditions, litter and topsoil properties (Tinya & Ódor, 2016).

In our study, analysis of the soil microbial characteristics showed considerable increase in N-mineralization, microbial biomass carbon (Cmic) and basal respiration following the CNFM. Moreover, microbial biomass was positively correlated with organic carbon content, in agreement with some previous observations (Allen & Schlesinger, 2004; Foote et al., 2015; Li et al., 2004). In contrast, in SFM stands characterized by multiply thinning silvicultural practices in the past, the activity of soil microbial communities was lower as indicated by microbial parameters. This situation is unlikely to change or improve after full harvest of the stand in the future, as suggested by the results of Pietikäinen and Fritze (1995) where basal soil respiration decreased after clear-cutting and clear-cutting followed by prescribed burning treatments. Similarly, Foote et al. (2015) found that forest harvesting practices that removed more than the tree bole significantly reduced total soil nitrogen, and microbial biomass carbon and nitrogen in loblolly pine forests. Moreover, these reductions were still evident 15 years after the treatments were applied. However, a recent meta-analysis conducted by Zhang et al. (2018) showed, that forest thinning did not significantly change soil carbon stocks and generally, increased soil respiration, but the effect of forest thinning on soil respiration varied by tree species. Moreover, soil respiration was initially increased but decreased gradually to the original level after thinning.

In addition to microbial indicators, a positive effect of CNFM on soil nematode communities has been observed. While nematode species diversity and number of species remained unaffected; total nematode abundance, abundance of community stabilizers of omnivorous (Om4) and predaceous (P3,4) nematodes as well as total nematode biomass considerably increased in CNFM stands, compared to SFM plantations. Moreover, a positive interaction between total nematode abundance, omnivores, predators and nematode biomass and soil moisture content were observed, suggesting on overall beneficial impact of CNFM on the plantation environment and its biodiversity. Unfortunately, to our knowledge’s the impacts of CNFM techniques on soil nematode communities has never been studied, neither in deciduous, mixed or coniferous forests, nor in different soil types to compare and discuss our findings.

It was found that another reasonable management practices can also help to increase forest biodiversity. For example, thinning where a proportion of saplings or small trees are removed to allow the remaining trees to grow faster and stronger, is widely used. Yin et al. (2021) reported, that thinning increased the density of soil nematodes, the number of omnivore-predator nematodes, the diversity (H′) of nematodes and enriched the food web structure of soil nematodes in Pinus massoniana plantation on Nitisol (yellow soil) in China. Similarly, Gibson et al. (2022) observed higher nematode densities in Pinus ponderosa forests two years after thinning; while nematode abundance was minimally affected in thinned black pine stands (Landi et al., 2020). In contrast, clear-cut harvesting considerably decreased overall nematode abundance (Bloemers et al., 1997); abundance of fungivores, predators and omnivores (Forge & Simard, 2000; Sohlenius, 2002), while bacterivores and plant parasites generally benefited from such management (Clausi & Vinciguera, 1999; Forge & Simard, 2000).

Conclusion

Trees directly or indirectly affect the input of heat, light or water to the soil surface as a result of the differences in the age of trees, stand composition and vertical and horizontal canopy. Numerous studies have investigated the impact of different silvicultural practices, as thinning or prescribed fires during silviculture leading to clear-cutting at the end on forest biota. Similarly, pine plantations with standard management practices used in our study will be harvested several years later, and new forests will be created from pine seedlings planted on cleared aeolian sands. However, our study showed that such plantations, decades after establishment had considerably poorer understory, soil properties, weakened microbial activity and less matured and structured nematode communities, compared to stands managed under close-to-nature approach recently established. It is generally accepted, that aeolian sands are harsh habitats for plants and biota. Chemically, aeolian sands are made of silicon and aluminum oxides, and belong to the substrates that have the poorest nutrient content and the lowest mineral strength. Development and growth in such habitats are very slow and the optimum stage of vegetation characterized by the highest stocking index and the lowest tree density (Durak et al., 2021) lasting for many years. Although data in our study are based on relatively few samples and pertain to a single temporal testing, nevertheless they have shown that the introduction of close-to-nature management in pine plantations on aeolian sands had a beneficial effect on particular soil properties and microbial activity. Additionally, although similar species number and nematode diversity in differently managed plantations pointed out to homogeneous nematode communities across the study locality. CNFM increased nematode abundance and biomass and improved the maturity and structure of the nematode food web. This underscores the general assertion that CNFM promotes components, structures and processes in managed forests, that are characteristic of natural forests.