1. Introduction
Soil is formed under the influence of bedrock, climate, vegetation, and topographic factors. However, the dominance of one factor over the others can lead to variations in soil properties and land productivity [
1]. Semiarid and arid ecosystems, for example, are primarily influenced by parent material and the continental climate, with summer drought being a critical stress factor [
2,
3]. The physical properties, mineral composition, and texture of the parent material significantly affect the soil formation rate and the characteristics of the resulting soil [
4]. The hardness of rocks is also crucial; soils formed on hard rocks have lower formation rates than the rate of soil loss through erosion, leading to poor habitat characteristics. Birkeland [
5] ranked bedrock stability against soil formation factors as follows: quartzite, chert > granite, basalt > sandstone, siltstone > dolomite, limestone.
The parent material contributes to the nutrient cycle by releasing nutrients through weathering and maintaining a balance between nutrient loss and storage [
1,
4]. After climate, plant and microorganism activities play the most important roles in soil formation. Trees, in particular, influence parent material through biomechanical, chemical, and biological effects [
1,
3]. Traditionally, studies have focused on the biochemical, ecological, and edaphic influences on soils, but the significance of biomechanical effects is increasingly recognized [
6,
7,
8]. Most research on the biomechanical effects of vegetation has centered on tree root growth, but trees also impact the physical displacement of regolith materials and the formation of root debris and exudates [
9]. Plant roots mechanically break down rocks during growth, accelerating soil formation processes. Additionally, root exudates and mechanical root activities help reduce erosion, stabilizing soil ecosystems [
10]. Organic acids and exudates produced by plants and microorganisms enhance the chemical weathering of minerals and the release of nutrients [
11].
Over the last 20 years, the Turkish Ministry of Agriculture and Forestry has conducted afforestation activities to increase forest cover to 30% of Turkey’s surface area. Scots pine and black locust are commonly used for erosion control due to their high tolerance to drought and poor soil conditions [
12]. Planting native tree species has accelerated natural succession [
13] and carbon sequestration [
14]. Plants, known as the organisms with the largest biomass in terrestrial ecosystems, contribute greatly to the formation and development of soils through litter residues, leaves, branches, roots, and exudates of their aboveground biomass [
15,
16]. In addition, vegetation affects the organic carbon (OC) concentration added to soils and the diversity, quality, and quantity of soil microorganisms [
17]. For example, the biomass of microbial organisms in soils converted from forest to agriculture decreases significantly [
18,
19]. Similarly, agricultural land contains less soil organic C (SOC) and C
mic compared to native forests [
20]. Vegetation type strongly influences soil-forming processes, contributing significantly to organic matter and microbial community development and activity [
21,
22]. Microbial stoichiometric indices such as microbial basal respiration (MR), microbial quotient (
qMic), metabolic quotient (
qCO2), and C mineralization are related to the SOC cycle in the environment. With the help of these stoichiometric indices, the effects of changes in land use or ecosystem on soil quality, microbial populations, and microbial activity rates can be estimated [
23,
24].
While many studies suggest that microbial biomass is controlled by soil physicochemical properties [
25,
26], others indicate that aboveground vegetation type has a stronger influence on microbial biomass and activity [
27,
28]. Kara et al. [
10] found positive effects of ecosystem changes on microbial biomass and activity in afforested areas in both the short and long term. Research on afforested ecosystems shows that vegetation enhances soil-forming processes, leading to organic matter accumulation and microbial community development [
29,
30,
31]. Trees provide nutrients to the soil through dead plant residues and root secretions, which microorganisms break down and mineralize, forming a usable nutrient source for plants [
32]. Microbial organisms decompose this litter, returning essential organic nutrients to the soil. This process enriches the growing environment, enhancing the productivity of the soil. As a result, plant growth parameters, such as tree height (TH) and diameter at breast height (DBH), improve significantly. The upper soil horizon, a reservoir for soil microbial biomass, is crucial for decomposition and nutrient cycling [
33]. This symbiotic relationship underpins the nutrient cycle, essential for ecosystem health.
Soil microorganisms are the most rapidly affected by environmental conditions, various forestry management practices, and sudden changes on or within the soil surface. Consequently, measuring microbial activities and properties following forestry management activities is essential for monitoring changes in soil fertility. Changes in microbial properties can provide early indications of potential ecosystem disruptions [
32,
34,
35,
36,
37]. Assessing microbial biomass in soils is crucial in many soil monitoring studies and programs [
35], but it should be considered alongside other habitat parameters for a comprehensive ecological assessment. Although recent studies focus on the effects of soil microbiological properties on soil fertility, there is still limited information on their relationship with tree growth. This research aimed to new perspectives on the effects of physicochemical, microbial properties, and stoichiometry of soils derived from different bedrocks on seedling growth parameters, highlighting the importance of evaluating soil quality parameters in forests.
4. Discussion
In this study, it was revealed that physicochemical and microbiological soil properties as well as plant growth parameters were significantly affected by the type of parent material. Limestone soils formed fine-grained soils with significantly lower sand and higher silt and clay contents. Some studies indicated that soils developed from limestone bedrock had lower sand and higher clay and silt contents than other bedrock types such as granite and acrostic [
4], and andesite [
1]. Similarly, [
3] determined that the grain sizes of soils from granite, granodiorite, and schist bedrocks were different.
The parent material difference also influenced the chemical properties of soils, such as pH, EC, SOC, TN, and CaCO
3, in both control and afforestation sites. Previous studies have reported significant variations in soil pH values due to differences in parent material [
1,
3,
4]. The EC values of soils from both parent materials at the study sites were below the critical salinity level of 0.20 dS cm
−1. SOC and TN content differed significantly depending on the parent material and were higher in afforestation sites compared to control sites. The increase in carbon and nitrogen sources in soils after afforestation affects the quality and quantity of organic material, including litter production [
49], root exudates [
50], and microbial community structure and activity [
15]. Plant residues such as leaves, cones, and branches contribute to increase SOC and TN levels in the ecosystem [
32,
51,
52]. Moreover, factors like higher clay content, tree presence [
53,
54], root exudates, and plant residues [
15] promote soil formation. While the effect of bedrock on soil properties was observed in control sites, afforestation, particularly with Scots pine, appears to have significantly contributed to the increases in SOC and TN. The concentration of SOC and TN in the study area may be lower than in mature forests, as only fifteen years have passed since afforestation. It may take a longer period for litter to accumulate on the soil surface and integrate into the soil. Typically, SOC and TN concentrations are higher in the topsoil layer (0-20 cm) due to litter accumulation [
4,
32]. As expected, CaCO
3 content was significantly higher in soils with limestone parent material, attributed to the inherent lime content of limestone. There was a notable decrease in CaCO
3 content after afforestation, likely due to plant influence on soil formation. It has been reported that afforestation with different species can reduce the amount of lime in the soil, particularly in the topsoil (Babur, 2019). Limestone soils are characterized by alkaline properties and texture variations between sand (sandstone) and clay (shale) [
55].
The chemical properties of soils such as pH, SOC, and TN and microbial properties of C
mic, N
mic, MR, and other indices are important sensitive indicators affected by several factors such as seasonality, tree species, bedrock properties, and soil properties [
16,
56]. The available C
mic in soils directly indicates the microbial biomass pool in the soil. The study results illustrated that the parent material significantly affects the C
mic concentrations in soils. Mahia et al. [
47] stated that schist was significantly different from granite parent material in C
mic, while Babur (2019) reported that soils developed from limestone parent material had higher C
mic than andesite. In addition, afforestation of bare land contributes to the nutrient cycle of the ecosystem, ecosystem productivity, soil quality, and the increase in the amount of soluble and storable organic C in the soil with the input of organic matter from litter and dead roots [
57,
58]. Increased organic carbon in soil also affects the biological properties of soils [
59]. Soil C
mic is generally dependent on soil organic matter as a substrate [
24]. In addition, exudates brought to the soil by plant roots can increase C
mic accumulation, especially in topsoil [
60]. In this article, it was confirmed that the increased organic matter input after afforestation also significantly increased the C
mic amount.
Microbial respiration is the most important microbial factor in monitoring decomposition [
35,
46]. Soil MR indicates the oxidation status of the soil by microbes and is considered one of the most important and sensitive indicators of the carbon cycle in soils [
61,
62]. Babur et al. [
32], in their study conducted in four different seasons, determined that m respiration in winter was 8.96 µg CO
2–C g
−1 h
−1, which was higher than in summer (6.19 µg CO
2–C g
−1 h
−1) and autumn (6.85 µg CO
2–C g
−1 h
−1). In addition, in the same study, it was determined that there were significant differences in C
mic and MR values of different tree species. Similarly, it was stated that the optimum soil temperature and moisture content, which accelerate microbial activities in different tree plantations, were in rainy seasons [
63].
The
qMic values of all soil samples were found to be within the range suggested by Anderson and Domsch [
64].
qMic can be used to quickly estimate the change in organic C [
65] and to compare soil quality parameters in ecosystems with different organic matter [
59]. The
qMic ratio is determined by using the relationship between microbial biomass carbon and soil organic carbon [
66]. The
qMic ratio of andesite soils of the study areas was lower than that of limestone. This provided more suitable conditions for the growth of soil microorganisms in the limestone bedrock. Although the average
qMic values increased after afforestation, this value remained low for fertile lands and did not reach equilibrium 15 years after afforestation. The
qMic value is estimated by microbial activity, i.e., soil basal respiration and organic carbon cycling within an ecosystem [
35,
67]. Higher MR and C
mic amounts in limestone soils indicated that it had more microbial activity, proving faster decomposition.
The
qCO
2 is often used to describe the soil stress response of an ecosystem [
67]. Environmental stresses, nutrient scarcity, and substrate resistance to decomposition can increase
qCO
2 [
68]. On the other hand, the productivity of the area can be increased by using natural vegetation in stressed areas [
69]. Although Anderson [
67] reported that
qCO
2 values in neutral soils ranged from 0.5 to 2.0 μg CO
2-C g
−1C
mic h
−1, in this study, the average
qCO2 values in areas afforested with andesite and limestone bedrocks were found to be 3.37 to 4.06 μg CO
2-C g
−1C
mic h
−1, respectively. This may cause microbial respiration to increase, decomposition to slow down and energy to be consumed in it. Low
qCO
2 rate in soils provides positive information about efficient carbon use for that soil, low environmental stress levels, usable nutrients being free in the area, microbial activity, and soil quality [
22].
The
qCO
2 and OC mineralization rates in soil constitute the basis of the carbon cycle in terrestrial ecosystems [
10,
70]. SOC content negatively correlated with the metabolic coefficient in both parent materials. Some studies have noted that OC and
qCO
2 in different forest soils are closely related [
1,
60]. As a result of the correlation analysis, it was determined that basal respiration was positively correlated with soil organic carbon in both parent materials. Similar results were found with Cheng and Xia [
71].
Differences in parent material and soil properties showed significant differences in plant growth parameters such as tree height and diameter at breast height. In particular, Scots pines on limestone bedrock were 20% and 25% larger in height and diameter than on andesite bedrock, respectively. There are many studies on the importance of the effects of habitat properties on plant yield and growth characteristics [
72,
73,
74]. Microbiological, physical, and chemical soil properties can improve as the age of the afforested area. Afforestation and rehabilitation practices in forest management contribute to the health and productivity of soils and support the restoration of soil microbial activities.
Available C fractions are one of the most important environmental factors affecting soil microbial activity [
22,
75]. Analyses performed on soil samples, particularly microbial biomass, MR, and other indices, may be responsible for the separation of different parent materials and control areas with DA because the parent material is an important factor in soil formation, and tall plants also provide more living space and substrate for microbial communities in soils with their aboveground and belowground biomass and waste. Improvement of microorganisms’ living environments can also be understood by the increase in
qMic value and decrease in
qCO
2 value [
10,
76]. In fact, from our research, it was determined that there was an increase in
qMic amount and a decrease in
qCO
2 amount in soils on both bedrocks after afforestation. These results provide strong evidence that afforestation on different bedrocks results in different soil properties and microbial communities compared to bare lands.