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Article

Effects of Afforestation on Soil Organic Carbon and Nitrogen Stocks in the Long Term in Semi-Arid Regions of Türkiye

by
Murat Sarginci
1,* and
Adem Seçilmiş
2
1
Faculty of Forestry, Düzce University, Düzce 81620, Türkiye
2
General Directorate of Forestry, Ankara 06560, Türkiye
*
Author to whom correspondence should be addressed.
Forests 2025, 16(10), 1524; https://doi.org/10.3390/f16101524
Submission received: 11 August 2025 / Revised: 17 September 2025 / Accepted: 23 September 2025 / Published: 28 September 2025
(This article belongs to the Special Issue Afforestation of Degraded Lands)
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Abstract

The black pine (Pinus nigra Arn.) is among the most preferred tree species for afforestation in Türkiye. This study aims to examine the effects of afforestation carried out in 1968, 1973, 1985, 1996, and 2002 on soil properties, especially soil organic carbon (SOC) and nitrogen (N) in a semi-arid region of Türkiye. Soil texture, electrical conductivity (EC), reaction (pH), Cation Exchange Capacity (CEC), lime (CaCO3) concentration, N, and inorganic-organic C contents were determined for each afforestation site. Although afforestation significantly increases SOC and TN stocks, the stand’s age did not affect the dynamics of the SOC stocks. But early stages of afforestation increased N stocks by more than 500%–600% compared to older ones. Our results show that afforestation combined with soil preparation increases the SOC and N contents, and soil tilling without plantation accelerates this process in the initial stages of afforestation. Rather than planting only one tree species, a plantation that mixes broadleaves and conifers with other annual and perennial plants may be more suitable for long-term C sequestration and the use of assisted natural succession in the revegetation of degraded arid and semi-arid regions as an alternative to large-scale afforestation should be paid more attention in the future.

1. Introduction

According to the United Nations Convention to Combat Desertification (UNCCD), 2.3 billion people lived in drylands, which comprised almost a third of all people on Earth in 2020, representing a two-fold increase from 1990. Most of these people reside in Asia and Africa [1]. These arid regions cover 41% of terrestrial ecosystems [2], store approximately 27% of all SOC [3], and are marked by chronic water scarcity, drought, significant climatic variability, and land degradation, which includes desertification and biodiversity loss [2]. As land degradation is a major challenge in these ecosystems [4], afforestation becomes one of the most essential forestry activities to restore these degraded areas [5]. The sustainability of livelihoods in these regions is threatened by a complex and interrelated range of social, economic, and ecological changes that cause challenges to researchers, policymakers, and, above all, rural land users [4]. Large-scale and time-spanning ecological restorations, such as afforestation, can change entire ecosystems and their services [5,6]. Using afforestation for ecological restoration changes the soil characteristics over time, soil fertility increases in parallel with the amount of organic matter entering the system, and, as a result, soil fauna and soil respiration also change [7,8]. Scientific detection and measurement of these changes are of great importance in the sustainable management of these ecosystems.
For centuries, the pressure of mankind on forest areas has caused the world’s forest cover to decrease, and the increasing pressure of the Industrial Revolution has caused a rapid decline since the 1800s [9]. These anthropogenic disturbances result in a lack of nutrients and carbon in forest ecosystems [10]. For instance, during the conversion of forests to agricultural lands in the Western Black Sea Region of Türkiye, 22% of the soil carbon, approximately 1/3 of the nitrogen, was lost in the first three years and 45% of the soil carbon ten years later [11], and the physical and chemical structure of the soil deteriorated. Soil fertility and production capacities decrease due to vegetation disturbances in arid and semi-arid areas. Since the disturbance of the structure and functions of ecosystems jeopardizes the sustainability of the services they provide to society, the structure of fragile ecosystems and the restoration of disturbed ecosystems have become one of the most critical concerns of natural resource managers [8]. Moreover, these regions play an essential role in the global C cycle and have the strong potential for C sequestration [3,12]. At the 12th Conference of the Parties to the United Nations Convention to Combat Desertification (UNCCD COP12), it was decided to determine the Land Degradation Neutrality (LDN) targets that countries have voluntarily stated until 2030 towards the United Nations Sustainable Development Goal 15.3 and to monitor the work carried out and land degradation trends [13]. The rehabilitation, restoration, and reforestation studies to be carried out will contribute to the carbon accumulation [14].
Türkiye is in the Mediterranean region and is one of the most vulnerable countries against climate change [15,16,17,18,19]. Semi-arid and arid areas constitute 35% of Türkiye’s surface area. However, when semi-humid areas prone to desertification are added, this figure increases to 60%. However, in continuing long-term anthropogenic disturbances like intensive cultivation, overgrazing, and land misuse for thousands of years over large terrestrial ecosystems, the naturally occurring plant cover has partially or totally disappeared [5,20]. Therefore, it is revealed that more than half of the country’s surface area is at risk of desertification [21,22]. Considering that large land areas of Türkiye are affected by drought and that most potential afforestation areas, including the study areas, are in these areas, it becomes clear that the scope of the study is parallel to the objectives of many action plans and R&D studies in Türkiye.
Black pine has a wide distribution area in the Mediterranean Basin, and Türkiye has an important share [23,24]. Türkiye has approximately 23.36 million hectares of forest area, and black pine is the third most widespread tree species after oak (Quercus spp.) and red pine (Pinus brutia) with a spread area of 4.1 million hectares, 1.26 million hectares of which are designated as degraded areas [25]. Afforestation has an important share in all forestry activities [25], and black pine is among the most used species from the past [26] to present [27,28,29] in Türkiye. According to the statistics kept by the General Directorate of Forestry in degraded areas, approximately 9.7 million hectares of afforestation were carried out between 1946 and 2023. Black pine has a large place in these afforestation activities, and approximately 1 billion black pine saplings were produced and planted in the fields between 2009 and 2016 [25].
Afforestation in degraded areas of arid and semi-arid regions causes changes in soil physical and chemical properties, including texture, pH, Cation Exchange Capacity (CEC), soil carbon, and nitrogen, in both the short and long term [30,31,32,33,34]. Various studies emphasize that afforestation increases the soil organic carbon (SOC) and nitrogen (N) content and decreases the high soil pH over time [5,6,35,36,37,38,39,40,41,42,43,44,45], but most afforestation studies in arid and semi-arid regions of Türkiye focus on survival rates and growth data; limited data are available about changes in soil properties [5,6]. Considering previous studies on arid- and semi-arid degraded lands, we hypothesized that over the long term, the afforestation of degraded lands improves soil properties and thus increases the SOC and N stocks in semi-arid regions of Türkiye. Therefore, this study aims to examine the soil properties, especially soil organic carbon (SOC) and nitrogen (N) changes, with the chronological order of black pine afforestation carried out at various years in the Nallıhan Region, which is a semi-arid transitional region between the humid Black Sea Region and the arid Central Anatolia Region. The assessed alterations in soil characteristics, including soil organic carbon (SOC) and nitrogen accumulation, provide some of the missing data and enable future afforestation, restoration, and rehabilitation efforts to be guided more scientifically within this framework.

2. Materials and Methods

2.1. Study Area

The study sites (Figure 1) are in Ankara Province, Nallıhan District, Nallıhan Forest Management Directorate, Nallıhan Forest Management Chiefship, and Uluhan Forest Management Chiefship (40°00′28″–40°19′22″ N 31°17′55″–31°45′04″ E), which has an approximately 81,623.6 ha area: 31,455 ha forested and 50,168 ha non-afforested [46,47]. Experimental sites were selected from the black pine (Pinus nigra Arn.) afforestation areas (Table 1) within the boundaries of Nallıhan Forest Management Directorate and Uluhan Forest Management Directorate above Atça Village, within the scope of the green belt afforestation of Ankara Regional Directorate of Forestry in 1968, 1973, 1985, 1996, and 2002.

2.2. Soil Properties

Calcareous soils dominate the area, and soil preparation is required in all afforestation sites [46,47,49]. A total of 80 percent of the study areas consist of andesite, lava, and tuff bedrock of volcanic origin formed in the 3rd geological period. The erosion period started after the Neogene, the valleys deepened, and the accumulation of lime, clay, sand, marl, and gravel on the lake and sea floors formed Neogene plateaus in the region and has preserved its continental form until today with the withdrawal of the sea in the Oligocene [46,47]. Black pine forests in the region are generally widespread on soil over limestone, andesite, and marl [50]. Soil depth ranges from 50 to 70 cm, measured on 25 soil profiles in the current study. The soil type is loam; it varies from place to place between clay, clayey loam, and sandy loam. These soils are brown forest soils according to the old European and American classification [50,51] and so-called calcic cambisols, xerosols, and calcisols in the FAO soil map [52]. Inceptisol soils are dominant in the area, and being in a transitional region, aridisols are present in the south and alfisols in the west, according to the USDA soil taxonomy [53].

2.3. Climate and Vegetation

This area is in the transition zone between the Euxine subflora section of the European–Siberian flora region and the Iran–Turan flora region, and a transition zone between the humid Black Sea climate and the arid continental climate [49,50]. The elevation of these sites varies between 900 and 1500 m, and according to Thornthwaite climate indices, it is in a semi-arid area under the influence of an effective drought, especially in the summer months [51]. The mean annual temperature is 12.4 °C, and the mean annual precipitation is 360 mm [46,47]. Black pine consists of 30.2% of the forest area in the region, 10.3% consists of red pine, and the rest is a mixture of other broad-leaved species [46,47]. Understory plants are Juniper (Juniperus oxycedrus L. subsp. oxycedrus), Astragalus (Astragalus schizopterus Boiss.), Alyssum (Alyssum minus (L.) Rothm. var. minus), Poisonous clover (Dorycnium pentaphyllum subsp. anatolicum), Windgrass (Apera spica-venti), Campanula (Campanula lyrata subsp. lyrata), Broomleaf toadflax (Linaria genistifolia subsp. linifolia), Italian catchfly (Silene italica), and rough bluegrass (Poa trivialis L.) [52].

2.4. Field Study and Sampling

We chose 50-, 40-, 30-, 20-, and 10-year-old black pine afforestation sites for this study (Figure 2). Three randomized sampling units (20 m × 20 m) were created in 2015 in each block (afforestation site). The forest management directorate plowed the topsoil (0–30 cm) of each afforestation site and ripped the subsoil (30–80 cm) with a three-shank ripper using a 4 × 4 rubber-tired tractor to prepare the site for planting [46,47,49] before afforestation. We collected three replicated litter and humus samples within 30 × 30 cm2 quadrats from randomly determined locations and needle samples from five randomly selected trees in each sampling unit. Then, we excavated three soil pits through bedrock in each sampling unit. Two sets of soil samples were from the same depth, one for bulk density, taken with a 100 cm3 cylinder (AMS Soil Core Sampler, AMS Inc., American Falls, ID, USA), and the other for soil physical and chemical analyses, 1 kg in weight, taken with a hand shovel from the soil surface to 50 cm at every 10 cm depth. For soil sample comparison, we took soil samples from three randomized locations (the control units) 50 m outside of each afforestation site. Control units were not afforested. However, during site preparation for afforestation, bulldozers cleared all above-ground vegetation from the entire area and plowed the topsoil, and thus, there was no undisturbed soil in the adjacent area. Therefore, only the topsoil of control units was plowed and not afforested. So, the control units are labeled as “tilled” (Figure 2).
We analyzed soil samples for soil bulk density, coarse fraction (dry screening Ø > 2 mm) texture, lime content, pH, salinity (EC), CEC, organic carbon (SOC), and total nitrogen (TN). We used the Bouyoucos hydrometer technique to measure the soil texture [53,54,55]. The acidity and electrical conductivity (EC) of the air-dried soil samples (<2 mm) were determined in a pure water soil mixture using a pH meter and an EC meter [56,57]. We calculated the Soil Inorganic Carbon (SIOC) concentration from the lime concentration determined via a Scheibler Calcimeter [58,59,60]. An NH4OAc extraction was used for the CEC [61]. CN concentration of soil, litter, humus, and needles was analyzed using the dry combustion method in an LECO CN analyzer [62,63]. The amount of soil C and N was calculated in Mg ha−1 by multiplying the C and N concentrations by the soil amount obtained from soil bulk density. The C and N amounts of litter and humus were calculated by multiplying the C and N concentrations by the amount of litter and humus in Mg ha−1.

2.5. Data Analysis

For a comparison of soil, litter, and humus variables among afforestation sites established in different years, we used a block design. The control values from outside each afforestation site were used as a covariate to eliminate the differences not caused by afforestation for soil analysis. We used the data to determine changes in SOC and N over afforestation periods. We used the analysis of variance (ANOVA) to evaluate the measured parameters. We used the SAS 1996 software [64] for the statistical analysis of the data, and the results were considered significant at a p-value of α = 0.05. The Tukey mean separation test was performed at a significance level of α = 0.05 to distinguish between the means. Visualization of SAS statistical analysis results was performed using RStudio v. 0.24.09.0+375 [65].
We applied structural equation modeling (SEM) to quantify the direct and indirect effects of soil properties, treatment, and treatment year on soil carbon and nitrogen pools. Analyses were conducted in R (v4.4.1) [66] using the lavaan (v048.i02) package [67]. Exogenous variables were treatment, treatment year, and measured physical and chemical soil properties, soil bulk density, sand concentration, clay concentration, silt concentration, lime concentration, pH, EC, and CEC. Endogenous group 1 was Soil Total Carbon (STC), SIOC, SOC, and N concentrations, and endogenous group 2 was STC, SIOC, SOC, and N stocks. For each endogenous group, we specified a saturated regression model in which every endogenous variable was regressed on all exogenous variables. In addition, residual covariances were allowed among endogenous variables within the same group. This yielded two models:
Model 1—exogenous variables predicting endogenous group 1.
Model 2—exogenous variables predicting endogenous group 2.
Models were fit using robust maximum likelihood estimation (MLR) with full information maximum likelihood (FIML) to handle missing data and standardized latent variable variances (std.lv = TRUE). Model fit was evaluated using conventional indices: χ2 test statistic, degrees of freedom, p-value, Comparative Fit Index (CFI), Tucker–Lewis Index (TLI), Root Mean Square Error of Approximation (RMSEA) with 90% confidence interval, and Standardized Root Mean Square Residual (SRMR). Standardized path coefficients (β), standard errors, and p-values were extracted from fitted models. Paths with p < 0.05 were considered statistically significant.
To enhance visualization and interpretability, the following was performed:
Heatmaps of standardized path coefficients were generated with ggplot2 (v 4.0.0) [68], showing the relative effect sizes of exogenous predictors on each endogenous variable.
Path diagrams were constructed with the igraph (v 2.2.0) package [69], where arrow thickness reflected effect size (|β|) and edge color indicated significance (black = p < 0.05; gray = non-significant). Node labels corresponded to the original descriptive variable names.

3. Results

3.1. Soil Cation Exchange Capacity (CEC), pH, Salinity (EC), and Lime (CaCO3) Concentration

No statistically significant difference was determined for soil CEC between the afforestation and control (tilled) units, and the values vary between 18.74 and 42.58 Cmolc kg−1 in afforestation sites and 18.32–38.60 Cmolc kg−1 in tilled sites. However, when compared among afforestation sites, the CEC of topsoil (0–10 and 10–20 cm soil depth) increased by approximately 50% from 50 years of afforestation sites to 20-year-old afforestation sites but then decreased to lower than initial levels in 10-year-old afforestation sites, and the same trend was observed for the control sites (Figure 3). The soil pH was alkaline (7.36–7.68 in the afforestation sites, 7.18–7.70 in the control sites). There was no statistically significant difference in 30-, 40-, and 50-year-old afforestation sites. For both, it decreased in topsoil (0–10 cm and 10–20 cm) between 20- and 10-year-old tilled and afforested sites (Figure 3). While the highest lime concentration was in the 50- and 40-year-old afforestation and tilled sites, it decreased dramatically in more recent years (Figure 3). There was no salinity problem for all sites, and the EC was lower than 4000 μS cm−1 (Figure 3).

3.2. Soil Texture (Sand, Clay, and Silt Concentrations) and Bulk Density (g cm−3)

Soil texture varied from sandy loam to clay loam. The dominant soil type was sandy clay loam in 50-, 40-, and 30-year-old sites, and it was sandy loam in 20- and 10-year-old sites. Sand, clay, and silt concentrations were statistically different between afforestation and control (tilled) sites. Especially, the clay content decreased by 60%–70% in 20-year-old tilled sites and by approximately 50% in 10-year-old afforestation and tilled sites compared to older ones (Figure 4). As the area ages, the clay content increases, and afforestation sites contain more clay than tilled sites. Afforestation did not change the soil bulk density (Figure 4). However, the soil bulk density for all soil depth levels, especially topsoil (0–10 cm, 10–20 cm soil depth), was ~1.4–1.5 g cm−3 and higher in the 10-year-old afforestation and tilled sites than in older ones. The soil bulk density gradually decreases as the years go back.

3.3. Soil Carbon and Nitrogen Concentrations and Stocks

Although there were no statistical differences among afforestation sites of various years at the same depth level, the SOC concentrations were 160% and 220% higher at 0–10 cm and 10–20 cm soil depths in 20-year-old tilled sites than in other tilled sites. Moreover, the SOC concentration was higher in the topsoil of the 50- and 40-year-old afforestation sites compared to tilled sites; the opposite was true for the newer afforestation sites. The SOC concentrations in afforestation sites at 0–10 cm were 35% and 54% lower and at a 10–20 cm soil depth were 31% and 69% lower in 30- and 20-year-old afforestation sites, respectively, than in the same year’s tilled sites (Figure 5). It decreased by 89% and 93% at soil depths of 20–30 cm and 30–40 cm in 20-year-old afforestation sites compared to the same year’s tilled sites, respectively. Quite the opposite of SOC concentrations, the SIOC and TSC concentrations of topsoil increase as the treatment year goes back (Figure 5). The highest SIOC and TSC concentrations were in tilled sites in 1973. While the SOIC concentration was lower in 50- and 40-year-old afforestation sites than in tilled sites, it was higher in 30- and 20-year-old afforestation sites.
The N concentrations of 10-year-old afforestation sites at 0–10 and 10–20 cm soil depths were ~630% and 525% higher than those of 50-, 40-, 30-, and 20-year-old afforestation sites (Figure 5). Furthermore, the soil N concentration difference with the highest value was in the 20-year-old tilled sites for five soil depth levels from surface to bottom (Figure 5), and it was ~875%, 840%, 1145%, 2310%, and 1730% higher than the same year’s afforestation sites, respectively. On the other hand, the N concentrations of both 20- and 10-year-old tilled sites at all soil depth levels were higher than those of the older tilled sites. It was ~1285%, 1520%, 1740%, 1970%, and 1790% more in the 10-year-old tilled sites and ~600%, 620%, 875%, 760%, and 920% more in the 20-year-old tilled sites at five soil depth levels from top to bottom, respectively, than in the 50-, 40-, and 30-year-old tilled sites (Figure 5).
Although there were no statistical differences for SOC stocks among the afforestation sites for all soil depths, it was 24% and 61% less in 30- and 20-year-old afforestation sites at a 0–10 cm soil depth, 72% less in 20-year-old afforestation sites at a 10–20 cm soil depth, and 140% and 190% less in 40- and 20-year-old afforestation sites at a 40–50 cm soil depth than the same year-old’s tilled sites (Figure 6). Additionally, the SOC stock was ~140% and 180% higher for the 0–10 cm and 10–20 cm soil depths of 20-year-old tilled sites than in the 50-, 40-, 30-, and 10-year-old tilled sites (Figure 6). The SIOC and TSC stocks gradually decreased from 50-year-old to 10-year-old treatments at all soil depth levels. The SOIC stocks of afforestation sites were ~1000% more in 40-year-old sites and ~500% more in 50-year-old sites; TSC stocks of afforestation sites were ~550% more in 40-year-old sites and ~300% more in 50-year-old afforestation sites at all soil depths than those of the 30-, 20-, and 10-year-old afforestation sites. The SIOC stocks of tilled sites were ~3500% more in the 40-year-old sites and ~2100% in the 50-year-old sites; the TSC was ~650% more in the 40-year-old sites and ~400% more in 50-year-old sites at all soil depths than those of the 30-, 20-, and 10-year-old tilled sites.
The N stocks of both afforestation and tilled sites decreased over the years. The N stocks of 10-year-old afforestation sites at 0–10, 10–20, 20–30, 30–40, and 40–50 cm soil depths were ~790%, 715%, 420%, 345%, and 900% more, respectively, than those of the 50-, 40-, and 30-year-old afforestation sites. Furthermore, the N stocks of 10-year-old tilled sites at 0–10, 10–20, 20–30, 30–40, and 40–50 cm soil depths were 750%, 815%, 910%, 730%, and 930% more, respectively, than those of the 50-, 40-, and 30-year-old tilled sites. The N stocks of 20-year-old tilled sites at 0–10, 10–20, 20–30, 30–40, and 40–50 cm soil depths were ~1250%, 1430%, 1370%, 1140%, and 1100% more, respectively, than those of the 50-, 40-, and 30-year-old tilled sites. The N stock of the 50-year-old afforestation site at the 0–10 cm soil depth was 34% less than the same year-old tilled site; it did not differ for 40-, 30-, and 10-year-old sites (Figure 6). It was ~1045%, 900%, 1050%, 1225%, and 1040% more at 0–10, 10–20, 20–30, 30–40, and 40–50 cm soil depths, respectively, in 20-year-old tilled sites than in the same year-old afforestation sites.
In the first 50 cm soil depth of 50-, 40-, 30-, 20-, and 10-year-old afforestation sites, the SIOC stocks were estimated at 129, 248, 26, 23, and 24 Mg ha−1, respectively; SOC stocks were 33, 14, 25, 16, and 28 Mg ha−1, respectively; and N stocks were 15, 8, 7, 9, and 62 Mg ha−1, respectively (Figure 7). The SOC and N stocks at a 50 cm soil depth in 20-year-old tilled sites were 320% and 830% more than the same year-old afforestation site (Figure 7). There were no differences for other years’ treatments for SOC stocks. The highest N stock was calculated in 10- and 20-year-old tilled sites and 20-year-old afforestation sites, and it was ~660% more than the rest of the treatments (Figure 7).

3.4. Carbon and Nitrogen Contents of Tree Needles, Litter, and Humus

The C concentration of pine needles did not change among afforestation sites, but the N concentration of 20- and 10-year-old afforestation sites was ~40% more than in 50-, 40-, and 30-year-old afforestation sites (Figure 8). The highest amount of litter (7.8 Mg ha−1) and humus (46 Mg ha−1) was in 50-year-old afforestation sites, and the litter was 79% more than that of 40-, 30-, 20-, and 10-year-old afforestation sites, respectively. Humus was 484% more than that of 40-, 30-, and 20-year-old afforestation sites, respectively. There was no humus in 10-year-old afforestation sites. While the mean C concentration of litter for all afforestation sites was ~47%, the C concentration of humus decreased from this level to 33% in the 50-year-old sites and 30% and 15% in the 40-, 30-, and 20-year-old afforestation sites, respectively (Figure 8). The highest N concentration in the litter and humus was 1.7% and 1.1%, respectively, in 50-year-old afforestation sites (Figure 8). The N concentration of the litter was ~276% higher in 50-year-old afforestation sites than in 40-, 30-, 20-, and 10-year-old afforestation sites. The N concentration of humus was ~125%, 270%, and 4681% more in 50-year-old afforestation sites than in 40-, 30-, 20-, and 10-year-old afforestation sites, respectively (Figure 8). The carbon stock of litter per hectare in 50-year-old afforestation sites was 72% more than that in 40-, 30-, 20-, and 10-year-old afforestation sites, and the carbon stock of humus per hectare was 490% more than that in 40-, 30-, and 20-year-old afforestation sites (Figure 9). The N stock of litter per hectare was ~700% more in 50-year-old afforestation sites than in 40-, 30-, 20-, and 10-year-old afforestation sites. The N stock of humus was ~290%, 1330%, and 16250% more in 50-year-old afforestation sites than in 40-, 30-, and 20-year-old afforestation sites, respectively (Figure 9).

3.5. Structural Equation Model (SEM) Analysis

While the soil depth, texture (concentrations of clay, sand, and silt), and lime concentration had the strongest negative correlation with SOC stocks, the pH and treatment (afforestation) had the strongest positive correlation. Treatment year longevity, soil bulk density, and the CEC did not change the SOC stocks (Figure 10 and Figure 11). Treatment year longevity had the strongest negative correlation with soil N stocks. The second strongest negative correlation was caused by the clay concentration. Although they were less effective than treatment longevity and clay concentration, CEC, EC, and soil depth also showed a negative correlation with N stocks. The treatment (afforestation) and soil bulk density had a positive correlation with N stocks (Figure 10 and Figure 11). Furthermore, the strongest positive correlation was observed for the lime concentration with SIOC and STC stocks (Figure 10 and Figure 11).

4. Discussion

Our results show that afforestation combined with soil preparation increases the SOC and N contents, and soil tilling without plantation (control sites) accelerates this process in the initial stages of afforestation. The highest SOC stock was calculated in the oldest afforestation sites (50-year-old) of the current study. Although afforestation significantly increases the SOC and TN stocks in arid and semi-arid regions, the plantation age did not affect the dynamics of SOC stocks, but early stages of afforestation, 20 years in the current study, increased the N stocks by more than 500%–600% compared to older ones, and our results are consistent with similar studies [38,70]. The increase in soil nitrogen was also determined in the needles of young stands and was 40% higher than in old stands. Afforestation in a similar area close to the current study also increased the SOC and N concentrations at the first 30 cm depth by 2.5 times after 10 years and approximately 0.3 times after 17 years, respectively, compared to the open area, and did not change in the afforestation sites for the following years [5]. It is well known that, depending on the management system of a terrestrial ecosystem, the seral stages of ecological succession can be reduced, eliminated, or increased [5,7,8,71], and this can alter the soil properties. Soil preparation and vegetation clearance in control sites before afforestation might have accelerated pioneer species colonization at the current site. These fast-growing pioneers might have a significant impact on the chemistry of the soil and produce more litter during the early stages of forest formation [7]. For example, using a systematic arrangement of grass, shrubs, young regenerated forest, and closed forest on karst landforms receiving about 900 mm of annual precipitation in Yunnan, S.W. China, increased the soil N and lowered the high pH as a result of forest restoration [72]. But using only one climax species, such as black pine in our study, increases the SOC and N concentrations at the initial stages, but they do not change for the following years of afforestation, and a meta-analysis on soil C and N stocks after afforestation in arid and semi-arid regions supports our results [38]. This study also concluded that afforestation with broadleaf trees resulted in a significant increase in the SOC stock, while afforestation with pines generally did not affect the SOC stock [38]. Afforestation created using a variety of plant species may result in variations in the chemistry of the soil [5,71,73]. For this reason, rather than planting only coniferous trees, a plantation that mixes broadleaves with other trees may be more suitable for long-term C sequestration [9,38,70,74]. By first choosing pioneer species with a high success rate, restoration initiatives for species at the advanced stage of succession in arid locations may be improved, and this assisted natural succession can be applied to accelerate the revegetation greatly [73]. Dense tree cover may suppress the productivity of understory plant species. Consequently, employing low-density forestry systems may be preferable to closed-canopy forestry systems, as they diminish ecosystem complexity and reduce the diversity and richness of plant species [75]. By doing this, the fertile soil conditions required for climax plants in the mid- and late-seral stages of succession [8,76,77,78,79] can be provided. R-strategic-like, light-dependent herb species (e.g., N-fixers) can exploit the high source of light before the closure of the canopy [74,80,81]. Consequently, a significant number of ruderals extant in ground vegetation during the initial phases of succession (stand development) may have enhanced the productivity of the soil in these previously degraded lands [7,8,82].
Degradation frequently coexists with a decline in soil fertility, which is necessary for trees to grow properly. Afforestation practices not only change the vegetation but also the soil properties [9]. Soil texture and OM are among the most important parameters determining both the physical and chemical properties of soil [6,83,84]. Using heavy machinery in afforestation operations can affect both the soil texture and bulk density and can cause compaction in the topsoil [5,6,83,85], while deep tillage with implements like rippers can have the opposite effect by increasing soil porosity [5,86,87,88]. The soil bulk density for all soil depth levels, especially topsoil (0–10 cm, 10–20 cm soil depth), was higher in 10-year-old afforestation sites than in older ones, and it gradually decreases as the years go back. Clay can be illuviated into lower horizons by this increased soil porosity, and the sand content in young stands may be higher due to clay illuviation by deep soil ripping. The current study showed that the older the afforestation site, the higher the clay concentration, and the clay concentration of soil at all depth levels was higher in afforestation sites compared to the control sites. The current study has similar results, showing that afforestation in degraded lands in the arid region of Türkiye initially decreases the CEC, then it increases for 10–20 years, slightly decreases, and stays stable for the following years [61,89]. While the highest lime and SIOC concentrations and EC values for all soil depths were in the 50- and 40-year-old afforestation sites, they decreased for the younger ones. This might have happened as explained before—deep tillage with implements like rippers might have increased soil porosity at the initial stages of afforestation [5,86,87,88], and this might have caused carbonates and salts to leach more easily than compacted soils. As the vegetation covers the area over time, it might have slowed down leaching [5,90]. It is stated that salinity problems occur in the soil at values greater than 4 ds m−1 (4000 μS cm−1) [91,92]. The EC was lower than 4000 μS cm−1 for all sites, and thus, there was no salinity problem in the current study areas. The soil lime concentration and pH decreased for 20 years but did not differ after 30 years in the current study; EC had the opposite trend and increased for 30 years and started to decrease after 40 years. A similar trend was observed in a study in the adjacent arid areas of inner Anatolia, and while the EC increased 15–20 years after, the soil pH at the first 30 cm had decreased by 0.15 units for the 25–27 year period compared to more recent afforestation periods [5]. Also, black pine afforestation in the Central Anatolia Region of Türkiye in the Gölbaşı/Ankara province, which is very close to the current study areas, decreased the soil pH both in the topsoil and subsoil [84]. A study in Picea crassifolia afforestation sites (limestone bedrock, 400 mm of rainfall) in the Gansu region of northwest China reported a drop in the soil pH at a depth of 70 cm after 33 and 45 years compared to neighboring steppe areas [37]. In another study of afforestation in India, the soil pH declined by 0.6 units in the afforested region compared to the control areas [93].
Not only did the soil chemistry and physical properties change, but the litter and humus stocks were also altered, and the highest litter (7.8 Mg ha−1) and humus (46 Mg ha−1) stocks were in 50-year-old afforestation sites, and they were much more than the 40-, 30-, 20-, and 10-year-old afforestation sites. Similarly, in an afforestation project established in the inner Anatolian region, the amount of litter was found to increase in parallel with the stand age regardless of species [5]. The initial land use of afforestation sites can alter the litter and humus accumulation. Most of the degraded areas in arid and semi-arid regions of Türkiye are barren lands and have sparse vegetation coverage and poor soil conditions. Soil tillage in the control sites, while preparing the entire area for afforestation, improved the soil properties and may have caused the rapid colonization of annual and perennial herbs in the control sites. Thus, the SOC and N could be rapidly increased after soil tillage [38,43,45]. Additionally, the increase in litter and humus stocks in old afforestation sites also increased the C and N stocks of the soil O horizon. While the highest soil and pine needles’ N concentrations were in young stands, it was vice versa for the litter and humus. The significant increase in soil nitrogen in the 10-year-old afforestation sites may have increased plant nitrogen uptake. Over time, the soil nitrogen decreased, and with litterfall, nitrogen in the soil surface (litter and humus layers) may have increased. This demonstrated a biogeochemical nitrogen cycle [7,8,94]. On the other hand, the nitrogen stocks in afforestation sites decreased faster than in the control (tilled) sites over time. Thus, it supports the idea that assisted natural succession can be applied to afforestation practices to greatly accelerate revegetation, plant nutrition, and successful restoration [73].
The aim of afforestation efforts in arid regions is to improve ecological services, including soil conservation and biodiversity enhancement. Due to the better maximization of the carbon-water ratio, pine species are increasingly selected for forestry plantations in areas with a higher drought risk [95]. Thus, black pine is a common [5,86,96] and recommended [23,87,95,97,98] species used for afforestation in the arid and semi-arid regions of Türkiye based on its water-use efficiency and successfully grows in the area [5,86,96]. It is reported that seedling survival rates for black pine in the arid region of Türkiye were 30% [86], 30%, 25%, and 20% [5] at the end of the 3rd, 10th, 15th, and 30th year, respectively. Although there is a decline in the mortality rate with stand age, it slows down after 20 years and grows faster than other species used in afforestation sites [5]. However, long-term regional data on the effects of afforestation on soil properties are scarce in Türkiye [5].

5. Conclusions

In general, the results of this study indicated that afforestation combined with topsoil (0–30 cm) plowing and subsoil (30–80 cm, depending on the depth) ripping significantly increased the SOC and N stocks. Although stand age did not affect the dynamics of the SOC, early stages of afforestation, 20 years in the current study, had 500%–600% more N stocks compared to older ones. However, soil tillage without afforestation doubled the SOC concentration in approximately 20 years, then decreased to previous levels in 30 years and maintained these levels until 50 years. The SOC concentrations were higher in the topsoil of the oldest afforestation sites compared to the control (soil tillage without afforestation) sites, and the opposite was true for more recent years. Moreover, the N concentration in tilled sites doubles in approximately 20 years, and a dramatic drop starts in the following years. Our results show that afforestation combined with soil preparation increases the SOC and N contents, and soil tilling without plantation accelerates this process in the initial stages of afforestation. For this reason, rather than planting only one tree species, a plantation that mixes broadleaves and conifers with other annual and perennial plants may be more suitable for long-term C sequestration, and the use of assisted natural succession in the revegetation of degraded arid and semi-arid regions as an alternative to large-scale afforestation should be paid more attention in the future. Furthermore, the assessed alterations in soil characteristics, especially soil organic carbon (SOC) and nitrogen accumulation, may supply some of the missing data and enable future afforestation, restoration, and rehabilitation efforts to be guided more scientifically.

Author Contributions

Conceptualization, M.S. and A.S.; methodology, M.S. and A.S.; software, M.S. and A.S.; validation M.S. and A.S.; formal analysis, M.S. and A.S.; investigation, M.S. and A.S.; resources, M.S. and A.S.; data curation, M.S. and A.S.; writing—original draft preparation, M.S. and A.S.; writing—review and editing, M.S. and A.S.; visualization, M.S. and A.S.; supervision, M.S.; project administration, M.S.; funding acquisition, M.S. and A.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Düzce University Scientific Research and Projects Coordination Office (DÜBAP) as a master’s thesis project, grant number 2016.02.02.509.

Data Availability Statement

Data is available upon request from the authors.

Acknowledgments

We thank Nallıhan Forest Management Directorate, Nallıhan Forest Management Chiefship crew, and Uluhan Forest Management Chiefship crew for their valuable help during field work. We also thank Mehmet Dayi from Düzce University for his valuable help with the SEM Analysis. The authors have reviewed and edited the output and take full responsibility for the content of this publication.

Conflicts of Interest

The authors declare no conflicts of interest.

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Forests 16 01524 g001
Figure 1. Distribution map of black pine. Subspecies Forests 16 01524 i001Pinus nigra subsp. salzmannii Forests 16 01524 i002 Pinus nigra subsp. laricio Forests 16 01524 i003 Pinus nigra subsp. nigra Forests 16 01524 i004 Pinus nigra subsp. dalmatica Forests 16 01524 i005Pinus nigra subsp. pallasiana [48] and Forests 16 01524 i006 study area (latitudes: 40°17′00″–40°13′00″ N and longitudes: 31°37′00″–31°09′00″ E) in Türkiye.
Figure 1. Distribution map of black pine. Subspecies Forests 16 01524 i001Pinus nigra subsp. salzmannii Forests 16 01524 i002 Pinus nigra subsp. laricio Forests 16 01524 i003 Pinus nigra subsp. nigra Forests 16 01524 i004 Pinus nigra subsp. dalmatica Forests 16 01524 i005Pinus nigra subsp. pallasiana [48] and Forests 16 01524 i006 study area (latitudes: 40°17′00″–40°13′00″ N and longitudes: 31°37′00″–31°09′00″ E) in Türkiye.
Forests 16 01524 g001
Forests 16 01524 g002
Figure 2. Sampling design of the afforestation sites selected for the study (Afforx50 is afforested 50 years ago, Tilledx50 is the non-afforested control unit where topsoil was initially tilled 50 years ago).
Figure 2. Sampling design of the afforestation sites selected for the study (Afforx50 is afforested 50 years ago, Tilledx50 is the non-afforested control unit where topsoil was initially tilled 50 years ago).
Forests 16 01524 g002
Forests 16 01524 g003
Figure 3. Soil CEC (a), pH (b), EC (c), and lime (CaCO3) concentration (d) in afforestation sites established in various years and adjacent nonplanted soil-tilled sites. Values are the mean ± standard error. Within the different treatments at the same time, means with a common lowercase letter are not significantly different at p = 0.05. Within the same treatment for various years, means with a common uppercase letter are not significantly different at p = 0.05. Afforx50 is afforested 50 years ago, Tilledx50 is the non-afforested control unit where topsoil was initially tilled 50 years ago.
Figure 3. Soil CEC (a), pH (b), EC (c), and lime (CaCO3) concentration (d) in afforestation sites established in various years and adjacent nonplanted soil-tilled sites. Values are the mean ± standard error. Within the different treatments at the same time, means with a common lowercase letter are not significantly different at p = 0.05. Within the same treatment for various years, means with a common uppercase letter are not significantly different at p = 0.05. Afforx50 is afforested 50 years ago, Tilledx50 is the non-afforested control unit where topsoil was initially tilled 50 years ago.
Forests 16 01524 g003
Forests 16 01524 g004
Figure 4. Soil texture (sand (a), clay (b), and silt (c) concentrations) and bulk density (d) of afforestation sites established in various years and adjacent nonplanted soil-tilled sites. Values are the mean ± standard error. Within the different treatments at the same time, means with a common lowercase letter are not significantly different at p = 0.05. Within the same treatment for various years, means with a common uppercase letter are not significantly different at p = 0.05. Afforx50 is afforested 50 years ago, Tilledx50 is the non-afforested control unit where topsoil was initially tilled 50 years ago.
Figure 4. Soil texture (sand (a), clay (b), and silt (c) concentrations) and bulk density (d) of afforestation sites established in various years and adjacent nonplanted soil-tilled sites. Values are the mean ± standard error. Within the different treatments at the same time, means with a common lowercase letter are not significantly different at p = 0.05. Within the same treatment for various years, means with a common uppercase letter are not significantly different at p = 0.05. Afforx50 is afforested 50 years ago, Tilledx50 is the non-afforested control unit where topsoil was initially tilled 50 years ago.
Forests 16 01524 g004
Forests 16 01524 g005
Figure 5. Soil Total Carbon (STC) (a), Soil Inorganic Carbon (SIOC) (b), soil organic carbon (SOC) (c), and nitrogen (N) (d) concentrations of afforestation sites and adjacent nonplanted soil-tilled sites. Values are the mean ± standard error. Within the different treatments in the same year for the same depth level, means with a common lowercase letter are not significantly different at p = 0.05. Within the same treatment for various years for the same depth level, means with a common uppercase letter are not significantly different at p = 0.05. Afforx50 is afforested 50 years ago, Tilledx50 is the non-afforested control unit where topsoil was initially tilled 50 years ago.
Figure 5. Soil Total Carbon (STC) (a), Soil Inorganic Carbon (SIOC) (b), soil organic carbon (SOC) (c), and nitrogen (N) (d) concentrations of afforestation sites and adjacent nonplanted soil-tilled sites. Values are the mean ± standard error. Within the different treatments in the same year for the same depth level, means with a common lowercase letter are not significantly different at p = 0.05. Within the same treatment for various years for the same depth level, means with a common uppercase letter are not significantly different at p = 0.05. Afforx50 is afforested 50 years ago, Tilledx50 is the non-afforested control unit where topsoil was initially tilled 50 years ago.
Forests 16 01524 g005
Forests 16 01524 g006
Figure 6. Soil Total Carbon (STC) (a), Soil Inorganic Carbon (SIOC) (b), soil organic carbon (SOC) (c), and nitrogen (N) (d) stocks of afforestation sites and adjacent nonplanted soil-tilled sites. Values are the mean ± standard error. Within the different treatments in the same year for the same depth level, means with a common lowercase letter are not significantly different at p = 0.05. Within the same treatment for various years for the same depth level, means with a common uppercase letter are not significantly different at p = 0.05. Afforx50 is afforested 50 years ago, Tilledx50 is the non-afforested control unit where topsoil was initially tilled 50 years ago.
Figure 6. Soil Total Carbon (STC) (a), Soil Inorganic Carbon (SIOC) (b), soil organic carbon (SOC) (c), and nitrogen (N) (d) stocks of afforestation sites and adjacent nonplanted soil-tilled sites. Values are the mean ± standard error. Within the different treatments in the same year for the same depth level, means with a common lowercase letter are not significantly different at p = 0.05. Within the same treatment for various years for the same depth level, means with a common uppercase letter are not significantly different at p = 0.05. Afforx50 is afforested 50 years ago, Tilledx50 is the non-afforested control unit where topsoil was initially tilled 50 years ago.
Forests 16 01524 g006
Forests 16 01524 g007
Figure 7. Soil organic carbon (SOC), Soil Inorganic Carbon (SIOC), Soil Total Carbon (STC) (a), and soil nitrogen (N) (b) stocks at 50 cm soil depth of afforestation sites and adjacent nonplanted soil-tilled sites. Values are the mean ± standard error. Within the different treatments in the same year, means with a common uppercase letter are not significantly different at p = 0.05. Within the same treatment for various years, means with a common lowercase letter are not significantly different at p = 0.05. Afforx50 is afforested 50 years ago, Tilledx50 is the non-afforested control unit where topsoil was initially tilled 50 years ago.
Figure 7. Soil organic carbon (SOC), Soil Inorganic Carbon (SIOC), Soil Total Carbon (STC) (a), and soil nitrogen (N) (b) stocks at 50 cm soil depth of afforestation sites and adjacent nonplanted soil-tilled sites. Values are the mean ± standard error. Within the different treatments in the same year, means with a common uppercase letter are not significantly different at p = 0.05. Within the same treatment for various years, means with a common lowercase letter are not significantly different at p = 0.05. Afforx50 is afforested 50 years ago, Tilledx50 is the non-afforested control unit where topsoil was initially tilled 50 years ago.
Forests 16 01524 g007
Forests 16 01524 g008
Figure 8. Carbon and nitrogen concentrations of needles (a,b), litter (c,d), and humus (e,f) in afforestation sites established in various years. Values are the mean ± standard error. Within the same treatments for different years, means with a common lowercase letter are not significantly different at p = 0.05. Afforx50 is afforested 50 years ago.
Figure 8. Carbon and nitrogen concentrations of needles (a,b), litter (c,d), and humus (e,f) in afforestation sites established in various years. Values are the mean ± standard error. Within the same treatments for different years, means with a common lowercase letter are not significantly different at p = 0.05. Afforx50 is afforested 50 years ago.
Forests 16 01524 g008
Forests 16 01524 g009
Figure 9. The carbon (Mg ha−1) and nitrogen (kg ha−1) stocks of litter (a,c) and humus (b,d) in afforestation sites. Values are the mean ± standard error. Within the same treatments for different years, means with a common lowercase letter are not significantly different at p = 0.05. Afforx50 is afforested 50 years ago.
Figure 9. The carbon (Mg ha−1) and nitrogen (kg ha−1) stocks of litter (a,c) and humus (b,d) in afforestation sites. Values are the mean ± standard error. Within the same treatments for different years, means with a common lowercase letter are not significantly different at p = 0.05. Afforx50 is afforested 50 years ago.
Forests 16 01524 g009
Forests 16 01524 g010
Figure 10. The heatmaps of standardized path coefficients, derived from the Structural Equation Model (SEM) for carbon and nitrogen stocks in afforestation sites of semi-arid regions of Türkiye.
Figure 10. The heatmaps of standardized path coefficients, derived from the Structural Equation Model (SEM) for carbon and nitrogen stocks in afforestation sites of semi-arid regions of Türkiye.
Forests 16 01524 g010
Forests 16 01524 g011
Figure 11. The path diagram, derived from the Structural Equation Model (SEM) for carbon and nitrogen stocks in afforestation sites of semi-arid regions of Türkiye.
Figure 11. The path diagram, derived from the Structural Equation Model (SEM) for carbon and nitrogen stocks in afforestation sites of semi-arid regions of Türkiye.
Forests 16 01524 g011
Table 1. The details of the afforestation sites selected for the study.
Table 1. The details of the afforestation sites selected for the study.
Afforestation YearAfforestation Age (~)Solum (cm)Coordinates (UTM)Elevation (m)
19685010040°17′21″ N 31°09′43″ E926
5010040°17′23″ N 31°09′42″ E950
5010040°17′27″ N 31°09′43″ E944
19734010040°14′04″ N 31°34′59″ E1086
4010040°14′01″ N 31°35′01″ E1077
4010040°14′00″ N 31°35′00″ E1074
1985305040°13′45″ N 31°35′52″ E1168
306040°13′46″ N 31°35′52″ E1164
305040°13′47″ N 31°35′51″ E1163
1996205040°13′15″ N 31°37′20″ E1193
208040°13′14″ N 31°37′19″ E1184
205040°13′13″ N 31°37′18″ E1180
2002107040°16′03″ N 31°34′25″ E1422
105040°16′04″ N 31°34′27″ E1429
105040°16′05″ N 31°34′29″ E1430
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Sarginci, M.; Seçilmiş, A. Effects of Afforestation on Soil Organic Carbon and Nitrogen Stocks in the Long Term in Semi-Arid Regions of Türkiye. Forests 2025, 16, 1524. https://doi.org/10.3390/f16101524

AMA Style

Sarginci M, Seçilmiş A. Effects of Afforestation on Soil Organic Carbon and Nitrogen Stocks in the Long Term in Semi-Arid Regions of Türkiye. Forests. 2025; 16(10):1524. https://doi.org/10.3390/f16101524

Chicago/Turabian Style

Sarginci, Murat, and Adem Seçilmiş. 2025. "Effects of Afforestation on Soil Organic Carbon and Nitrogen Stocks in the Long Term in Semi-Arid Regions of Türkiye" Forests 16, no. 10: 1524. https://doi.org/10.3390/f16101524

APA Style

Sarginci, M., & Seçilmiş, A. (2025). Effects of Afforestation on Soil Organic Carbon and Nitrogen Stocks in the Long Term in Semi-Arid Regions of Türkiye. Forests, 16(10), 1524. https://doi.org/10.3390/f16101524

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