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Article

Limited Impacts of Activated Carbon and Mycorrhizal Amendments for Pinus echinata Reforestation on Strip-Mined Soils

by
Casey Iwamoto
1,*,
Courtney Siegert
1,
Joshua J. Granger
1,
Krishna P. Poudel
1,
Adam Polinko
1 and
Zachary B. Freedman
2
1
Department of Forestry, Mississippi State University, Starkville, MS 39759, USA
2
Department of Soil Science, University of Wisconsin-Madison, Madison, WI 53706, USA
*
Author to whom correspondence should be addressed.
Forests 2025, 16(8), 1316; https://doi.org/10.3390/f16081316
Submission received: 13 June 2025 / Revised: 6 August 2025 / Accepted: 11 August 2025 / Published: 12 August 2025
(This article belongs to the Special Issue Advances in Forest Restoration and Applied Conservation: Bridging Science and Practice)
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Abstract

Strip mining creates widespread degraded landscapes that have low soil pH, high bulk density, impacted hydrologic processes, and an accumulation of heavy metals that limit revegetation efforts. To improve soil conditions and restoration success, soil amendments paired with native trees provide a potential solution. However, limited empirical studies have been conducted on the success of soil amendments to facilitate shortleaf pine (Pinus echinata Mill.) growth in the southeastern US. To fill this knowledge gap, a field trial was established on a reclaimed coal-mined site. Shortleaf pine seedlings were planted in a complete randomized block design with two soil amendment treatments: activated carbon and mycorrhizal inoculation, applied at a rate of 3.36 g/m2 and 42.5 g per tree, respectively. Soil treatment did not impact tree survival which concluded with a 69 ± 3% (mean ± standard error) survival rate. Activated carbon increased soil electrical conductivity (p = 0.037) and the mycorrhizal amendment led to increased soil Ca content (p = 0.004). After the first growing season, trees in the mycorrhizal-amended soil were 12% shorter (p = 0.016) than trees in the activated carbon treatment. While soil amendment resulted in minimal improvements to soil parameters, shortleaf pine was found to be an effective species choice for post-mined site reforestation.

1. Introduction

Forest restoration is a growing priority in the face of widespread anthropogenic land use change and soil degradation. In the southeastern US, strip mining has led to decreased forest coverage and widespread alterations of soil chemical, physical, and biological functioning [1]. To address these challenges, the planting of a native species, shortleaf pine (Pinus echinata Mill.), in conjunction with soil amendments provide a potential restoration method for post-mined degraded sites [2,3].
Surface mining includes strip and open-pit mining which removes the upper layers of soil to access mineral deposits underneath [4], causing significant degradation to soil conditions. The Surface Mining Control and Reclamation Act (SMCRA) of 1977 requires remediation efforts to restore the topography and vegetation to previous conditions [5]. However, following reclamation, post-mined soils have increased soil bulk density from heavy machinery which decreases soil aeration, water holding capacity, and the growth of plant roots [1,6,7]. Additionally, the removal of vegetation leads to increased soil erosion [6]. These physical changes to soil structure lead to alterations in soil pH, which controls the mobilization of nutrients and heavy metals [1,8]. Mining also reduces soil organic matter (SOM) through disrupting the soil profile [8], which is vital for soil structure, nutrient cycling, and nutrient storage [9]. This results in the reduction in soil multifunctionality, whereby ecosystem services such as water storage, carbon (C) sequestration, soil biodiversity, and nutrient dynamics are reduced [10]. Therefore, to improve soil conditions for reforestation on post-mined sites, restoration efforts focused on soil physical, chemical, and biological characteristics are required.
One restoration method is artificial regeneration in conjunction with soil amendments to improve tree establishment and restore soil multifunctionality [11]. One such soil amendment is activated carbon (AC), which is a thermally activated feedstock that is used for soil remediation [12]. AC has a porous structure, high surface area, and strong adsorption properties which decreases soil bulk density, increases soil water holding capacity, and increases cation exchange capacity [13,14,15,16,17]. Soil electrical conductivity (EC), which is an indicator of soil salinity and ion capacity, is therefore also expected to increase [18]. On acidic mined soils, AC can increase soil pH through dilution [19,20] and through the addition of C, cations, and anions [21]. AC has generally been found to induce physical and chemical changes to the soil that increases aboveground plant biomass, soil microbial biomass, and soil C and nitrogen (N) content [22,23]. The growing use of AC is commonly utilized for wastewater treatment and contamination remediation [15,24] while biochar, a similar pyrolyzed charcoal product, has been successfully utilized for soil [25] and forest restoration [26]. While these amendments can have similarities in production methods, key differences include differing feedstocks and AC requiring a form of activation specifically to increase surface area [12,27]. However, AC as a soil amendment for restoring degraded soil has not been well documented [28], especially in the southeastern region of the US [29].
The use of mycorrhizal amendments (MA) can also improve soil biological characteristics by reintroducing microorganisms to post-mined depleted soil. Specifically, MA contains ectomycorrhizal fungi which form a necessary symbiotic relationship with pine species [30]. On post-mined sites, these fungi have been shown to improve pine establishment through increased access to water [31], tolerance against pathogens, and access to nutrients [32,33]. In forest ecosystems, selecting mycorrhizal amendments that match the restoration site and target species is critical to leverage the benefits of MA [34,35].
Furthermore, AC promotes fungal formation on roots through adsorbing inhibiting chemicals and providing habitat for fungal populations [36]. Previous work has found that AC can synergistically improve mycorrhizal colonization [37] which increases nutrient availability and microbial activity [23,38,39]. Therefore, the combination of AC and MA (A×M) are expected to support plant growth more effectively in combination than individually [40,41,42,43]. However, further empirical studies utilizing A×M are needed to understand species specific responses [22], especially in a post-mined context.
Shortleaf pine is a potential candidate for reforesting strip-mined sites as it is an early successional species that regenerates well in high-light and bare mineral soil environments [44]. It also has the largest range of any pine species in the southeastern US and provides habitat for many wildlife species including the threatened red-cockaded woodpecker (Leuconotopicus borealis) [45]. Due to the economical and ecosystem benefits, this species is a part of many restoration efforts [3]. However, shortleaf pine faces many challenges including being estimated to be at less than 10% of its historical range [45,46]. This is largely due to being displaced by commercial loblolly pine (Pinus taeda L.) plantations. However, impacts from climate change, altered fire regimes, disease, and pests also contribute to the decreased spread [3]. Additionally, large knowledge gaps on shortleaf pine as a whole limit the restoration potential of this species.
In this study, commercially sourced AC and MA were utilized with consideration for accessibility and economic feasibility for land managers restoring post-mined sites to conditions suitable for reforestation. The first objective was to quantify the changes in shortleaf pine growth through measurements of survival, groundline diameter (GLD), and height from the application of soil amendments. The second objective was to evaluate the impacts on soil function indicators: bulk density, pH, EC, C content, N content, nutrient content, and SOM from the application of soil amendments. A×M was hypothesized to improve soil physical and chemical parameters that support fungal populations and tree growth more effectively than individual treatments. This study provides baseline measurements to guide future reforestation efforts on degraded soil using commercially available soil amendments.

2. Materials and Methods

2.1. Study Area

This study was conducted on privately owned land in Winston County, Alabama that borders the William B. Bankhead National Forest (Figure 1). The average annual precipitation was 150–155 cm, average annual temperature was 16–17 °C, and the elevation of the site was 229 m [47]. The land was surface mined in the late 1990s and early 2000s for brown coal with no remediation beyond the requirements of the SMCRA, except for one corner of the study area which had one lime application (Landowner, personal communication). The soil is classified as a part of the Brilliant series which is well drained and formed on alkaline coal mine spoil. The soil classification is a loamy-skeletal, mixed, active, nonacid, thermic Typic Udorthents [48]. This site was surrounded by a loblolly pine plantation that was established in 2004 and had understory vegetation including sericea lespedeza (Lespedeza cuneata (Dum. Cours.) G. Don.), Rubus spp. L., and greenbriar (Smilax spp. L.).

2.2. Field Experimental Design

Two treatments were applied independently and as a combination in four replicates to the study site: control (CT), AC, MA, A×M. Each of the sixteen plots were ~30.5 m × 30.5 m with a 3.0 m buffer from each other. However, due to mowing interference, no samples or measurements were collected in block one plot four. A commercially available granular organic amendment was utilized as the AC treatment (Dri-Carbon, Monty’s Plant and Soil Products, Louisville, KY, USA). The AC amendment was derived from brown coal with 49% humic acid. The AC manufacturer recommended an application rate of 1.12 g/m2; however, this rate was increased to 3.36 g/m2 to magnify intended effects on soil. AC was applied using a bag seeder at a selected feeding rate for an even distribution. The entire study area was disked to ~25 cm both before and after the AC application.
Prior to tree planting and soil amendment application, initial soil measurements were collected. Soil sampling then occurred every three months for two growing seasons, and once at the end of the third growing season. Each sample collection included 30 soil cores using a hammer-type soil probe. The survival, height, and GLD of the one-year-old shortleaf pine seedlings were measured five weeks after planting to capture establishment and at end of each growing season for three years. Only the inner 36 trees were measured for tree survival and growth from each plot. Due to mowing interference, 534 trees were utilized for the analysis of this study.

2.3. Plant and Soil Characteristics

The bareroot shortleaf pine seedlings were sourced from Native Forest Nursery in Chatsworth, GA. 100 trees were hand planted at a spacing of 3.05 m × 3.05 m within each plot in March 2021 following recommended guidelines artificial shortleaf pine regeneration [49]. MA was applied directly after the shortleaf planting. A commercially available mycorrhizal inoculant was used as the MA treatment (PHC Tree Saver Mycorrhizal Fungi Inoculant, Plant Healthcare, Inc., Pittsburg, PA, USA) containing Pisolithus tinctorius (Mont.) E. Fisch. Following the company recommended application rate, 42.5 g of the inoculant was applied 10 cm deep for each seedling using a dibble bar.
Additionally, one month after tree planting, native grasses and herbaceous plants were hand-sowed. This was to achieve savannah-open woodland conditions in which shortleaf pine was historically observed before wildfire suppression shifted forests to closed oak-hickory forests. Seeds were sourced from Prairie Nursery (Westfield, WI, USA) and Native American Seed (Junction, TX, USA). Seeds were applied at 5.29 g/m2 across all blocks, following the company recommended maximum application rate. In block two, seeds were applied at a rate of 6.23 g/m2 due to space constrictions.

2.4. Laboratory Analysis

All soil samples were oven-dried at 105 °C, ground, and sieved to <1700 µm except the pretreatment soil samples that were sieved to <650 µm. Dry bulk density was measured in the upper 0–10 cm of soil
B D d r y = M d r y V
where BDdry is dry bulk density in g/cm3, Mdry is mass in g after being oven-dried, V is the volume of soil in cm3. Bulk density samples were only collected up to 18 months due to sampling constrictions.
EC and pH were measured using HI5521 and HI5522 bench meters (Hanna Instruments, Leighton Buzzard, UK). pH and EC were measured using a slurry of 1:2 and 1:5 oven-dried soil and deionized water, respectively.
An ECS 4010 CHNO-S elemental analyzer (Costech Analytical Technologies, Valencia, CA, USA) was used to quantify soil C content, N content, and carbon/nitrogen (C:N) ratio. Detecting overall changes to total C content during this three-year period was expected to be low due to the slow rate of change in SOM [46]. Therefore, SOM was divided into particulate organic matter (POM) and mineral associated organic matter (MAOM) to identify changes in partitioning of C in the soil [50]. In this study, MAOM was considered as soil at <650 µm in 2020 and at <1700 µm for all other samples. The different sieving levels between sampling periods did not significantly impact results and were due to sampling constraints. Total C for POM and MAOM were then found using the ECS 4010 CHNO-S elemental analyzer.
Soil was further analyzed for macronutrients, micronutrients, and metal content through the Mississippi State Chemical Laboratory. Each plot was analyzed for each sampling period. Samples were digested using the MARS 6 Express Microwave Digestor (CEM Corporation, Matthews, NC, USA) with the Fertilizer AOAC Method 2006.03 methods. Digested soil samples were then analyzed by the 7900 Inductively Coupled Plasma-Mass Spectrometry (Agilent, Santa Clara, CA, USA) and 5110 VDV Inductively Coupled Plasma-Optical Emission Spectrophotometer (Agilent, Santa Clara, CA, USA).

2.5. Statistical Analysis

The experimental design was a fully randomized and full factorial design with two treatments: AC and MA, including their interaction. Mixed effects models were utilized through the package lme4 [51]. Dependent variables (tree productivity, soil parameters) were regressed as a function of soil amendment treatments (fixed effects), with blocks as random effects to account for spatial variation. Tree survival analysis specifically used glmer [51] with a binomial family to account for repeated binary survival data. Sidak’s post hoc test was conducted through the emmeans package [52] to identify significant interactions. Statistical analysis for soil nutrients was conducted on the concentrations for each analysis. Z-scores were then used to standardize soil nutrient measurements across soil amendment treatments. The average Z-score for each treatment was then calculated and displayed in radar charts. All data analysis and plots were completed using R software version 4.3.2 [53]. The following data will be presented in mean ± standard error format.

3. Results

3.1. Survival

After three growing seasons, shortleaf pine seedling survival was 69 ± 3% (Table 1). Overall, soil treatment did not affect survival (p = 0.109). However, during the second growing season, plots with MA had lower survival (59 ± 10%) compared to AC (77 ± 3%) (p = 0.015), while AC was not significantly different from CT (73 ± 9%) or A×M treatments (74 ± 4%) (p > 0.05).

3.2. Height and Groundline Diameter

Treatment did not impact tree height (p = 0.053) and final GLD (p = 0.089). After the first growing season, tree height was greater in AC amended plots (36.2 ± 1.0 cm) than MA plots (32.4 ± 1.1 cm) (p = 0.016) (Figure 2). However, plots with AC in the first growing season did not differ in height from the CT (p = 0.978) or A×M treatments (p = 0.535) (Table S1).

3.3. Bulk Density

Soil amendment treatment did not affect bulk density (p = 0.471) while sampling period did (p = 0.002) (Table S1). At the onset of the study, average bulk density was 1.46 ± 0.02 g/cm3. This slightly increased to 1.59 ± 0.02 g/cm3 at the end of the study with no differentiation between soil amendment treatments (p = 0.174). The final sampling found that MA and AC plots had more compact soil at 1.63 ± 0.04 and 1.62 ± 0.05 g/cm3, respectively, while the A×M and CT plots had lower bulk density at 1.57 ± 0.03 and 1.53 ± 0.03 g/cm3 (Figure 3).

3.4. pH and Electrical Conductivity

Sampling period was significant for both soil pH (p < 0.001) and EC (p = 0.015), while the application of soil amendment treatments only impacted soil EC. During this experiment, soil pH on this site ranged from 3.32 to 6.70 pH with the average pretreatment soil pH being 5.60 ± 0.10 pH. After one growing season, soil pH was 17.22% lower in A×M-treated soils (4.71 ± 0.24 pH) compared to MA-treated soils (5.69 ± 0.25 pH) (p = 0.007) (Figure 4). However, during this period, soil pH in MA-treated soils did not differ from CT or AC-treated soils (p = 0.310 and p = 0.216, respectively). Overall, AC had higher levels of soil EC compared to A×M (p = 0.049) and CT (p = 0.037).

3.5. Soil Nutrients

Sampling period impacted the soil content of Al (p = 0.006), Ba (p = 0.014), Cr (p = 0.002), Cu (p < 0.05), Mg (p = 0.030), P (p = 0.043), and K (p = 0.027) (Table S2, Figures S1 and S2). The soil amendment treatment impacted soil Ca content (p = 0.003). MA had 39% more Ca in the soil at 1278 ± 94 ppm compared to CT (p = 0.004), and 5% more than A×M (p = 0.007). 30 months after soil treatment application, AC had 31% and A×M had 32% more soil Ca in comparison to CT treatments (p = 0.044 and p = 0.042, respectively) (Figure 5). Additionally, Mg content in the final sampling period was impacted by soil treatment (p = 0.020) where plots with AC (p = 0.034) and A×M (p = 0.035) both had 28% more Mg soil compared to the CT plots (Figure 6).

3.6. Carbon, Nitrogen, and Carbon/Nitrogen

Soil amendment did not induce changes in soil C (p < 0.641) or N (p < 0.964). Increases in soil C led to the C:N ratio increasing from 15.85 ± 0.32 at pretreatment to 17.42 ± 0.45 (Figure 7). Sampling period did influence the soil C:N ratio (p < 0.001) with the July 2022 C:N ratio being significantly larger (p < 0.05) than all other dates except October 2023 (p > 0.902).

3.7. Soil Organic Matter Proportion

The proportion of SOM composed of POM and MAOM was not impacted by soil amendment treatments (p = 0.184). During pretreatment, mean POM and MAOM were 53.9 ± 1.8% and 46.1 ± 1.8%, respectively, while the final sampling period had a mean POM 51.8 ± 1.9% and MAOM of 48.2 ± 1.9% (p = 0.319) (Figure 8). Additionally, total C in both fractions increased from the 2020 samples across this experiment with MA having the largest total C increase by 26.2% (p > 0.05). This is due to a 37.3% increase in POM and a 9.3% increase in MAOM (Figure 8).

4. Discussion

This study had a final survival of 69% after three years, which is comparable to earlier studies with shortleaf pine on post-mined sites [35,54]. In the second growing season, trees with the AC treatment had increased survival compared to those with MA. However, this trend was not found in the first or third season where all treatments had comparable survival to the CT. This suggests that the application of AC was more effective at improving survival in comparison to MA. This was unexpected as MA treatments have been found to increase or have neutral impacts to seedling survival [35,55]. However, severe detrimental impacts on survival using specific species of mycorrhizal fungi in A×M have been observed [38]. Previous studies have found that biochar [56,57] and AC have short-term positive and neutral [58,59,60] impacts on survival which aligns with the findings from this study. Therefore, while this work provides evidence of successful seedling establishment on post-mined soils, limited differences between treatments are attributed to high survival rates under control conditions [61]. This likely limited the capacity for increased survival using soil amendments on this site.
Height after the first growing season was significantly higher in AC plots compared to MA. However, AC plots were not significantly different from CT and A×M, indicating that plots with MA were not as effective in supporting the height of the trees. This was unexpected, as MA has been found to increase tree height and diameter growth [62].
The decreased height growth could be attributed to a change in energy allocation to belowground growth due to the application of MA. Although not significant, these plots had an increased proportion of POM, which is largely derived from plant inputs including fine roots [50]. The decreased aboveground biomass could indicate increased belowground biomass, which was not directly measured in this study. This would align with the lower soil pH observed under the MA treatment, as increased root biomass from mycorrhizal fungi may release more root exudates, acidifying the soil [63,64]. This decrease in soil pH could then alter nutrient accessibility for plants and microorganism populations [62,65,66]. This is critical as MA increased soil Ca content, which has been found to have strong negative impacts on shortleaf pine growth [67]. However, shortleaf pine root growth is strongly correlated to aboveground growth indicating that changes in biomass investment due to amendment application were likely unchanged [68]. Therefore alternatively, MA could have inhibited belowground growth through soil chemical and biological changes, limiting aboveground growth and briefly increasing mortality. Additionally, ectomycorrhizal fungi can have a large C demand during initial formation, [23,69], decreasing the initial growth of pines due to competition for soil C. However, A×M indicates that the AC ameliorates this potential setback through the addition of soil C, the adsorption of organic compounds, or through reducing the effectiveness of fungal populations [70]. Shortleaf pine planted in this study were also found to grow at comparable rates in the southeastern US under conditions without additional herbaceous plantings. This suggests that the limited impacts of MA were not ecologically significant and that there was little increase in competition for shortleaf pine establishment and early growth [54]. While further studies on the potential impact of herbaceous plantings on tree establishment should be carefully considered, this aligns with previous findings on the limited impact of herbaceous control for shortleaf pine regeneration [71].
The lack of long-term effects from soil treatments on tree productivity measurements from this study is mainly attributed to the small application of soil amendments and conifers having a lower response to charcoal [72]. This aligns with previous studies that found limited short-term impacts on tree diameter in field [56,73,74,75] and greenhouse experiments [59], due to the length of studies. This aligns with a long-term study that found that the application of biochar improved Scots pine (Pinus sylvestris L.) growth over nine years [60]. However, regardless of the variability of results when using soil amendments for improved tree productivity, this study indicates that shortleaf pine grows at comparable rates on degraded sites, indicating its strong potential for post-mined site reforestation.
Bulk density on this site was expected to be 1.60 g/cm3 [48] and aligns with pretreatment measurements of 1.46 g/cm3. Measurements between 1.30 and 1.55 g/cm3 are considered fairly compacted [76] and impact the root growth of the seedlings. The AC utilized in this study had a bulk density of 0.70 g/cm3 and was expected to reduce compaction following application due to high porosity, high surface area, and increase in soil C [14]. However, even with the increased application of AC, disking, and planting of pine, a decreased bulk density was not observed. This was surprising, as generally, the more charcoal product that is applied, the more soil bulk density is reduced [77]. However, a lack of change in soil compaction with the application of biochar has been recorded [78], and was attributed to using a lower application level in this study.
Soil pH controls nutrient availability for plants, heavy metal mobility, and microbial populations [79,80,81]. This site was expected to be moderately acidic at a pH of 6.5 [48], which was within the observations in this study ranging between 3.32 and 6.70 pH. This has been identified as favorable soil pH for shortleaf pine growth and aligns with other strip-mined sites. [45,54,82].
In this study the application of A×M was found to temporarily decrease the soil pH, which was likely due to an interaction between the two amendments as neither amendment impacted soil pH individually. AC influences soil pH depending on a variety of factors including feedstock and activation method [20,22]. In this study, AC was expected to increase soil pH due to its alkaline nature compared to the field soil [19,29] while MA could decrease soil pH due to increased root exudates [83]. The decrease in soil pH in A×M amended plots was surprising as changes in soil pH were expected to be driven more strongly by the application of AC. However, this change in soil pH was limited, suggesting that soil amendment application had a weak impact on pH.
Soil EC levels in this study were low and considered non-saline [84] indicating favorable conditions for tree growth [82]. EC also describes the nutrients available to trees in the soil solution which directly impact tree survival and growth. The increase in soil EC with application of charcoals aligns with increases in nutrient availability [29,85], indicating support for shortleaf pine productivity long term. Additionally, the lowest EC observed in the final sampling period indicated decreased nutrient availability which, while statistically significant, was likely not ecologically significant as further analysis of soil nutrients and tree growth parameters were not impacted.
The availability of nutrients and heavy metals act as indicators for the capacity of soil to support reforestation. Specifically, the application of MA increased soil Ca, while AC and A×M had short-term increases in soil Ca and Mg content. The addition of macronutrients to the soil observed in this study aligns with previous findings and was attributed to AC reducing nutrient leaching and increasing nutrient availability [29,85,86]. However, shortleaf pine have been found to be sensitive to soil with a high Ca content, which likely contributed to the increased mortality and decreased tree height in plots with MA [87,88]. Ectomycorrhizal fungi can increase Ca concentrations in the soil through increased mobilization of Ca in the rhizosphere [89,90,91], providing a pathway for the application of MA to increase soil Ca content. Both Ca and Mg are more available in neutral soil, but the lack of change in soil pH due to amendments indicates that it was not this soil characteristic that drove the change in macronutrients [92]. Additionally, mining disrupts the parent material, which likely provided adequate amounts of rock-derived nutrients, including Ca and Mg, before the application of MA [93]. Charcoal has been found to decrease the leaching of Ca and decrease Mg uptake into plants in tropical soils due to adsorption [94], providing an explanation for the temporary increases in Mg and Ca in AC applied plots. Therefore, the increases in soil Mg were likely due to the production methods of AC, indicating the need for tailored products for successful restoration impacts. Additionally, the soil content of Cu decreased after the planting of shortleaf pine seedlings, which has been previously observed for pine seedlings [95]. This was likely due to plant uptake and not differences in soil pH or soil amendments. This indicates that the application of soil amendments has the potential to increase soil Ca and Mg content while shortleaf pine seedlings decrease soil Cu content through uptake.
C is often used to indicate soil quality on post-mined sites [96] where mining and reclamation deteriorates soil organic C [8]. AC was added to directly increase recalcitrant C to the soil to improve chemical and physical soil qualities [72,97]. AC was expected to increase total C while MA could contribute through fungal biomass and root exudates. However, long-term changes in stable C change slowly, which explains why no significant increase in soil C was observed [98].
Previous soil N content in strip mining has been found to range from 0.05 to 0.11% [8], which aligns with the soil N observed in this study. After the planting of shortleaf pine, a slight decrease in soil N was observed, likely due to plant uptake [99]. Although, AC applications have been found to increase plant uptake of N [100], it has also been found to not have an effect on soil N [101,102]. MA could also contribute to this decrease through creating competition for soil N from soil fungal populations in the soil [103]. However, measurements of fungal communities were not collected during this experiment. Therefore, it is difficult to determine if the lack of effects on soil N were from the short-term nature of the experiment or if the MA inoculation was unsuccessful at forming ectomycorrhizal communities on this degraded site.
Immediately after reclamation, strip-mined sites have been found to have C:N ratios of 10.3:1 which was within the range observed in this study [8]. Additionally, many reclamation sites can recover soil organic C back to pre-mined levels after 20 years, which aligns with the timeline of this site [9]. Therefore, the larger C:N ratio in this study could be due to the extended recovery time following reclamation and planted pines, grasses, and herbaceous plants.
Total C and N were expected to see little change over three years in this experiment [98], but with additional observations of the proportion of POM and MAOM, more short-term variation in SOM composition were monitored. Patterns in the proportion of organic matter in SOM describe the characteristics of the soil C. Specifically, POM is primarily from plant matter and has a high C:N ratio [50], while MAOM is mainly produced from microbes that are chemically bound to minerals and has a lower C:N ratio [104]. In this study, the application of soil amendments did not significantly improve tree growth and aligned with the lack of change in POM. Additionally, this study found no changes to the proportion of MAOM with the application of amendments, indicating that they did not significantly impact the C associated with microbial populations. However, the variability between soil treatments SOM proportions increased during the experiment (Figure 8). This suggests that soil treatments may increase the proportion of POM in the future which is associated with increased tree growth and contributes to the moderate C:N found on this site.

5. Conclusions

In this three-year study, commercially available soil amendments did not significantly improve soil restoration or shortleaf pine productivity measurements. For AC, this is likely attributed to the low application rate utilized in this study. While amendments individually increased soil nutrients, the use of A×M did not lead to stronger responses as hypothesized. Specifically, each amendment seemed to minimize the effect of the other, which, when paired with unique species-specific interactions, indicates the importance of thoughtful amendment application restoration projects. Further studies under different site and species conditions could utilize these amendments for restoration to take advantage of soil nutrients and chemical changes. Extended experimental studies are needed to understand the long-term soil remediation and reforestation impacts when using soil amendments. On degraded soil, larger than commercially recommended levels of amendment application, along with utilizing site-specific AC, are likely to amplify effects to aid reforestation efforts. However, this increases the economic burden for landowners restoring post-mined sites. Therefore, while this study found that shortleaf pine is a strong choice for reforestation on post-mined sites, it also suggests that AC and MA at economically viable levels do not increase restoration success.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/f16081316/s1, Figure S1: Soil heavy metal content; Figure S2: Soil micronutrients; Table S1: Statistics for tree and soil, Table S2. Statistics for soil nutrients.

Author Contributions

Conceptualization, C.S., J.J.G. and K.P.P.; Formal analysis, C.I.; Funding acquisition, C.S. and J.J.G.; Investigation, C.I.; Methodology, C.S., J.J.G. and K.P.P.; Resources, C.S. and J.J.G.; Supervision, C.S. and J.J.G.; Validation, C.I., C.S. and A.P.; Writing—original draft, C.I.; Writing—review and editing, C.I., C.S., J.J.G., K.P.P., A.P. and Z.B.F. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded through the National Fish and Wildlife Foundation (1904.20.067768) and supported by the National Institution of Food and Agriculture, US Department of Agriculture, McIntire-Stennis capacity Grant # MISZ-032100. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the US Department of Agriculture. This material is a contribution of the Forest and Wildlife Research Center, Mississippi State University.

Data Availability Statement

The experimental data and results that support the findings of this study are available in Scholars Junction at https://scholarsjunction.msstate.edu/fwrc-publications/17.

Acknowledgments

Thank you to Mark and Maggie Johnston for their hospitality and support of our research on their property.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
CTControl
ACActivated carbon
MAMycorrhizal amendment
A×MActivated carbon and mycorrhizal amendment mixture
C/NCarbon to nitrogen ratio
SOMSoil organic matter
POMParticulate organic matter
MAOMMineral associated organic matter
GLDGroundline diameter
ECElectrical conductivity

References

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Forests 16 01316 g001
Figure 1. Study site with applied treatments in Alabama, USA.
Figure 1. Study site with applied treatments in Alabama, USA.
Forests 16 01316 g001
Forests 16 01316 g002
Figure 2. Mean (a) height and (b) groundline diameter across three growing seasons between treatments control (CT), activated carbon (AC), mycorrhizal amendment (MA), and the mixture of AC and MA (A×M). Different letters indicate significant differences at p < 0.05 within a growing season. No letters indicate no differences between treatment.
Figure 2. Mean (a) height and (b) groundline diameter across three growing seasons between treatments control (CT), activated carbon (AC), mycorrhizal amendment (MA), and the mixture of AC and MA (A×M). Different letters indicate significant differences at p < 0.05 within a growing season. No letters indicate no differences between treatment.
Forests 16 01316 g002
Forests 16 01316 g003
Figure 3. Dry bulk density compared between treatments control (CT), activated carbon (AC), mycorrhizal amendment (MA), and the mixture of AC and MA (A×M) over 18 months.
Figure 3. Dry bulk density compared between treatments control (CT), activated carbon (AC), mycorrhizal amendment (MA), and the mixture of AC and MA (A×M) over 18 months.
Forests 16 01316 g003
Forests 16 01316 g004
Figure 4. (a) Soil pH and (b) electrical conductivity between treatments control (CT), activated carbon (AC), mycorrhizal amendment (MA), and the mixture of AC and MA (A×M). An * indicates significant differences (p < 0.05) between soil treatments within a sampling period.
Figure 4. (a) Soil pH and (b) electrical conductivity between treatments control (CT), activated carbon (AC), mycorrhizal amendment (MA), and the mixture of AC and MA (A×M). An * indicates significant differences (p < 0.05) between soil treatments within a sampling period.
Forests 16 01316 g004
Forests 16 01316 g005
Figure 5. Radar graphs across treatments (a) control (CT), (b) activated carbon (AC), (c) mycorrhizal amendment (MA), and (d) the mixture of AC and MA (A×M). Soil nutrients were standardized using the calculated Z-scores. Black indicates pretreatment levels (2020), and gray indicates 30 months post soil amendment application (2023).
Figure 5. Radar graphs across treatments (a) control (CT), (b) activated carbon (AC), (c) mycorrhizal amendment (MA), and (d) the mixture of AC and MA (A×M). Soil nutrients were standardized using the calculated Z-scores. Black indicates pretreatment levels (2020), and gray indicates 30 months post soil amendment application (2023).
Forests 16 01316 g005
Forests 16 01316 g006
Figure 6. Soil macronutrients (a) calcium, (b) magnesium, (c) phosphorous, and (d) potassium with soil amendment treatments: control (CT), activated carbon (AC), mycorrhizal amendment (MA), and the mixture of AC and MA (A×M). An * indicates significant differences (p < 0.05) between soil treatments within a sampling period.
Figure 6. Soil macronutrients (a) calcium, (b) magnesium, (c) phosphorous, and (d) potassium with soil amendment treatments: control (CT), activated carbon (AC), mycorrhizal amendment (MA), and the mixture of AC and MA (A×M). An * indicates significant differences (p < 0.05) between soil treatments within a sampling period.
Forests 16 01316 g006
Forests 16 01316 g007
Figure 7. Soil (a) carbon, (b) nitrogen, and (c) carbon/nitrogen ratio compared between treatments control (CT), biochar (AC), mycorrhizal amendment (MA), and the mixture of AC and MA (A×M) over 30 months.
Figure 7. Soil (a) carbon, (b) nitrogen, and (c) carbon/nitrogen ratio compared between treatments control (CT), biochar (AC), mycorrhizal amendment (MA), and the mixture of AC and MA (A×M) over 30 months.
Forests 16 01316 g007
Forests 16 01316 g008
Figure 8. Total soil carbon divided between (a) POM and (b) MAOM across the treatments control (CT), biochar (AC), mycorrhizal amendment (MA), and the mixture of AC and MA (A×M).
Figure 8. Total soil carbon divided between (a) POM and (b) MAOM across the treatments control (CT), biochar (AC), mycorrhizal amendment (MA), and the mixture of AC and MA (A×M).
Forests 16 01316 g008
Table 1. Mean survival at the end of each growing season by soil amendment (mean ± standard error). Different letters indicate significant differences at p < 0.05 within a growing season.
Table 1. Mean survival at the end of each growing season by soil amendment (mean ± standard error). Different letters indicate significant differences at p < 0.05 within a growing season.
Growing SeasonTreatmentMean Survival (%)Overall Survival
0CT96 ± 2 a98 ± 0.5
AC98 ± 1 a
MA98 ± 1 a
A×M99 ± 1 a
1CT78 ± 8 a76 ± 2
AC79 ± 3 a
MA71 ± 10 a
A×M74 ± 5 a
2CT73 ± 9 ab71 ± 4
AC77 ± 3 a
MA59 ± 10 b
A×M74 ± 4 ab
3CT70 ± 10 a69 ± 3
AC74 ± 5 a
MA60 ± 10 a
A×M72 ± 4 a
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MDPI and ACS Style

Iwamoto, C.; Siegert, C.; Granger, J.J.; Poudel, K.P.; Polinko, A.; Freedman, Z.B. Limited Impacts of Activated Carbon and Mycorrhizal Amendments for Pinus echinata Reforestation on Strip-Mined Soils. Forests 2025, 16, 1316. https://doi.org/10.3390/f16081316

AMA Style

Iwamoto C, Siegert C, Granger JJ, Poudel KP, Polinko A, Freedman ZB. Limited Impacts of Activated Carbon and Mycorrhizal Amendments for Pinus echinata Reforestation on Strip-Mined Soils. Forests. 2025; 16(8):1316. https://doi.org/10.3390/f16081316

Chicago/Turabian Style

Iwamoto, Casey, Courtney Siegert, Joshua J. Granger, Krishna P. Poudel, Adam Polinko, and Zachary B. Freedman. 2025. "Limited Impacts of Activated Carbon and Mycorrhizal Amendments for Pinus echinata Reforestation on Strip-Mined Soils" Forests 16, no. 8: 1316. https://doi.org/10.3390/f16081316

APA Style

Iwamoto, C., Siegert, C., Granger, J. J., Poudel, K. P., Polinko, A., & Freedman, Z. B. (2025). Limited Impacts of Activated Carbon and Mycorrhizal Amendments for Pinus echinata Reforestation on Strip-Mined Soils. Forests, 16(8), 1316. https://doi.org/10.3390/f16081316

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