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Systematic Review

The Impact of Ecological Restoration on Soil Quality in Humid Region Forest Habitats: A Systematic Review

by
Wendy Jiba
,
Alen Manyevere
* and
Chuene Victor Mashamaite
Department of Agronomy, University of Fort Hare, Private Bag X1314, Alice 5700, South Africa
*
Author to whom correspondence should be addressed.
Forests 2024, 15(11), 1941; https://doi.org/10.3390/f15111941
Submission received: 30 September 2024 / Revised: 29 October 2024 / Accepted: 31 October 2024 / Published: 4 November 2024
(This article belongs to the Special Issue Forest Soil Physical, Chemical, and Biological Properties)
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Abstract

:
Ecological restoration is widely recognized as an essential technique for addressing soil degradation, biomass decline, and biodiversity loss. Improving and maintaining soil quality is critical to ensuring environmental sustainability and successful forest recovery. This systematic review aimed to assess the impact of ecological forest restoration efforts on soil quality in humid regions, as well as to compare the effectiveness of various ecological restoration strategies on soil quality indicators. Subsequently, a systematic search on various databases (e.g., Scopus and Google Scholar) yielded 696 records, of which 28 primary studies met the inclusion criteria. The results emphasized that chemical and physical soil properties are the key indicators for assessing ecosystem performance during forest restoration. The most commonly measured parameters were soil carbon, nitrogen, phosphorus, pH, bulk density, and soil porosity. It was shown that the restoration process required a longer duration to reach a comparable level of recovery as seen in mature forests, particularly in terms of fully restoring soil quality. Additionally, it has been noted that prior land use influences the length of time needed for soil quality recovery. In planted sites, soil quality may keep improving as the site ages, though it tends to stabilize after a certain period.

1. Introduction

Natural disturbances such as wind, fire, and snow have an impact on forest structure on both regional and local levels, resulting in forest degradation [1]. Moreover, the spread of alien invasive species, soil and water erosion, and human exploitation pose global risks to natural ecosystems, resulting in biodiversity and habitat loss [2]. According to the key findings of the Millennium Ecosystem Assessment, globally, ecosystem services are declining [3]. The study by [4] estimated that the extent of land modification and degradation accounts for 66% of global land surface. As a result, it is critical to restore these degraded forests in accordance with the United Nations’ Sustainable Development Goal 15 (SDG 15) of “Life on Land”, which strives to protect the environment and soils [5].
Ecological restoration is commonly regarded as a rehabilitation approach for addressing the challenges of forest degradation and its consequences [6]. Restoration of degraded lands is a global priority, driven by ambitious international commitments [7]. For example, the Bonn Challenge and the New York Declaration on Forests are global initiatives aimed at restoring 350 million hectares of degraded forests by 2030 [7]. In Africa, the AFR100, a regional project, aims to rehabilitate 100 million acres of degraded forests [8]. Also, South Africa has implemented nature-based solutions such as forest landscape restoration (FLR), which reverses the effects of degradation and restores the ecological, climatic, and economic benefits of forests.
Ecological restoration (active and passive) comprises forest development, forest rehabilitation, and other activities that fall under the purview of eco-system services [9]. Active and passive restoration are two primary methods for repairing large, degraded lands [10]. Active encompasses management measures including the planting of seeds or seedlings, and when utilizing this procedure, it is common to need to amend and rectify the soil [9]. Passive restoration occurs when activities that cause environmental harm, such as agriculture or grazing, are discontinued, allowing secondary succession to commence with little active management of the soil or vegetation [11].
Forest recovery efforts have primarily focused on vegetation structure, species diversity, ecosystem processes [12], ecosystem productivity, and invasion susceptibility [13]. While the mechanisms are clear, the feedback interactions between plants and soil, as well as the development and regulation of plant communities, remain unclear [14]. Understanding these feedback links is crucial for projecting future scenarios under changing conditions [15].
The relationship between soil and plant has consistently evolved and developed together over time and has been recognized as a critical factor for forest succession and growth [16]. The relationship has the capacity to constantly alter the course of restoration. Previous studies have demonstrated that forests, as ecosystem builders, have effects on the soil environment that are not limited to individual species [17]. This includes soil nutrients, moisture, and plant structure [18], but also influences ecosystem productivity and plant diversity [19]. Prior research on soil components under forest restoration has focused on primary elements; however, the cycling and feedback impacts of mineral nutrients within above- and below-ground forest ecosystems are complex [20].
Soil is a vital component of the forest ecosystem that is required for successful forest regeneration [21]. Improving and maintaining soil quality is critical for ensuring environmental sustainability and forest recovery [22]. Root penetration and the amount of gas in the soil have an impact on tree species dispersal and growth, as well as microfauna interactions [23]. As a result, the availability of water and nutrients, as well as physicochemical properties, can be altered [23]. The assessment of changes in soil chemical and physical properties is regarded as critical to understanding the ecological implications of vegetation regeneration [24]. They can be utilized as indicators of soil quality and ecosystem functioning. Although the soil is a critical component in the process of restoring a forest eco-system, the changes or development of the ecological features of the soil during the restoration process have not yet been thoroughly investigated [25]. As a result, it is critical to understand how soil properties respond during the time of forest reestablishment after restoration efforts.
Given the growing number of projects that combine forest restoration with soil management, the research work done on the impact of ecological forest restoration attempts on soil quality in humid regions was systematically reviewed. This review aimed to fill the current knowledge gaps regarding soil recovery and the effectiveness of various restoration strategies in humid forests. Furthermore, assessing the recovery of soil quality indicators is critical for directing effective restoration strategies and enhancing ecological outcomes. Therefore, this review compared the studies on the impacts of active and passive restoration on soil quality indicators, emphasizing the necessity of taking environmental circumstances and prior land use into consideration. It was hypothesized that forest restoration in humid regions, through both active and passive strategies, could significantly improve soil quality indicators, with active restoration recovering soil chemical properties faster than passive restoration.

2. Methodology

The systematic review was carried out using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline [26]. Numerous articles were reviewed to gather data on the impact of ecological restoration on the soil quality of forest habitats in humid regions within restoring and reference sites. This review designated reference areas using a range of criteria, including natural vegetation cover, natural forest, and native forest. Reference sites were defined as mature or undisturbed forests in their initial or late stages of succession.

2.1. Data Sources, Literature Search Strategies and Selection Criteria

From October 2023 to October 2024, a comprehensive search was undertaken across two renowned electronic databases: Scopus and Google Scholar. The literature search was conducted using the following keywords; “ecological restoration” OR “restoration strategies” AND “active restoration” OR “passive restoration” AND “soil quality” OR “soil management” OR “soil health” OR “soil nutrients” OR “soil properties” OR “soil physical properties” OR “glomalin-related protein” AND “forest management” OR “forest habitat” OR “forest restoration” OR “forest recovery” OR “forest regeneration” OR “forest structure” OR “forest health” OR “forestry” OR “afforestation” OR “reforestation” AND “humid regions”. The use of the terms “afforestation” and “reforestation” assumed that they could represent a form of active forest restoration. All duplicates were eliminated electronically and physically. The study’s titles were reviewed, and the abstracts were examined for relevancy.
We also reviewed the reference lists of the published studies to enhance sensitivity and identify additional relevant research that could have been missed during the search. The software tools such as Mendeley and Excel were used to export and easily manage the extracted articles obtained from databases. The Mendeley software proved essential for managing large article collections, including automated citation production, seamless cooperation with scholars, and paper importing. To improve data quality, duplicates were identified and eliminated, resulting in the establishment of a more streamlined reference library.

2.2. Study Selection, Exclusion, and Inclusion Criteria

This study focused on ecological restoration attempts that were carried out in humid regions. Therefore, all articles published outside of humid regions were excluded, as well as those that were not written or translated into English. Moreover, articles were excluded if they only assessed the effects of deforestation or reported only on changes in tree species growth. We also excluded articles that studied biomes other than forests. Furthermore, in order to be included in our systematic review, studies must be conducted in forest ecosystems and examine several sampling sites (replicates) in both restored and reference forests. As such, they should include quantitative data on soil parameters from both restored and reference forests.
The titles and abstracts were evaluated according to the above-mentioned inclusion and exclusion criteria. If the inclusion or exclusion criteria could not be determined from the title and abstract, full-text publications were reviewed, and the judgment was made accordingly. The following criteria were used to select studies: research location, type of restoration approach, year of publication, study overview, and findings on each soil property data. For the studies that contained both restoration strategies, we treated each strategy as independent research. Therefore, data were extracted separately for each restoration strategy. We only considered the first soil depth in the results, in which data were collected under several soil depths. The findings were assessed to see if the soil parameters were high or low under reference sites and restoration sites.

2.3. Risk of Bias Assessment

The Crowe Critical Appraisal Tool (CCAT) was used to determine the methodological quality of all studies. The CCAT assessed studies based on eight criteria: preliminary, introduction, design, sampling, data collection, ethical considerations, outcome, discussion, and conclusion [27].

2.4. Data Extraction and Data Analysis

After screening the articles, duplicates were detected and excluded by comparing author names, titles, and ecosystem types. The study’s location, soil type, and outcomes were reported. Subsequently, each study was subjected to a methodological assessment, followed by data extraction. The two database searches yielded a total of 696 records. After removing duplicates, there were 642 records remaining. The remaining 535 articles were considered unsuitable due to their titles and abstracts. Of the 107 publications that qualified for a full-text evaluation, 79 were excluded because they did not meet the review’s eligibility criteria. Ultimately, 28 articles met the selection criteria and were included for data extraction in the study (Figure 1). The first author completed the data extraction form, and others validated the extracted information. A thorough analysis of soil physicochemical properties was carried out to assess the impacts of ecological forest restoration on soil quality in humid region forest habitats. The findings were presented in tables, with descriptive statistics displayed in bar graphs.

3. Results and Discussion

3.1. Publication Trends and Geographic Distribution

Figure 2 depicts the frequency of studies published on the ecological restoration of soil properties, spanning the years 2002 to 2024. Publications have increased significantly in recent years, particularly between 2015 and 2024. These studies were conducted between 2000 and February 2024 in humid regions of various countries. Chemical properties were the most commonly measured among these studies, followed by physical properties. Active restoration was assessed in ten studies, passive restoration in nine, and only three included both restoration strategies. These studies were conducted in various countries across the world, including Australia (n = 1), Brazil (n = 9), Colombia (n = 1), China (n = 7), Ethiopia (n = 1), Ghana (n = 1), Malaysia (n = 1), Mexico (n = 1), Nepal (n = 1), Poland (n = 1), the United States (n = 3), and Turkey (n = 1). One study was conducted in a ranch [28], while the other 19 studies were conducted on large regional scales. These few studies indicate the noticeable literature gaps regarding the effect of forest restoration on soil quality.

3.2. Effect of Passive and Active Restoration on Soil Chemical Properties

3.2.1. Soil pH

Soil pH at both passive and active restoration sites did not differ significantly between reference and restoration forests [29,30], with mean values ranging from 5.7 to 6.2. Although the authors did not explain their non-significant results, it is possible that this was due to the age of the sites or the need for more time to reduce soil pH. Other studies demonstrated that pH was significantly higher in reference than at passive [31,32] and active restoration sites [32,33]. The mean values for passive and active restoration ranged from 4.5 to 7.4 and 4.4 to 5.7, respectively. The observation that soil pH was lower or acidic (4.5) in restoration forests indicated the presence of acidic-causing invasive species. Planting acacia and pine plants in restoration locations, for example, can make the soil acidic since pine needles and acacia leaves are both components of soil humus. The humus created from pine needles and acacia leaves contains carboxylic acids (COOH), which when combined with water contribute to soil acidity. The dissociation of carboxylic acid in water releases hydrogen ions (H+), increasing the acidity of the soil. In addition, the pH decrease has been linked to rainfall-induced cation leaching, leaving behind stable Al and Fe oxides in the soil solution [34,35]. The H+ ions and pH have an inverse relationship; an increase in H+ ions contributes to a decrease in pH. In general, humid forest soils are acidic and depleted in fertility. The chemical interactions of Fe and Al oxides with water in soil are shown below [36].
Fe(OH)2+ + H2O ↔ Fe(OH)3 + H+
Al(OH)2+ + H2O ↔ Al(OH)3 + H+
Kassaye et al. [37] on passive restoration and [28,38] on active restoration sites found that pH was significantly higher in restoration forests than in reference sites. The mean values ranged from 6.5 to 8.3, which is moderately basic and within the acceptable soil pH range. The restored sites showed a significant increase in soil pH, reaching levels comparable to the reference forest sites. However, the presence of chemical components such as carbonates resulted in a pH of up to 8.3 in these restoration sites in forests with good vegetation cover. This is based on Delgado and Gomez’s [39] findings that carbonate chemicals raise soil pH to 8.5. When calcium carbonate (CaCO3) dissolves in soil water, it releases Ca2+ and CO32− ions. Carbonate ions react with hydrogen ions (H⁺) in soil. When they react, they neutralize hydrogen ions, lowering the soil’s acidity and raising its pH.
CaCO3 + CO2 + H2O → Ca(HCO3)2 → Ca2+ + 2HCO3
2HCO3− + 2H+ → 2H2O + 2CO2

3.2.2. Soil Organic Matter and Soil Organic Carbon

Soil organic matter is recognized as a key determinant of soil quality due to its multifaceted roles in soil functioning and ecosystem health [40]. One of the crucial functions of soil organic matter is its ability to bind soil particles together, resulting in the development of soil aggregates and improved soil structure and stability [41]. The presence of stable soil aggregates enhances soil porosity and water penetration while also reducing soil erosion and compaction [41]. The findings presented in Table 1 indicate a consistent trend of decreased soil organic matter levels in restoration forests compared to reference forests, with mean values ranging from 1.33 to 7.98%. There is much organic matter on reference sites because they are mature forests, and there is more litter than in restoration forests. This finding implies that the restoration process may not fully replicate the natural accumulation of organic matter levels in the soil. Several factors could contribute to this discrepancy. Firstly, disturbances associated with land-use history or restoration activities, such as soil compaction, erosion, or vegetation removal, may disrupt organic matter accumulation processes [42]. Secondly, the establishment of vegetation cover in restored forests may not yet be mature, resulting in a limited supply of organic matter from litter fall and root turnover [43]. Lastly, the restoration sites might have altered environmental conditions, such as changes in microclimate or hydrology, which could influence decomposition rates and organic matter turnover [44]. The slower accumulation of organic matter in restoration forests underlines the need for longer timeframes to achieve comparable levels of soil organic carbon to those found in reference ecosystems.
Significant increases in soil organic carbon were observed between reference and restoration sites in both active and passive restoration (Table 1), with mean values ranging from 4.40 to 6.52%, falling within the acceptable organic carbon range for soil. This suggests that both active and passive restoration may lead to a slow increase in soil organic carbon levels due to the disruption of organic matter decomposition processes during soil disturbance and restoration-related management practices. In most studies, the high levels of soil organic carbon detected in reference forests compared to restoration forests suggest that the restoration process may take some time to show results. This aligned with the study conducted by [45], which found higher soil organic carbon in natural mature forests than in managed vegetation restoration. This could be explained by vegetation cover in mature forests, which promotes litter decomposition, resulting in the accumulation of soil organic matter decomposition and an increase in carbon pools. Eclesia et al. [46] showed that planted trees had to build up biomass during their first years, and only a small amount of soil organic carbon was accumulated due to the high carbon retention in standing biomass. Therefore, a longer period is necessary to increase the accumulation of organic carbon in active restoration to improve soil quality. However, others reported that there was no significant effect between reference and restoration sites in passive restoration [47,48]. Higher and lower total organic carbon in restored forests depend on the history of land use and the predominant vegetation type.

3.2.3. Total Nitrogen and Phosphorus

The current review showed a consistent trend of lower significant levels of total soil nitrogen in restoration sites when compared to reference sites for passive restoration (Table 1). Similarly, total nitrogen was significantly different across reference and restoration sites on active restoration, with mean values ranging from 0.10 to 0.44%, respectively [49,50]. This observation underscores the impact of restoration processes on nitrogen cycling in soils, which may have implications for ecosystem functioning [51]. The accumulation of nitrogen in restored soils could be attributed primarily to microbial decomposition of litter and roots during vegetation recovery. This highlights the interaction between ecological restoration and soil nutrient dynamics [52]. The recovery is more pronounced on the upper layer (0–20 cm), which is easily affected. The high NO3−-N levels at the reference site suggest limited NO3−-N absorption in mature woods. Nitrate concentrations are typically low in forests due to small populations of nitrifying microorganisms, high concentrations of allele chemicals that produce inhibition, and high concentrations of tannins, a group of polyphenolic compounds (C76H52O46, Ellagitannin) and tannin derivatives that inhibit nitrification [31]. Furthermore, NO3−-N is more easily absorbed and leached from soil than NH4+-N.
Studies by Damptey et al. [33] and Liu et al. [29] found that total nitrogen increased significantly on restoration sites compared to reference sites on passive restoration, with mean values ranging from 0.22 to 0.61%. This significant increase can be linked to restoration interventions that encouraged vegetation growth and development, which had a direct impact on nitrogen concentration. The use of exotic species capable of fixing atmospheric nitrogen contributes to nitrogen accumulation in restored sites. For example, Damptey et al. [33] used Leucaena leucocephala in the restored area, which is a leguminous plant that releases nitrogen into the soil, resulting in high nitrogen levels. Leucaena leucocephala improves the soil by fixing atmospheric nitrogen with symbiotic bacteria in its root nodules and then releasing nitrogen back into the soil. The findings of this study are consistent with those of [53], which indicated a significant increase in total nitrogen approximately after 15 years of restoration compared to matured forests.
Several studies, on the other hand, found that the total available organic phosphorus differed significantly between reference and restoration sites in both passive and active restoration [29,54]. The mean values of these studies ranged from 2.95 to 10.16, suggesting potential differences in phosphorus availability between these forests. Numerous studies on passive and active restoration found no significant differences in available organic phosphorus between reference and restoration sites, with mean values ranging from 10.50 to 25.50, suggesting relative stability in available organic phosphorus levels following restoration interventions [33,37]. Changes in available organic phosphorus levels in restoration sites may be caused by changes in soil phosphorus cycling processes, such as mineralization, fixation, and immobilization, which are affected by a variety of factors (e.g., vegetation recovery and soil properties) [55]. Soil pH has been recognized as a critical determinant of phosphorus availability, with acidic conditions often leading to phosphorus immobilization and reduced plant accessibility [56].
Our review corroborates these findings by indicating a reduction in soil available organic phosphorus in restoration forests, coinciding with lower soil pH levels. This emphasizes the importance of considering soil acidity as a primary factor influencing soil phosphorus availability in restored ecosystems. The findings regarding soil pH, soil organic carbon, total nitrogen, and available phosphorus in active restoration highlight the complex interactions between restoration activities and soil properties. Active restoration strategies can have significant effects on soil chemical properties, which in turn can impact soil quality. The review indicated that the active restoration could lead to changes in soil chemical properties over time.
Table 1. Shows the effect of forest restoration on soil chemical parameters.
Table 1. Shows the effect of forest restoration on soil chemical parameters.
ReferencesStudy
Location		ObjectiveSoils TypeRestoration StrategyReference SitesRestoration Sites
[37]EthiopiaTo examine the impacts of forest restoration with area closures on vegetation and soil property changing aspects.Eutric CambisolsPassivepH (−) *, SOC (+) *; SOM (+) *, TN (+) *, AvP nspH (+) *, SOC (−) *, SOM (−) *, TN (−) *, AvP ns
[31]ChinaTo determine the most effective vegetation recovery pathway for enhancing soil quality.Mollic InceptisolsPassivepH (+) *, SOM (+) *,
TN (+) *, AvP (+) *		pH (−) *, SOM (−) *, TN (−) *, AvP (−) *
[11]BrazilTo assess and compare soil parameters between two restoration strategies in the early stages of restoration.Acrisols and FluvisolsPassivepH ns, SOM ns,
TN ns		pH ns, SOM ns, TN ns
[29]ChinaTo evaluate soil property changes caused by various land use types following passive restoration.Humic acrisolsPassivepH ns, SOM (+) *, TN (−) *, AvP (−) *pH ns, SOM (−) *,
TN (+) *, AvP (+) *
[32]BrazilTo evaluate soil quality in restoration areas and compare them to mature forests.MollisolsPassivepH (+) *pH (−) *
[47]BrazilTo evaluate the impact of forest restoration on surface saturated hydraulic conductivity.Ultisols and EntisolsPassiveSOC nsSOC ns
[57]ChinaTo examine the accumulation of glomalin related soil protein and its contribution to SOC sequestration under various forest restoration strategies.LeposolPassiveSOC (+) *SOC (−) *
[58]BrazilTo determine the impacts of disturbance and forest succession on lowland forest.UltisolsPassivepH (−) *, AvP nspH (−) *, AvP ns
[59]United StatesTo investigate the carbon (C) and nitrogen (N) cycling processes in contemporary pine forests that are relatively unmanaged.Mollic EutroboralfsPassivepH (−) *, SOC (−) *, TN nspH (+) *,SOC (+) *,
TN ns
[60]MalysiaTo determine the extent of soil nutrient deterioration and the rate of biomass recovery during shifting cultivation in a degraded forest.EnceptisolsPassivepH (+) *, SOC (+) *,
TN (+) *, AvP (+) *		pH (−) *, SOC (−) *,
TN (−) *, AvP (−) *
[61]BrazilTo quantify the C sequestration potential of several restoration efforts in the Atlantic Forest habitat.Ferrosols and AcrisolsPassiveSOC (+) *SOC (−) *
[49]BrazilTo assess C storage, GHG fluxes, and the quantity, quality, and provenance of SOM in two sites with ongoing forest restoration.UltisolsActiveSOC nsSOC ns
[62]AustraliaTo compare the recovery of attributes and functions in throughout different forest restoration.OxisolActivepH nspH ns
[33]GhanaTo examine soil parameters and be-low- and above-ground biomass qualities in a restored former gravel mine region two decades after active restoration.UltisolsActivepH (+) *, SOM ns, TN (−) *, AvP nspH (−) *, SOM ns,
TN (+) *, AvP ns
[63]BrazilTo examine the recovery of above ground and soil carbon stocks in restoration forests.–PassiveSOC nsSOC ns
[11]BrazilTo assess and compare soil parameters between two restoration strategies in the early stages of restoration.Ultisols ActivepH (−) *, SOM ns, TN nspH (+) *, SOM ns, TN ns
[64]TurkeyTo examine the impacts of afforestation on the chemical and physical soil properties and plant growth.AlfisolActivepH ns, TN (+) *pH ns, TN (−) *
[65]BrazilTo investigate how the physical and hydraulic properties of soil have recovered in an active restoration forest and a secondary old-growth forest.Ultisols ActiveSOC (+) *SOC (+) *
[66]United StatesTo evaluate the mine soils and vegetation present, compare their soil qualities with those that had been reforested successfully, and assess the effects of site age on the measured properties and reforestation sites.–ActivepH (−) *pH (+) *
[30]ChinaTo examine how the synergistic interaction between GRSP, SOC, and soil aggregate stability over time following various stages of plant restoration.Eutric CambisolsActivepH ns, SOC (+) *,
TN (+) *		pH ns, SOC (−) *,
TN (−) *
[38]ChinaTo explore the accumulation kinetics of glomalin and amino sugars inside aggregates and assess their respective contribution to the SOC pool during forest restoration.LeptosolsActivepH(−) *, SOC(+) *, TN (+) *pH (+) *, SOC (−) *,
TN (−) *
[28]MexicoTo evaluate the impact of active restoration on forest structure, tree diversity, and soil qualities as indicators of restoration success.Umbric AndosolsActivepH (−) *, SOC (+) *,
TN ns, AvP ns		ph (+) *, SOC (−) *,
TN ns, AvP ns
[48]United statesTo examine the key indicators of ecosystem function in restored floodplain forests.–ActiveSOC (+) *, TN (+) *SOC (−) *, TN (−) *
[50]ChinaTo investigate the impact of vegetation restoration on soil physicochemical properties and saturated hydraulic conductivity at various land uses.–ActiveSOC (+) *, TN (+) *SOC (−) *, TN (−) *
[54]BrazilTo examine the impact of long-term forest-to-sugarcane conversion and following forest restoration on soil indicators and to establish a structured soil health evaluation.OxisolActivepH (−) *, AvP (−) *, TN (+)pH (+) *, AvP (+) *,
TN (−)
SOC = soil organic carbon; SOM = soil organic matter; TN = total nitrogen; AvP = available phosphorus; GHG = greenhouse gas; and pH = soil pH. The positive (+) and negative (−) marks indicate if soil parameters were positively or negatively changed. A superscript * indicates a significant difference, whereas a superscript ns indicates no significant difference.

3.3. The Effect of Passive and Active Restoration on Physical Properties

3.3.1. Soil Bulk Density and Soil Porosity

Soil bulk density and porosity are linked with soil compaction, and their rise is detrimental to soil quality [67]. The current review showed that soil bulk density did not alter significantly between reference and restoration sites on active restoration [28]. However, several studies observed that soil bulk density was considerably lower in reference sites compared to restorative sites for both passive and active restoration [29,47]. The mean bulk densities in passive restoration ranged from 0.72 to 1.23 g/cm3, while in active restoration they ranged from 1.10 to 1.60 g/cm3 [29,33,50]. These ranges are within the permissible 1.33 g/cm3 levels, suggesting that the restoration positively impacts the bulk density. The higher bulk density on restoration forest soils can be attributed to the use of machinery during planting. Soil compaction can also reduce soil infiltration and decrease soil hydraulic conductivity. In general, increased organic matter results in lower bulk density and vice versa.
De Vasconcelos et al. [32] reported that there was no significant variation in total soil porosity between reference and restoration sites for both passive and active restoration sites. This implies that restoration has little or no effect on soil porosity. In contrast, numerous studies have indicated that total soil porosity in active and passive restoration was considerably higher in reference sites than in restoration sites that varied from 52 to 60%, respectively [47,51,68], which is within the acceptable range of soil porosity. Mature forests generally have more developed ecosystems with deeper root systems, higher microbial activity, and greater soil fauna presence, all of which contribute to higher humus content and better soil structure [69]. This increased biological activity and organic matter content lead to higher soil porosity and more continuous pore systems, which is consistent with the findings of higher soil porosity in reference forests compared to restoration sites. Therefore, active and passive restoration may have varying effects on restoring soil quality.

3.3.2. Soil Macro- and Soil Micro-Porosity

Macro-porosity, which represents the larger pore spaces in soil, is critical for water infiltration, root penetration, and gas exchange [70]. Micro-porosity refers to the smaller pore spaces in soil that are important for nutrient retention, microbial activity, and soil aeration [70]. The comparison of macro-porosity between reference and restoration sites in both active and passive restoration reported conflicting results. Notably, Zhang et al. [32,50] found that macro-porosity was significantly higher in reference sites than in restoration sites, with mean values varying from 20 to 29%. This indicates that restoration efforts may be limited in restoring this soil parameter. Conversely, Lozano-Baez et al. [47] reported that there was no significant difference in macro-porosity between reference and restoration sites in active restoration. The inconsistency in the findings suggests variations in restoration outcomes are influenced by site-specific aspects such as soil type, previous land use, and climatic conditions. Similar to macro-porosity, comparing micro-porosity between reference and restoration sites under active and passive restoration showed inconsistent results. Lozano-Baez et al. [47] observed no significant difference in micro-porosity between reference and restoration sites in active restoration. In contrast, research studies by Zhang et al. [50] and de Vasconcelos et al. [32] found significantly higher micro-porosity in reference sites compared to restoration sites, ranging from 17 to 31%, respectively. This indicates the potential challenges in achieving comparable soil quality in restored sites. Low macro- and micro-porosity in forest soils reduces water infiltration, root development, and nutrient exchange, thus affecting ecosystem functioning.

3.3.3. Soil Water Repellence and Soil-Saturated Conductivity

Soil water repellence adversely affects soil hydrological parameters, lowers infiltration capacity, and causes flow surface runoff and erosion. Our systematic analysis revealed that the comparison of soil water repellence between reference and restoration sites had a higher significant difference in reference sites compared with restoration sites in both passive restoration [71,72] and active restoration [68,73]. These findings could indicate that restoration activities successfully minimized the negative impacts of soil water repellence, contributing to the improvement and maintenance of soil quality at the restoration sites. Many factors can contribute to soil water repellence, such as soil conditions following forest fires or dry spells, as well as amphiphilic molecules made by plants and organisms. Amphiphilic molecules such as saponins (ginsenoside, C42H70O13), phospholipids (phosphatidylcholine, C44H84NO8), and fatty acid amides (anandamide (N-arachidonoylethanolamine), C22H37NO2). Soil attributes such as texture, temperature, pH, water content, and soil organic carbon all have an impact on soil water repellence as well [74]. Nevertheless, information on factors that enhance soil water repellents is scarce, and many tropical forest soils need to be comprehensively investigated.
Soil saturated hydraulic conductivity, which is a measure of soil’s ability to transfer water, showed no significant difference in passive and active restoration between reference and restoration sites. This suggests that restoration has no effect on saturated hydraulic conductivity, with a mean ranging from 322 to 469 mm h−1 [50,71]. The dearth of substantial reactions could be attributed to severe soil compaction, as the study showing no significant impacts had high soil bulk density and low organic carbon on the restoration site [75]. Soil-saturated hydraulic conductivity is an important property in the soil infiltration process because it regulates the water movement through the soil matrix. It is known to vary by several orders of magnitude, particularly in forest soils, and depends on highly variable soil structures. It is also known to vary several orders of magnitude, especially on forested soils [76]. This suggests that passive and active restoration may not have a substantial effect on soil water movement and drainage.

3.3.4. Glomalin-Related Soil Protein

The glomalin-related soil protein (GRSP) acts as a binding agent within soil aggregates, enhancing soil structure stability, water retention capacity, and nutrient cycling processes [77]. Several studies shown in Table 2 reported significant differences in total GRSP between reference site and restoration (both passive and active). The mean values ranged from 0.2 mg g−1 to 1.9 mg g−1, which is attributed to the successional restoration. It was observed that the restoration of vegetation can increase the accumulation of total GRSP. These studies demonstrated that forest soils accumulate more GRSP than herb or shrub soil. In addition, easily extractable (GRSP) is constituted of generated or rapidly deteriorated glomalin-related soil protein in soil [78]. On passive restoration, Zhang et al. [57] observed that easily extractable GRSP had a significant difference across restoration sites and reference sites. Also, Li et al. [38] and Sun et al. [30] indicated that easily extractable GRSP had a significant difference across restoration sites and reference sites in active restoration. This may be linked to the rise in arbuscular mycorrhizal fungi (AMF) releasing more GRSP or increasing microbial biomass, progressively repairing and improving soil fertility in restoration sites [57]. However, some studies discovered that AMF increased greatly during successional restoration and the function of the AMF community improved consistently, promoting EE-GRSP production and conversion to T-GRSP [57,79]. This supports the efficacy of forest restoration in enhancing soil microbial activity and improving soil quality.

3.3.5. Soil Particle Size

The current review showed that soil texture does not necessarily change between reference and passive restoration sites. This was shown by Lozano-Baez et al. [47], who demonstrated that sand and clay content did not differ significantly between these components in reference and restoration sites, suggesting that passive restoration may not alter these components of soil texture. However, they observed an increase in silt content. This suggests passive restoration practices, which often involve allowing natural processes to regenerate the land without active human intervention. As such, this might lead to an accumulation of finer particles like silt over time. Damptey et al. [33] found that soil texture had no significant effect across reference and restoration sites. This could be attributed to the soil types found at these two sites. This could indicate that the physical properties of the soil, which are largely determined by the soil type, remain unchanged even after active restoration. According to Wiesmeier et al. [80], soil type is an important characteristic that is impacted by factors that include parent material, climate, and topography over long periods. It determines the soil texture, structure, and overall capacity to support vegetation. On the other hand, Yao et al. [68] and De Vasconcelos et al. [32] reported significant differences in soil texture among reference and restoration sites. This suggests that the effects of forest restoration on soil properties might be extremely diverse and possibly affected by the type of vegetation and previous land use [38]. Different plant species have different root structures and interactions with soil microorganisms, which influences soil texture and structure. While passive and active restoration can cause changes in certain soil properties, such as increased silt content, their overall impact on soil texture varies. As a result, the recovery of soil quality occurs on both passive and active restoration, but mainly on the topsoil since in this study we only looked at the first depth and it takes time to see the changes.

3.4. Differences Among Restoration Strategies

The effectiveness of restoration strategies (whether passive or active) is influenced by several factors, including the duration of restoration efforts and the history of previous land use such as cattle pastures, agricultural land, abandoned mining land, and other land uses. Understanding these site-specific factors is crucial for determining the most suitable restoration approach for a given ecosystem. Previous land use practices leave lasting imprints on ecosystems, which can persist for decades or centuries. Matzek et al. [48] and Trujillo-Miranda et al. [28] published studies that showed that the previous land use before restoration was cattle pastures. Furthermore, all the restoration sites exhibited higher soil pH, while carbon content was lower compared to the reference sites. Forest restoration in these studies had been undergone for less than a decade; this indicates a gradual recovery of soil quality following the agricultural use of land as cattle pasture. Foster et al. [81] proposed that the traces of previous land use on the environment may linger for decades or even centuries. For instance, following restoration, the influence of previous land use on soil properties may extend for more than a decade [31,33,48]. These findings emphasize the significance of considering past land usage and its implications for soil recovery when evaluating the successional trajectories and recovery processes of ecosystems post-restoration.
The restoration age has been previously acknowledged as one of the key factors of forest restoration success, biodiversity, and vegetation cover [82]. The current review showed that the more mature the forest restoration, the better the recovery. It highlighted the positive link between organic matter carbon inputs and soil recovery and vegetation diversity. For soil recovery, it was also noted that soil quality improved with the duration of planting (active restoration) [57]. Organic matter inputs directly affected the chemical and physical attributes of soil when forest cover was established through different successional phases [83]. Contradictory results emerged when comparing active and passive restoration strategies, particularly regarding their effects on soil chemical properties. While both approaches aim to enhance ecosystem recovery, their impacts on soil quality may vary. The findings suggested that active restoration may have diverse effects on soil properties, potentially affecting soil quality in different ways. The active and passive restoration in some of the studies in this review has been going on for a decade or less than a decade. These discrepancies highlighted the need for careful consideration of soil attributes in restoration planning and management to minimize adverse impacts on soil quality and ensure the long-term health of ecosystems.

4. Limitations of the Study, Research Gaps, and Recommendations

The current review was designed to focus on humid forest regions; hence, several locations of the world, including Southern Africa and Australia, are missing or have tiny sample sizes. As such, more studies are required in forest and other biomes and should evaluate several sampling locations (replicates) in both restored and reference sites. During the literature search, some papers were not written in English. This posed a challenge in the attempts to quantify all the studies that focused on forest ecological restoration. There is considerable variability in the methodologies used across studies, including differences in measured parameters (e.g., soil organic matter, soil water repellence) and experimental designs. This inconsistency can complicate comparisons and synthesis of results. Additionally, there is a disproportionate focus on active restoration strategies compared to passive restoration, resulting in a limited understanding of the effectiveness and ecological outcomes of allowing ecosystems to recover naturally.
Firstly, the response of soil biological properties was not frequently measured in the literature. Given the significance of soil biological properties and their function in soil ecosystems and health, future research should extensively investigate the influence of restoration activities on soil biological characteristics. These properties are the first to be immediately impacted by degradation, as habitat degradation impacts the survival of microorganisms in the soil. Thus, further research should focus on comparing the recovery of soil biological properties after forest restoration.
More comparative studies between different restoration strategies (active vs. passive) and their impacts on soil and ecosystem health are needed to inform best practices. Research should aim to uncover the underlying mechanisms through which different restoration strategies affect soil attributes and ecosystem functions, including studies on soil hydro-physical attributes and nutrients.
Finally, a narrow focus on long-term effects may not capture the full potential of any restoration over time. Long-term monitoring and further research are required to comprehend the mechanisms driving shifts in both soil chemical and physical attributes and to create successful techniques for restoring soil quality. Understanding these mechanisms is crucial for designing more effective restoration strategies tailored to specific ecosystems. There is a need for more long-term studies to evaluate the sustainability and resilience of restored ecosystems over time, particularly in relation to climate change and other environmental stressors. Moreover, establishing standardized methodologies for measuring soil health and ecosystem recovery to improve comparability across studies and regions is required.

5. Conclusions

The current review showed that soil chemical and physical attributes were the primary indicators utilized to assess ecosystem performance in forest restoration in humid regions. However, passive restoration in degraded forests takes longer to attain similar recovery to that of mature forests and to fully recover soil quality, particularly its chemical qualities. The previous land use has an impact on how long the recovery of soil quality may take. Soil quality in planted sites can continue to enhance with age, but it may plateau after a few years. In summary, our results showed that active and passive restoration in humid regions significantly increased when compared to mature forests. The soil quality recovery occurred during both passive and active restoration, although mostly on the topsoil (0–20 cm), which is mostly affected by restoration activities; however, changes require time to develop. Nevertheless, it is still early to find definitive patterns considering few soil data are available in restoration research, particularly soil biological properties. Moreover, several limitations and gaps in the literature were identified, including inconsistencies in findings, the absence of long-term studies, a limited understanding of restoration mechanisms, and site-specific considerations. Addressing the inconsistencies in soil data and understanding restoration mechanisms is critical for defining efficient strategies in forest restoration efforts. This is important for guaranteeing long-term recovery of ecosystem functioning and soil quality in humid environments.

Author Contributions

W.J. writing—original draft, writing—revision, methodology, data curation, formal analysis, investigation; A.M. conceptualization, writing—review and editing, funding acquisition, methodology, project leadership, project administration, supervision, validation; C.V.M. conceptualization, writing—review and editing, methodology, supervision, validation. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Govan Mbeki Research and Development Centre of the University of Fort Hare (P774), South Africa. We are also grateful to the National Research Foundation of South Africa for awarding Wendy Jiba the scholarship (PMDS230524109549).

Data Availability Statement

No underlying data were collected or produced in this study.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Flow diagram of the selection process used to identify studies for inclusion in this review based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) Guideline. The letter n = number of studies.
Figure 1. Flow diagram of the selection process used to identify studies for inclusion in this review based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) Guideline. The letter n = number of studies.
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Figure 2. The total quantity of published studies considered in the systematic review.
Figure 2. The total quantity of published studies considered in the systematic review.
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Table 2. Impacts of forest restoration on soil physical attributes.
Table 2. Impacts of forest restoration on soil physical attributes.
ReferencesStudy LocationObjectiveSoil TypeRestoration StrategyReference SitesRestoration Sites
[32]BrazilTo evaluate soil quality in restoration areas and compare them to mature forests.ArgisolsPassiveTP ns, Mac ns,
Mic (+) *, CL (−) *,
SN (+) *, ST (−) *		TP ns, Mac ns, Mic (+) *,
CL (+) *, SN (−) *, ST (+) *
[47]BrazilTo evaluate the impact of forest restoration on surface saturated hydraulic conductivity.Ultisols and EntisolsPassiveBD (−) *, TP (+) *,
Mac ns, Mic ns, CL ns, SN ns,
ST (+) *		BD (+) *, TP (−) *, Mac ns,
Mic ns, CL ns, SN ns,
ST (+) *
[29]ChinaTo evaluate soil property changes caused by various land use types following passive restoration.Humic AcrisolsPassiveBD (−) *BD (+) *
[71]ColombiaTo evaluate the saturated hydraulic conductivity and some hydro-physical soil properties in four land-uses.EntisolsPassiveBD ns, Ks ns,
WDPT (+) *		BD ns, Ks ns, WDPT (−) *
[11]BrazilTo assess and compare soil parameters between two restoration strategies in the early stages of restoration.Acrisols and Fluvi-solsPassiveCL ns, SN ns,
ST ns		CL ns, SN ns, ST ns
[72]PolandTo investigate the moisture content, persistence, and strength of soil water repellence on sandy soil that had been excluded from agriculture and was then subjected to spontaneous afforestation.Albic PodzolsPassiveWDPT (+)WDPT (−)
[57]ChinaTo examine the accumulation of glomalin related soil protein and its contribution to SOC sequestration under various forest restoration strategies.LeposolPassiveT-GRSP (+) *,
EE-GRSP (+) *		T-GRSP (−) *,
EE-GRSP (+) *
[11]BrazilTo assess and compare soil parameters between two restoration strategies in the early stages of restoration.Acrisols and Fluvi-solsActiveCL ns, SN ns,
ST ns		CL ns, SN ns, ST ns
[54]BrazilTo examine the impact of long-term forest-to-sugarcane conversion and following forest restoration on soil carbon storage, soil physical, chemical, and biological health indicators, and to establish a structured soil health evaluation.OxisolActiveBD ns, Mac (−) *,
Mic (+) *		BD (−) *, Mac (+) *,
Mic (−) *
[33]GhanaTo examine soil parameters and below- and above-ground biomass qualities in a restored former gravel mine region two decades after active restoration.Ultisols and AcrisolsActiveBD (−) *, CL ns, SN ns, ST nsBD (+) *, CL ns, SN ns,
ST ns
[62]AustraliaTo compare the recovery of attributes and functions in throughout different forest restotation.OxisolActiveBD nsBD ns
[32]BrazilTo evaluate soil quality in restoration areas and compare them to mature forests.MollisolsActiveTP ns, Mac ns,
Mic (+) *, CL (−) *,
SN (+) *, ST (+) *		TP ns, Mac ns, Mic (−) *,
CL (+) *, SN (−) *, ST (−) *
[38]ChinaTo explore the accumulation kinetics of glomalin and amino sugars inside aggregates and assess their respective contribution to the SOC pool during forest restoration.LeptosolsActiveT-GRSP (+) *,
EE-GRSP (+) *		T-GRSP (−) *,
EE-GRSP (+) *
[64]TukeyTo examine the impacts of afforestation on the chemical and physical soil properties and plant growth.AlfisolActiveBD ns, CL ns,
SN ns, ST ns 		BD ns, CL ns, SN ns,
ST ns
[29]ChinaTo evaluate soil property changes caused by various land use types following passive restoration.Humic AcrisolsActiveBD (−) *BD (+) *
[49]United statesTo examine the key indicators of ecosystem function in restored floodplain forests.–ActiveBD (−) *BD (+) *
[30]ChinaTo examine how the synergistic interaction between GRSP, SOC, and soil aggregate stability over time following various stages of plant restoration.Eutric CambisolActiveT-GRSP (+) *,
EE-GRSP (+) *, 		T-GRSP (−) *,
EE-GRSP (+) *
[66]United StatesTo evaluate the mine soils and vegetation present, compare their soil qualities with those that had been reforested successfully, and assess the effects of site age on the measured properties and reforestation sites.–ActivepH (−) *pH (+) *
[28]MexicoTo evaluate the impact of active restoration on forest structure, tree diversity, and soil qualities as indicators of restoration success.Umbric AndosolActiveBD nsBD ns
[68]ChinaTo assess the impacts of vegetative restoration and hydrophobic dissolved organic carbon fractions on soil hydrological and mechanical stability.–ActiveTP (+), SWR (+) *, CL (−) *, SN (−) *, ST (+) *TP (−), SWR (−) *, CL (+) *, SN (+) *, ST (−) *
[73]NepalTo analyze the trade-offs between changes in water use going from a severely degraded pasture to a mature coniferous tree plantation to the increases in soil water reserves afforded by improved rainfall infiltration after plantation.CambisolActiveKs (+) *Ks (−) *
[50]ChinaTo examine the impact of vegetation restoration on soil physicochemical properties and saturated hydraulic conductivity at various land uses.–ActiveBD (−) *, TP (+) *, Mac (+) *,
Mic (+) *, Ks ns 		BD (+) *, TP (−) *, Mac (−) *, Mic (−) *, Ks ns
BD = bulk density, CL = clay. EE-GRSP = easily extractable glomalin-related soil protein; KS = saturated conductivity. Mac = macro-porosity, Mic= micro-porosity, SN = sand, ST = silt, SWR = soil water repellence, T-GRSP = total glomalin-related soil protein, and TP = total porosity. The positive (+) and negative (−) marks indicate if soil parameters were positively or negatively changed. A superscript * indicates a significant difference, whereas a superscript ns indicates no significant difference.
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Jiba, W.; Manyevere, A.; Mashamaite, C.V. The Impact of Ecological Restoration on Soil Quality in Humid Region Forest Habitats: A Systematic Review. Forests 2024, 15, 1941. https://doi.org/10.3390/f15111941

AMA Style

Jiba W, Manyevere A, Mashamaite CV. The Impact of Ecological Restoration on Soil Quality in Humid Region Forest Habitats: A Systematic Review. Forests. 2024; 15(11):1941. https://doi.org/10.3390/f15111941

Chicago/Turabian Style

Jiba, Wendy, Alen Manyevere, and Chuene Victor Mashamaite. 2024. "The Impact of Ecological Restoration on Soil Quality in Humid Region Forest Habitats: A Systematic Review" Forests 15, no. 11: 1941. https://doi.org/10.3390/f15111941

APA Style

Jiba, W., Manyevere, A., & Mashamaite, C. V. (2024). The Impact of Ecological Restoration on Soil Quality in Humid Region Forest Habitats: A Systematic Review. Forests, 15(11), 1941. https://doi.org/10.3390/f15111941

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