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Review

Global Trajectories of Forest Soil Acidification: A Scientometric Synthesis of Drivers, Impacts and Sustainable Solutions

by
Yujie Zhang
,
Jiangmin Zhou
and
Hualin Chen
*
College of Life and Environmental Sciences, Wenzhou University, Wenzhou 325035, China
*
Author to whom correspondence should be addressed.
Forests 2025, 16(5), 733; https://doi.org/10.3390/f16050733
Submission received: 12 March 2025 / Revised: 13 April 2025 / Accepted: 24 April 2025 / Published: 25 April 2025
(This article belongs to the Section Forest Soil)
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Abstract

:
Global forest soil acidification has become a significant environmental concern, making it essential to gain a comprehensive understanding of research hotspots in this field. Acidic substances in forest soil originate from both external and internal factors. To investigate this issue, we conducted a visual analysis of 2325 papers published between 2004 and 2024 using the Web of Science Database, along with the visualization and analysis tools CiteSpace and VOSviewer. Over the past 20 years, the number of publications on global forest soil acidification has steadily increased. China and the United States have far more publications than any other country. Key research hotspots include soil acidification, atmospheric deposition, nitrogen deposition, heavy metals, soil pH, plant growth, impacts and governance, each displaying distinct characteristics at different stages. This review offers a comprehensive overview of recent advances in global forest soil acidification research and serves as a valuable reference for both research and practical applications. It examines the current state of this global environmental problem, the long-term effects of acidification and forest succession, and the eco-environmental effects associated with soil acidification. It also proposes sustainable solutions to mitigate forest soil acidification and outlines potential future research topics. These efforts aim to support the stable development of forest ecosystems and promote ongoing research in this critical area.

1. Introduction

Forest ecosystems play vital roles in terrestrial ecosystems, but they are increasingly threatened by global soil acidification, an environmental problem characterized mainly by declining soil pH levels. Soil pH cab be categorized as follows: values below 4.5 indicate very acidic conditions, 4.5–5.5 indicate strongly acidic conditions and 5.6–6.5 indicate acidic conditions. Stabilizing soil pH is crucial for maintaining ecosystem structure, function and sustainability [1]. It plays a vital role in supporting plant growth, preserving biodiversity and regulating the biogeochemical cycling of carbon and nitrogen [2].
The sources of acid in forest soil can be roughly divided into two types (Figure 1). One source is external, including acid deposition and fertilization [3,4,5,6]. In natural and protected forests, fertilization is unlikely to cause significant soil acidification. However, in forests with both natural and managed soils, fertilization becomes a key factor contributing to soil acidification. In order to ensure the healthy growth of forest trees and maintain their nutrient requirements [7,8,9], nitrogen, phosphate, potassium and organic fertilizers are commonly applied to the soil [10]. While these inputs are beneficial, excessive application of nitrogen fertilizer can lead to soil acidification. This occurs in two ways: first, trees release H+ when absorbing nitrogen fertilizer [3]; second, ammonium-based nitrogen undergoes nitrification, a process that also releases H+ [11]. Furthermore, when nitrate is leached by rain, it carries away base ions, reducing the acid-neutralizing capacity of the soil and potentially worsening soil acidification [3]. Despite this, the role of fertilization in forest soil acidification has received relatively little attention. Acid deposition is recognized as the main source of this problem. Research has shown that acid rain and nitrogen deposition aggravate forest soil acidification, alter soil nutrient content and enzyme activity, and significantly impact overall forest soil health [12]. Wu et al. [13] demonstrated that acid rain increases the leaching of base ions and simultaneously lowers soil pH. Therefore, acid rain is expected to remain a major driver of forest soil acidification well into the future. Currently, research on acid rain-induced forest soil acidification focuses mainly on its effects and possible treatment. However, most studies rely on simulated acid rain experiments and lack long-term, large-scale regional field data.
Forest soil acidification can also result from various reaction mechanisms within forest ecosystems, and these include the decomposition of organic matter, microbial metabolism and plant root secretions [14,15,16]. Acidic substances produced during the decomposition of dead branches and leaves can be introduced into the soil through rainfall or leached as acidic organic matter already present in the soil. Some soluble substances may penetrate groundwater, further increasing soil acidity. Additionally, the decomposition of forest litter produces large amounts of organic acids [17,18]. Denitrifying bacteria release metabolites such as carbon dioxide and bicarbonate [19], which lower the pH of forest soil. Plant roots are an important source of organic acids in the forest soil [14,15]. During nutrient absorption, some plants release more organic acids to balance the uptake of excess base ions (e.g., Ca2+, Mg2+ and K+) [20]. This process increases the consumption of base ions in the soil while simultaneously raising the concentration of exchangeable H⁺, thereby intensifying soil acidification [21].
Forest soil acidification is a growing concern in most countries and regions, with its ecological and environmental impacts becoming increasingly severe. Therefore, it is crucial to explore future trends and identify research priorities relating to forest soil acidification. The core aim of this review is to systematically clarify the sources, impacts and sustainable solutions to soil acidification with the help of scientometric analysis, and to propose future research priorities that will contribute to further forest soil research.

2. Historical Summary of Forest Soil Acidification

2.1. Data Sources

The literature search was conducted using the Web of Science Core Collection database with the search term “forest soil acidification”. The search covered publications from 2004 to 2024, yielding a total of 2325 records of various types. The records were exported by selecting “Full Record and Cited References” as the content option and saved in .txt format for further analysis.

2.2. Research Methods

The exported data were imported into CiteSpace 6.3.R1 software for analysis. The time slice was set to one year, and two separate analyses were performed: one with the node type set to “Country” and the other to “Keyword”. To include a broader range of nodes, the g-index scale factor (k) was set to 25 for the Country analysis and 15 for the Keyword analysis. All other settings were left at their default values. This approach allowed for the identification of publication trends by country and the extraction of high frequency keywords at different stages of research. Additionally, VOSviewer 1.6.18 software was used to generate a keyword density map for visual analysis. GraphPad Prism 9 was then used to plot trend graphs illustrating the annual publication output, the number of articles published by different countries and the occurrence of high-frequency keywords over time.

2.3. Contribution of the Countries

An analysis of the top 15 contributing countries to forest soil acidification research (Figure 2) revealed that China leads with 624 publications, followed by the United States (504), Germany (206), the Czech Republic (201) and Sweden (192). These results demonstrate that these countries have made significant contributions to the field, highlighting both their active research engagement and the importance they place on forest soil acidification studies.

2.4. Research Focus

As shown in Figure 3, most research in the field of forest soil acidification has focused on soil acidification, atmospheric deposition, nitrogen deposition, heavy metals, pH, plant growth, impacts and governance, among which atmospheric deposition is extremely important.
As shown in Figure 4, global research on forest soil acidification has shown a clear upward trend from 2004 to 2024, which can be divided into two distinct phases. The first phase, spanning from 2004 to 2017, was characterized by a gradual increase in the number of publications. Although there was a noticeable spike in 2005, the overall growth in annual publication volume during this period remained relatively slow. The second phase, from 2018 to 2024, marked a period of rapid growth, with the number of publications increasing exponentially. This trend indicates a rising global interest and growing academic focus on the issue of forest soil acidification.
As shown in Figure 5, substantial interest has been devoted to acidification, soil acidification, deposition, nitrogen deposition, atmospheric deposition, acid deposition and plant growth, indicating that researchers have paid great attention to forest soil acidification caused by atmospheric deposition and its effects on plant growth. The high frequencies of the keywords chemistry, aluminum, ecosystems, dynamics and critical loads in publications from the first stage indicate that substantial attention has focused on changes in the soil physicochemical properties (abiotic), the responses of forest ecosystems to soil acidification, the dynamics of forest soil acidification, and the critical loads in forests. During the second phase, there were new high-frequency keywords (such as carbon, responses, organic matter, diversity, pH and climate change), indicating that the impact of acidification on the carbon cycle of forest ecosystems has become a recent focus in the context of carbon neutralization and peaking. The response of forest soil microbial community diversity to acidification, soil organic matter, and blocking and controlling forest soil acidification has become a hot topic of research.

2.5. Scope and Level of Research

The scope of research on forest soil acidification spans both spatial and temporal dimensions. Spatially, most previous studies have focused on specific forest areas, lacking broader regional or global assessments. There is still a need for large-scale evaluations of forest soil acidification at the national and global levels. In China, acid deposition is a widespread issue, especially in the southern regions, including Zhejiang, southwestern Chongqing and parts of Guangdong. Consequently, research on Chinese forest lands has primarily been concentrated in these areas. Previous studies have shown that from the 1990s to the 2010s, the proportion of land in China affected by acid rain (defined as precipitation with a pH < 5.6) increased from 22.53% to 30.45%, indicating a steady expansion of acid rain-affected areas [22]. According to the China Ecological Environment Bulletin (2022), acid deposition remains a serious issue in China. Precipitation monitoring across 468 cities in China showed the annual average pH ranged from 4.60 to 7.93, with a national mean of 5.67. Moreover, the number of cities experiencing acid rain increased by 3.0% compared to 2021, highlighting the ongoing severity of the problem.
In other countries, research on the acidification of forest soils also focused on areas with more severe acid deposition. In the United States, acid deposition has historically been a significant issue, and although it has improved in recent years, it remains a concern. For example, in Ohio, precipitation in 2019 recorded a pH of 5.33 ± 0.21 [23]. Similarly, in far-eastern Russia, acid precipitation has led to a notable decline in stream water pH, decreasing from an average of 7.12 during 2007–2011 to 6.90 during 2015–2019. The lowest pH recorded was 6.35 in February 2019, accompanied by significantly elevated SO42− and NO3− concentrations [24]. These findings indicate that acid deposition continues to be a global issue. In terms of soil acidification research, most studies have focused on surface soil (0–20 cm), with fewer studies addressing the deep soil layers. Furthermore, many studies are based on short-term observations, lacking long-term investigations within the same forest area. There is a need for further research to understand how persistent acid deposition contributes to forest soil acidification and to assess changes over extended periods.
Most current research on forest soil acidification primarily addresses its effects on abiotic properties, such as changes in soil physicochemical characteristics, including changes in the levels of exchangeable aluminum [25,26], salt-based ions [27] and heavy metals [28], among other soil components. Increased forest soil acidity can enhance the solubility and concentration of aluminum, especially in the form of Al3+ [29]. Additionally, enhanced acidification of forest soils also exacerbates the leaching of exchangeable salt-based ions (Ca2+, Mg2+, K+, and Na+) [30]. While plants [31], animals [32] and micro-organisms [33] all play important roles in forest ecosystems, research on the effects of acidification on biotic characteristics remains limited. Most studies to date have predominantly focused on abiotic aspects, leaving a gap in understanding how acidification impacts living organisms within these ecosystems.

3. Overall Global Forest Soil Acidification

3.1. Overall Soil Acidification

Currently, the acidification of forest soils is prevalent on the global scale. Previous data have shown that the pH of forest soils in the areas studied typically ranges from 3.6 to 4.5 (with a few values higher than 5.0) at varying soil depths. This indicates that forest soils generally exhibit strongly acidic conditions [34]. For example, Mao et al. [35] reported a mean pH of 3.98 for forest surface soils within the Dinghu Mountain Forest Reserve (Guangdong Province, southern China). In Tieshanping Forest Park (Chongqing, southwest China), which lies within a subtropical region, the soil pH ranges from 3.80 to 4.54 [36]. In Republic of Korea, the 2018 soil environment analysis reported that the pH of surface soil of the Rear Garden Forest (located in Changdeokgung), ranged from 4.16 to 5.37, with an average value of 4.39 [37]. Another report found that, in a survey of soil pH across 22 forests in Switzerland, the majority had pH levels below 4.2 [38]. Acid forest soils have also been documented in similar studies conducted in Germany [31], and in northern Belgium [39], acidic forest soils have also been documented. In northern Belgium, the soil pH typically ranges from 4.2 to 5.5 when different soil depths are considered, further confirming that soil acidification is a global phenomenon.
The above findings indicate that global forest soil acidification is a serious and persistent ecological issue. Although acid deposition has decreased in some regions, atmospheric acid loading in China remains a severe issue [40,41]. Most countries and regions in the world continue to experience the damaging effects of acid rain, including soil erosion and other related problems. As a result, forest soil acidification remains a prominent and ongoing challenge on a global scale.

3.2. Differences in Acidification in the Same Geographical Area

Soil acidification can vary significantly within the same forest area, depending on factors such as forest stand type, soil depth, elevation and slope direction. For instance, studies have shown that the mean pH values of forest soils dominated by oak, Japanese cedar and pine trees at different soil depths are 5.3, 5.7 and 4.9, respectively [42]. Similarly, previous research demonstrated that the top soil pH of deciduous forests is significantly higher than that of spruce forests [43]. At different soil depths, spruce forests exhibit lower soil pH values compared with beech forests and felled areas [29]. Additionally, Clesse et al. [44] highlighted that the acidification of forest soils varies under different tree species.
Soil acidification also varies by depth within the same forest area. Previous research has indicated that soil pH generally increases with depth, with values at a depth of 25–35 cm being higher than those at a depth of 0–5 cm [43]. However, few studies have investigated the differences in acidification across varying elevations and slope directions within the same forest area. Some studies suggest that surface soil pH decreases sharply at low elevations over time and decreases less or remains stable at high elevations [45]. This difference may be attributed to variations in rainfall pattern brought about by altitudinal differences, as higher elevations tend to receive larger amounts of precipitation.

3.3. Differences in Acidification in Different Geographical Areas

Forest soil acidification exhibits strong geographical variations, largely influenced by acid precipitation patterns. Areas that frequently experience acid rain tend to show more severe acidification. For example, acid rain occurs more frequently and intensely in southern China than in northern China. A large-scale data study from 2006 to 2010 [46] reported soil pH values of 5.47, 5.60, 4.96, 5.35, 6.66 and 7.51 in southwest China, northeast China, south-central China, east China, north China and northwest China, respectively. Similarly, studies conducted in three different woodlands in Norway revealed variations in soil pH when soil depth was taken into account [47]. An analysis of representative woodlands in four provinces of China found soil pH values ranging from 4.2 to 5.9 [48].
Beyond acid deposition, other factors also contribute to geographical differences in soil acidification. Forest ecosystem characteristics and human activities, such as logging and land clearing, can disturb forest soils, leading to soil erosion and degradation. These disturbances reduce the organic matter content of the soil [49], thereby decreasing its acid-buffering capacity [50].

4. Long Time-Scale Effects of Acidification and Forest Succession

4.1. Long Time-Scale Effects of Acidification

Forest soil acidification can be mitigated by a combination of mineral weathering [51], organic matter [20] and other natural factors. However, research has shown that soil acidification is a natural process within forest ecosystems that can be accelerated by acid deposition and anthropogenic factors [46,52,53]. A study by Zhu et al. [46] found that the combined effects of atmospheric deposition and forest harvesting—with atmospheric deposition contributing 84%—caused a substantial decline in forest soil pH in China. From 1981 to 1985 and 2006 to 2010, forest soil pH decreased by an average of 0.36 units per year, after accounting for soil depth. The most severe acidification occurred in southwest China, with a pH decline of 0.63 units, followed by the northeastern region (0.55 units), the south-central region (0.50 unit), northern China (0.44 units) and eastern China (0.25 unit).
Forest soil acidification continues to worsen in many regions of the world, with acid deposition remaining the primary driver of accelerated acidification [46,54]. One study reported that the surface soil pH in Chinese forests decreased significantly between 1980 and 2019, dropping from 5.64 to 5.08, with an average decline of 0.56 pH units [40]. In South Korea, long-term monitoring of Kwanak Mountain from 1972 to 2010 revealed an increase in acidification of forest soil. The pH declined from 5.40 to 4.50, while the concentration of exchangeable potassium (K+) concentration and calcium (Ca2+) decreased from 0.67–0.25 cmol kg−1 and 3.20–0.87 cmol kg−1, respectively [55]. Similarly, data from the Eastern Sudetes in the Czech Republic indicated that between 1941 and 2003, 80%–90% of forest soil pH values decreased by an average of 0.7 units across 20 monitoring sites [45]. Another study predicted that sustainable development practices combined with high-intensity emission reduction measures in China from 2020 to 2050 could reduce the decline in forest soil pH by 60%. However, these measures would still not completely reverse the overall trend of decreasing forest soil pH levels [40].

4.2. Impacts of Forest Succession and Weather Extremes on Acidification Processes

Although numerous studies have shown that the rapid enhancement of forest soil acidification is primarily driven by atmospheric acid deposition from natural ecosystems [46,54], this is not the sole contributing factor. Other factors, such as forest succession and changes in land use, also play significant roles. These factors can lead to shifts in dominant species, which in turn alter nutrient uptake rates and contribute to further acidification of forest soils [40]. Research examining forests at three different successional stages (plantation forests, secondary forests and primary forests) revealed significant changes in soil pH levels [56]. As forests matured from plantation to secondary and then to primary stages, the pH of surface soil (0–10 cm) decreased from 5.08 to 4.10, and that at a 10–20 cm depth it decreased from 5.52 to 4.64. These findings suggest that forest succession may intensify soil acidification.
Extreme weather, such as storms and typhoons, can also significantly impact the physical, chemical and biological properties of the soil, further affecting its fertility [57]. In Sweden, storms are among the most frequent disturbances in forest ecosystems [58]. A study on Norway spruce forests in southwestern Sweden found that between 1997 and 2009, soil pH decreased from 4.54 in 1997 to 3.86 [57]. Storms typically involve strong winds and heavy rain, which can break the tree canopy and expose the soil to greater acid input. The canopy normally acts as a barrier, reducing the amount of acid that reaches the soil, thereby helping to buffer acidification. One study showed that a mature spruce forest canopy reduced atmospheric acid loading by 27% to 28% [59]. Similarly, in subtropical regions of China, different plantation forest canopies have shown varying abilities to retain acid rain. For instance, longan plantation canopies retained more acid rain compared to those of Acacia mangium, leading to a greater decline in soil pH in A. mangium plantations—approximately 1.4 pH units lower than in longan plantations [60]. In addition to the effects on canopy, heavy rainfall can increase leaching, accelerating the loss of base cations and further heightening the risk of soil acidification. A related study in southwestern Japan showed that intense precipitation significantly altered the chemical properties of cedar forest soils, leading to a marked decrease in pH and changes in cation exchange capacity in the topsoil over time [61].

5. Eco-Environmental Effects of Forest Soil Acidification

5.1. Effects of Forest Soil Acidification on Soil Physicochemical Properties

The acidification of forest soils accelerates the leaching of cations such as Ca2+, Mg2+, K+ and Na+ from forest soils, leading to reduced soil fertility [30]. Data from the Daniel Boone National Forest (US) show that between 1994 and 2012, salt-based saturation levels across all soil layers decreased by at least 66% as a result of acidification [62]. These changes can significantly affect nutrient uptake, fertility and buffering capacity of the soil. Moreover, in strongly acidic forest soils, lower pH levels can also inhibit soil respiration and microbial activity, leading to increased accumulation of organic matter [63]. In addition, acidification of forest soils has been linked to accumulated levels of soil organic carbon [64].
Soil acidification can also affect the behavior of heavy metals. As soil pH decreases, the mobility of metals such as Cd, Cu and Zn increases, resulting in higher concentrations of these heavy metals in the soil [65]. Similar and related findings have shown that soil acidification significantly enhances the potential mobility and bioavailability of metals like Cu and Pb [28]. Consequently, acidified soils may increase the risk of heavy metal contamination, posing ecological hazards.
From a practical standpoint, southern China is predominantly forested, with most farmland located near forested areas. Organic acids produced by the decomposition of forest litter can be transported into adjacent agricultural lands via surface runoff. This process may increase the accumulation of heavy metals in crops. When such crops are consumed by humans, they could inevitably cause considerable harm to human health (Figure 6). Despite these concerns, few studies have investigated how organic acid production from forest litter affects the availability of heavy metals in surrounding farmland soils. Understanding and quantifying these dynamics is crucial for predicting the impact of heavy metal pollution on the surrounding farmland ecosystems [66] and food security.

5.2. Effects of Forest Soil Acidification on Plant Growth

Under normal conditions, both excessively acidic and alkaline soils can hinder the absorption and utilization of forest soil nutrients by the plant roots, consequently disrupting the normal growth and development of plants by inhibiting the growth of the root system. Moreover, acidification can lead to the formation of primary and secondary Al-rich minerals, which release Al into the soil [67]. Elevated Al levels are toxic to plants and can even lead to plant death [68]. Acid deposition further restricts plant growth by promoting the release of Al ions from the soil, which has been shown to reduce biomass and inhibit root elongation in Pinus sylvestris seedlings [69]. Achilles et al. [31] showed that soil acidification would have impacts on tree productivity and resilience against diseases. Other studies have reported that lower pH can lead to more severe negative effects on the growth, net photosynthesis and leaf-nutrient content of beech seedlings [70].
Over the past decades, forest soil acidification has been recognized as an important cause of the decline of Sargasso pine forests in Chongqing, southwestern China [71]. Similarly, acid rain has been linked to increased defoliation in horsetail pine forests [72]. Ponytail pine, a widely distributed and economically important species in subtropical China, may also be vulnerable to these effects [73].

5.3. Effects of Forest Soil Acidification on Soil Fauna

Forest soil fauna, which include earthworms, ants and nematodes [74], are important organisms that play crucial roles in nutrient cycling and energy flow within forest ecosystems [75,76,77]. These organisms are highly sensitive to changes in soil conditions, particularly pH levels. Variations in forest soil pH can significantly affect the abundance, diversity and biomass of forest soil fauna. One study that used American toads as subjects found that after 90 days, their numbers were significantly higher in plots with an elevated forest soil pH [78].
Similarly, soil acidification has been shown to negatively impact nematode communities. In a monitoring study conducted in 2010 and 2011, acid additions led to a 49% decrease in total nematode numbers and a 50% reduction in species richness by 2011 [2]. Soil acidification can also alter the trophic structure of nematode populations [79], further highlighting its significant effects on forest soil fauna.

5.4. Impact of Forest Soil Acidification on Microbial Diversity

Soil micro-organisms play important roles in forest ecosystem functions [80], are among the richest components of terrestrial biodiversity, and serve key roles in ecosystem processes such as soil formation and carbon and nitrogen cycling [81].
Soil micro-organisms are highly sensitive to changes in soil pH. Heavily acidic conditions can alter the compositions of microbial communities [82]. In one study, soil pH was found to be the primary factor shaping bacterial diversity and composition, explaining 68.6% and 69.9% of the variation, respectively, while the bacterial communities also became more phylogenetically concentrated as the soil pH increased [83]. In tropical agroforestry systems in China, pH was similarly identified as the main driver of bacterial community composition [84]. These findings demonstrate that forest soil acidification has a profound impact on microbial diversity, potentially disrupting essential ecological processes.

5.5. Impact of Forest Soil Acidification on the Fluxes of Major Greenhouse Gases

Understanding how forest soil acidification influences greenhouse gas emissions (CO2, CH4 and N2O) is critical in the context of global climate change. Forest ecosystems represent an important type of natural ecosystem in China [85], and they play important roles in terrestrial ecosystems.
One study showed that CH4 emissions decreased by 81.94% with increasing soil pH [86]. Similarly, another study confirmed a similar decrease in CH4 fluxes under elevated soil pH [87]. In addition, in N-saturated subtropical forest, increased nitrogen deposition was found to increase N2O emissions [88], indicating that soil acidification promotes N2O emissions. Therefore, greenhouse gas emissions may increase with increasing forest soil acidification, which would have a more serious impact on the quality of the atmospheric environment. Currently, there are few studies on the effect of forest soil acidification on greenhouse gas emissions. Conducting more in-depth research in this area is essential to advancing our understanding of climate change, the global carbon and nitrogen cycle, and sustainable forest management.

6. Sustainable Solutions

6.1. Source Control: Reduce Acid Input

To address acid deposition at its source, several strategies can be implemented across key sectors. Firstly, the industrial and energy transition sector should promote the use of clean energy sources such as wind and solar to replace fossil fuels in order to reduce emissions of sulfur oxides (SOX) and nitrogen oxides (NOX) [89]. At the same time, actions should be taken to enforce the mandatory installation of desulfurization and denitrification equipment in industrial facilities and to encourage the development and deployment of carbon capture, utilization and storage (CCUS) technologies. Secondly, the agriculture and transportation sector should reduce the use of nitrogen fertilizer and promote alternatives like slow-release or organic fertilizer. In transportation, support should be given to the adoption of electric vehicles and the expansion of green public transport to lower vehicle emissions. Lastly, in the trans-boundary pollution control sector, international cooperation should be strengthened through agreements such as the Convention on Long-Range Transboundary Air Pollution to manage and reduce the cross-border transport of acid rain pollutants.

6.2. Soil Remediation and Improvement

To mitigate the impacts of forest soil acidification, targeted remediation strategies can be employed, such as the application of alkaline amendments, wood ash, organic matter and biochar, and microbial and fungal interventions. In the application of alkaline amendments, site-specific soil testing is used to guide the local application of alkaline materials like lime (CaCO3) or dolomite powder (CaMg(CO3)2) in severely acidified areas. These substances neutralize soil acidity while replenishing calcium and magnesium [6,90,91]. However, large-scale applications should be avoided to prevent negative effects such as soil compaction or microbial community destruction. In wood ash application, research has shown that applying wood ash can raise the soil pH from 3.2 to 4.8 and increase alkali saturation from 30% to 86% [92]. Adding organic matter, biochar, straw or compost can improve soil buffering capacity, promote humus formation, and help stabilize pH levels [93,94,95]. Microbial and fungal regulation relies on the introduction of acid-resistant micro-organisms such as azotobacter or phosphorus-solubilizing bacteria, and mycorrhizal fungi such as ectomycorrhiza can improve plant stress tolerance and nutrient uptake efficiency. There have been studies indicating that with ongoing human disturbances, soil pH continues to decline, and bacterial diversity in extremely acidic soils (pH < 4.5) decreases across various forest types [96].

6.3. Ecological Adaptive Management

To restore and maintain soil function in acidified forest ecosystems, adaptive ecological strategies are essential, and these include optimization of tree species, stand structure adjustment and hydrological regulation. Optimization of tree species is a strategy that introduces acid-resistant tree species such as spruce and fir or pioneer tree species like alder to gradually restore soil conditions. Establishing mixed forests (dominated by a mixture of broadleaf trees and conifers) and using litter from broad-leaved trees can help neutralize acidity and promote nutrient cycling [97]. The adjustment of stand structures involves maintaining appropriate forest density through thinning and selective cutting to reduce litter accumulation, which can contribute to acidification [98]. Retaining dead trees and fallen trees can slow surface runoff, minimize rain erosion and promote humus accumulation. In the case of hydrological regulation, ecological ditches or wetlands are constructed in catchment areas to filter acidic runoff using plant-microbial systems.

6.4. Long-Term Monitoring and Early Warning Systems, Policy and Economic Incentives, Public Participation Education

A soil health database has been established, and the key indicators, such as pH, aluminum ion concentration and organic matter content, have been regularly monitored to evaluate the status of acidification in soil. An intelligent early warning platform, combined with remote sensing technology and ground sensors, real-time tracking of acid rain precipitation and soil changes can help to determine the precise treatment [99,100]. Ecological compensation programs, such as the EU Agri-Environmental Schemes (AES), subsidize forest farms and communities that implement soil remediation. The promotion of carbon trading and ecological certification by linking forest carbon sequestration capacity to soil health within carbon markets can also encourage sustainable forest management. Community science popularization should be carried out to promote the concept of “low intervention” forest management and reduce human interference (e.g., over-cutting and improper fertilization) [101,102]. To achieve this, it is essential to encourage citizen science initiatives that engage the public in soil sampling, monitoring and data collection, thereby fostering grassroots involvement in forest conservation.

6.5. Key Considerations

Avoid single-strategy solutions: Relying solely on one method, such as excessive lime application, can lead to unintended consequences like soil compaction. Therefore, a combination of chemical, biological and ecological approaches is recommended.
Adapt to local conditions: Acidification mechanisms vary between forest types; for instance, tropical forests may require targeted measures to manage aluminum toxicity, whereas temperate forests may respond differently. Therefore, strategies should be customized to local ecological conditions.
Maintain a long-term perspective: Soil restoration is a gradual process that may take decades, requiring sustained investment, continuous monitoring and adaptive management.
Ultimately, the sustainable restoration of forest and soil health can be achieved through multi-scale interventions, from global emission reduction efforts to site-specific remediation and multi-stakeholder collaboration involving governments, research institutions and local communities.

7. Conclusions

Most research on forest soil acidification has primarily focused on acid deposition and its risks to forest ecosystems. However, several important areas remain underexplored and warrant further investigation.
(1)
Scale of study and applicability of results: Existing studies often focus on relatively small areas within forests, limiting the broader applicability of their findings. To improve the scientific validity and generalizability of the conclusions of the studies, future research should expand to larger regional or global scales, both temporally and spatially. Integrating long-term observational data with predictive modeling, particularly in relation to forest succession and extreme weather events, will help improve the universality and scientific validity of the conclusions.
(2)
Complex interactions within forest ecosystems: Forest soil acidification is a complex process that occurs within forest ecosystems, and it is affected by both internal and external factors. Therefore, future research should more rigorously investigate the interactions among key factors—such as acid deposition, litter decomposition and root exudation—to better understand their combined effects. In particular, the impacts of acid deposition in driving forest soil acidification should be thoroughly explored.
(3)
Buffering mechanisms in forest ecosystems: The effects of acid deposition and anthropogenic factors, among others, can accelerate forest soil acidification. The mechanisms by which forest ecosystems buffer against acid deposition have been explored in studies focusing on topics such as ion-exchange reactions, the weathering of soil minerals and soil biology. Moreover, changes in the diversity of soil micro-organisms can be used as a sensitive indicator of ecosystem health. Changes in soil acidity can significantly affect micro-organisms in the soil, which may adapt to such changes through a series of internal biochemical reactions, which may then impact the acidification process in forest soils. Therefore, research on the mechanisms underlying acid-buffer deposition in forest ecosystems should be strengthened.
(4)
Cross-ecosystem impacts of acid deposition: The acidification of forest soils can have harmful effects on both biotic and abiotic components of forest ecosystems. Moreover, under the influence of acid deposition, acidic substances originating in forest ecosystems may be transported to adjacent ecosystems, such as agricultural ecosystems, via surface runoff. These cross-ecosystem impacts remain poorly understood. Therefore, strengthening long-term field monitoring and sample plot studies will be essential to quantify these environmental hazards and support the sustainable development and management of interconnected ecosystems.

Author Contributions

Conceptualization, H.C. and J.Z.; methodology, Y.Z., H.C. and J.Z.; software, Y.Z.; formal analysis, Y.Z.; data curation, Y.Z.; writing—original draft preparation, Y.Z.; writing—review and editing, Y.Z. and H.C.; supervision, H.C. and J.Z.; project administration, H.C.; funding acquisition, H.C. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the Science and Technology Department of Zhejiang Province (No. 2019C54002), and was supported by the Master’s Innovation Foundation of Wenzhou University (No. 3162024004094).

Acknowledgments

We thank Alan K. Chang from the College of Life and Environmental Sciences at Wenzhou University for his kind effort in revising the language of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Sources that contribute to forest soil acidification.
Figure 1. Sources that contribute to forest soil acidification.
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Figure 2. The top 15 countries contributing to forest soil acidification research publications.
Figure 2. The top 15 countries contributing to forest soil acidification research publications.
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Figure 3. Keyword density map related to global forest soil acidification research.
Figure 3. Keyword density map related to global forest soil acidification research.
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Figure 4. Trends in annual publications on global forest soil acidification research.
Figure 4. Trends in annual publications on global forest soil acidification research.
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Figure 5. High-frequency keywords for different stages of global forest soil acidification research.
Figure 5. High-frequency keywords for different stages of global forest soil acidification research.
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Figure 6. Eco-environmental effects of forest soil acidification.
Figure 6. Eco-environmental effects of forest soil acidification.
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MDPI and ACS Style

Zhang, Y.; Zhou, J.; Chen, H. Global Trajectories of Forest Soil Acidification: A Scientometric Synthesis of Drivers, Impacts and Sustainable Solutions. Forests 2025, 16, 733. https://doi.org/10.3390/f16050733

AMA Style

Zhang Y, Zhou J, Chen H. Global Trajectories of Forest Soil Acidification: A Scientometric Synthesis of Drivers, Impacts and Sustainable Solutions. Forests. 2025; 16(5):733. https://doi.org/10.3390/f16050733

Chicago/Turabian Style

Zhang, Yujie, Jiangmin Zhou, and Hualin Chen. 2025. "Global Trajectories of Forest Soil Acidification: A Scientometric Synthesis of Drivers, Impacts and Sustainable Solutions" Forests 16, no. 5: 733. https://doi.org/10.3390/f16050733

APA Style

Zhang, Y., Zhou, J., & Chen, H. (2025). Global Trajectories of Forest Soil Acidification: A Scientometric Synthesis of Drivers, Impacts and Sustainable Solutions. Forests, 16(5), 733. https://doi.org/10.3390/f16050733

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