The Effect of Vegetation Restoration in Soil Organic Carbon Storage

This review paper has been made to assess the past studies reviewed regarding vegetation restoration and its impact on soil organic carbon content. A Vegetation Restoration is an influential technique that can be used to respond to these effects. As a response to the global biodiversity crisis, more restoration actions have been taken. The European Union Council’s results on kinds of diversity after 2010 highlight words like stopping biodiversity loss and the breakdown of ecological systems in the European Union. The United Nations Conference on Biological Diversity’s growth strategy for 2022, which includes restoring at least 15% of degraded ecosystems, has made this possible. Soil types are among the most vulnerable resources on the planet due to factors such as climate change, land degradation, and the reduction of biodiversity. Organic Carbon, the top meter of soil, could potentially store three times as much carbon as is found in the air and almost twice as much as in plants. For the systematic literature review, past papers on vegetation restoration have been extracted from the latest papers of 2013 and onwards to 2022. The summary of results included key findings of the papers, the interpretation of papers reviewed, and the relevant references. Thirty papers were reviewed and selected from authentic databases and assessed that vegetation restoration significantly affects soil organic carbon (SOC). The findings also exhibit that the primary sources of prediction for SOC dynamics include changes in soil properties, quality, the number of carbon inputs, and the composition of the C pool. Vegetation restoration also plays an important role in improving the services of ecosystems such as controlling the erosion of


Abstract
This review paper has been made to assess the past studies reviewed regarding vegetation restoration and its impact on soil organic carbon content. A Vegetation Restoration is an influential technique that can be used to respond to these effects. As a response to the global biodiversity crisis, more restoration actions have been taken. The European Union Council's results on kinds of diversity after 2010 highlight words like stopping biodiversity loss and the breakdown of ecological systems in the European Union. The United Nations Conference on Biological Diversity's growth strategy for 2022, which includes restoring at least 15% of degraded ecosystems, has made this possible. Soil types are among the most vulnerable resources on the planet due to factors such as climate change, land degradation, and the reduction of biodiversity. Organic Carbon, the top meter of soil, could potentially store three times as much carbon as is found in the air and almost twice as much as in plants. For the systematic literature review, past papers on vegetation restoration have been extracted from the latest papers of 2013 and onwards to 2022. The summary of results included key findings of the papers, the interpretation of papers reviewed, and the relevant references. Thirty papers were reviewed and selected from authentic databases and assessed that vegetation restoration significantly affects soil organic carbon (SOC). The findings also exhibit that the primary sources of prediction for SOC dynamics include changes in soil properties, quality, the number of carbon inputs, and the composition of the C pool. Vegetation restoration also plays an important role in improving the services of ecosystems such as controlling the erosion of soil and increasing the carbon sequestration. Moreover, some papers concluded that vegetation restoration positively influences on the SOC. Moreover, to increase the generalizability of the study, implications and future research indications have also been included in the end.

Background
Environmental damage in the last few decades can be linked to the growth of agriculture in many parts of the world and the intensification of farming practices that went along with it. For example, Mehrabi et al. [1] research showed that 14 of the 21 most essential biome types are used in agriculture. In 2011, 491 billion hectares (ha) were used for farming. It is the same as about 38% of the Earth's total surface area [2]. Because of this, the natural plant cover was lost. In addition, more people and more land are being used for farming, which means farming practices are becoming more harmful. A Vegetation Restoration is a powerful tool that can be used to counteract these effects. As a response to the global biodiversity crisis, more restoration actions have been taken. The European Union Council's conclusions on species diversity after 2010 include words like stopping biodiversity loss and the breakdown of ecological systems in the European Union. The United Nations Conference on Biological Diversity's growth strategy for 2022, which includes restoring at least 15% of degraded ecosystems, has made this possible. Even though these policy initiatives are helpful, these understandings raise questions about how well humans can manage and restore ecosystems so that they continue to provide a wide range of ecosystem services and support different kinds of biodiversity [3]. It is often required to choose between, for example, agricultural output that satisfies the demands of society for food and fiber and, for example, the supply of other activities and the preservation of biodiversity [4].
Soil types are among the most vulnerable resources on the planet due to factors such as climate change, land degradation, and the reduction of biodiversity.
There is a greater quantity of carbon stored in the Earth's soils than in the atmosphere or all of the plants that grow on land. It is because soil includes a natural ingredient called organic carbon. According to the soil organic carbon, the top meter of soil could potentially store three times as much carbon as is found in the air and almost twice as much as is found in plants (SOC) [5]. The constant movement of carbon through the world's ecosystems is called the global carbon cycle. Soil organic carbon (SOC) plays a small but essential part in this more significant cycle. More carbon is stored in SOC in the top meter of soil than in the atmosphere (about 800 PgC) and in land plants (around 500 PgC). So, the Soil's Organic Carbon Storage is more than the air and plants on land combined [6]. Soil Organic Carbon Storage is dynamic, moving into and out of different molecular shapes in the many carbon pools worldwide [7]. Small changes in the amount of carbon in the soil can significantly affect the amount of CO 2 in the  [8], approximately fifty-six percent more SOC is stored in the layer of soil that extends from zero to three meters than in the layer of soil that extends from zero to one meter. Only thirty-four percent of the 136 Gt of carbon found in the zero to eight-meter soil profile is located in the zero-to-the-one-meter soil layer. While looking into the gully area, Liu et al. [9] found that almost eighty-five percent of the SOC storage in the zero to the two-meter layer was in the zero to four-meter layer. The effects of vegetation restoration in soil organic carbon are crucial to forest ecosystems in areas like providing nutrients, carbon storage, managing water, building soil structure, and increasing biodiversity. Plants are the primary source of soil organic carbon for soils because they contribute on and below the surface. It also significantly impacts the Earth's temperature, moisture, pH, makeup, and activity levels, among other things [10]. So, in the agriculture field study, it is essential to systematically review the effects of vegetation restoration on soil organic carbon storage. It is possible to gain a better understanding of the patterns of vegetation restoration, soil organic carbon, and the storage of soil organic carbon over the past few years by reviewing several studies conducted on the following topic. Doing so would assist the authors in gaining a better understanding of the patterns that link the variables and vegetation restoration's effects on carbon matter. In addition, by studying previous studies, authors can better differentiate between the many methods that have been utilized in the past to make the vegetation restoration process more efficient in storing soil organic carbon.
Moreover, it will assist in making them comprehend the benefits acquired by employing those approaches in the real world and the downsides of those techniques, which will help them make better decisions in the future. The difficulty is that no systematic review has been presented of the literature, which can clearly explain the direct benefits and loopholes to the experts. It has been a problem since it arose because no such kind of systematic review has been submitted.
The objectives of this study are: to explore reported literature regarding the impact of vegetation restoration on organic carbon content in the soil; to investigate the literature on factors influencing the vegetation restoration; to evaluate the variation of soil organic carbon decomposition rates by vegetation restoration as reported by past studies; to explore the literature reports regarding the observed changes in the proportion of new and old soil organic carbon (SOC) and to report the explored factors controlling SOC.

Significance of the Study
This thorough review will have significant repercussions, not only in the academic realm but also in the broader world. There are very few systematic reviews conducted in the past on topics such as the process of restoring vegetation, the storage of organic carbon in the soil, and the influence that vegetation has on the storage of organic carbon in the ground. Given the circumstances, this indicated will be used in the context of future investigations conducted in this environment. This research will be of great use to anyone who wants to understand more about how the storage of organic carbon in soil and vegetation restoration work together to assist farmers in increasing crop yields. Improving the overall performance of the plants is essential for the whole country because it helps minimize various types of air pollution, which is beneficial for people's health in the long term. It is why improving the overall performance of the plants is vital.
This paper will encourage both environmental companies and the government to take essential steps toward implementing effective ways to help plants grow, which ultimately helps to breathe well in the less polluted country.

Conceptual Boundaries
Despite agriculture's role as a significant source of environmental deterioration, a new study finds that restoring vegetation on farmland can help farmers increase yields while benefiting local wildlife and providing other ecosystem services. It was discovered that this is the case even though agriculture is one of the leading causes of environmental devastation. However, these three benefits would only accrue on the landscape or global scales if vegetation restoration was carried out by soil separation, mainly secondary succession after agriculture abandonment and tree plantations. Moreover, field-level agriculture suffers when this restoration is implemented [11]. On the other side, the fundamental goal of vegetation restoration is to bring back the original mix of species and community structures. The primary purpose of rehabilitation is to "rebuild ecosystems," while the primary goal of restoration is to "repair, processes, production, and services" [12]. Both "reclamation" and "rehabilitation" can be used to talk about putting a piece of land that was once useless back to good use, but "reclamation" is usually used to talk about the process in more detail. Usually, the first step in any rehabilitation or restoration project is revegetation, which means putting plants back on land once covered in trees. Soil Organic Carbon Storage is the process of absorbing carbon dioxide from the atmosphere and storing it in the soil. Photosynthesis, which occurs in plants and results in the storage of carbon as SOC, is the primary way plants contribute to this process. However, even in dry and semi-dry areas, this process extracts carbon from the soil at a much slower rate. As a result, soil carbon dioxide (CO 2 ) can be preserved in inorganic compounds such as secondary carbonates [13]. Another study found that carbon is present in all living things and is required for all life on Earth. Plant biomass, soil organic matter, and carbon dioxide are the most common forms of carbon in nature (CO 2 ). Carbon dioxide (CO 2 ) can exist in both the air and water. The long-term storage of carbon in natural reservoirs such as seas, soils, plants (particularly forests), and rocks is known as carbon storage. Even though the waters carry more carbon, soils contain 75 percent of the world's organic carbon. The volume of the land pool is three times

Review Aims
The main emphasis and focus of the study are to wisely classify and combine the concerning literature to assess the different kinds of factors that can be considered in analyzing the impact of vegetation restoration on soil organic carbon storage. Moreover, the study also intended to identify various factors controlling soil organic carbon, the impact of vegetation restoration on organic carbon content in the soil, factors influencing the vegetation restoration, soil organic carbon decomposition rates by vegetation restoration along with changes in the proportion of new and old soil organic carbon.

Review Design
This review is based on a systematic literature review (SLR). Such a review is considered with the motive of detecting, selecting, and analytically assessing research on a specific topic to ensure the provision of an answer to a clearly expressed question as explained in the previous section [15]. The researcher is comprised on the design of this SLR on the methodology of PRISMA which is the abbreviation of, Preferred Reporting Items for Systematic Reviews and Meta-Analyses that reviews the minimal substances that are accurate to be included in a review or a meta-analysis paper. PRISMA is a commonly known and much-used technique for the writing and reporting of systematic reviews. It assists and supports a clear and transparent recording of research findings in a review.

Review Search Methods
The failure to perform a detailed and comprehensive search for a systematic literature review can be a consequence of biases in the results reported and can be a cause of decreasing authenticity of the decisions of review. [16]. It is advisable and wise to search different databases for extraction of related references. However, this task is thorough and burdensome [17]. To develop an authentic and accurate database selection and search strategy for this review, the researcher has reviewed the databases available to select the ones having the most related papers on the topic of vegetation restoration and soil organic carbon content. After analyzing different databases like SCOPUS, Web of Science, EMBASE, Pro-Quest, Ebsco, ScienceDirect, and Google Scholar. To make the collected data reliable and valid, the experts wrote articles published in various journals and also revised by different experts before the publication, so the data collected was reliable and complete. As for the timeline of the papers collected, the researcher confined it from 2013 and onwards i.e., the research between 2013 and 2020 was viewed more and relevant papers were involved. Moreover, the researcher

Inclusion Exclusion Criteria for Review
The researcher holds detailed research in this review. The papers that are included are mostly quantitative but qualitative were also included as they were English language were also excluded from the data set of those papers which were included in this review.

Search Outcomes of the Review Process
The detailed research to collect relevant papers resulted in the collection of 132 papers. At the end of the process of overviewing and removal of papers that were photocopies or whose full text could not be assessed, 62 papers were left of the initial research 132. All the saved references from the different databases included in the study were then further added to the software which is used in this study for the management of references, i.e. Endnote Version 9.2. Further elimination after the researcher had reviewed the abstracts and titles of papers led to the deletion of 15 articles, leaving a set of 55 articles. The detailed review of the remaining 55 studies took to the dropping off of further 10 studies by the reviewers which were based on the fact that they did not meet the required category of the review. Finally, a set of 45 studies was extracted which was further reduced to 30 papers after full screening and views. Figure 1 given below exhibits the PRISMA flow diagram that is followed in this study which clearly illustrates the data extraction and the steps of selection.

Data Abstraction and Synthesis Techniques
The data extracted for this review comprised of the following information: key findings, interpretation, and concerning references. This data was firstly extracted and then sorted using Excel. The researcher used Nvivo software to read,  code, and mark the research papers for the concerned information extraction and categorization of the papers. The researcher has not achieved meta-analysis in this review as it does not comprise the objectives of the current study and the studies which were included in heterogeneous, due to which a more detailed and thematic analysis approach was measured to be suitable in this study. To assess the study characteristics, the researcher used VosViewer software.

Characteristics of Study
The above table, i.e., Table 1 Figure 2 shows the types of studies that are included in the review. By observing the figure, it can be assessed that more studies were related to experimental type and as compared to that meta-analysis and quantitative studies were very few.

Type of Papers
The papers that were reviewed were based on experimental types.

Impact of Vegetation Restoration on Organic Carbon Content in the Soil
In the study by Gong Li, Liu Gouhua, Wang Meng, Ye Xin, Wang Hao, And Li Zhongshan as referenced in Table 2, it was concluded that vegetation restoration has been projected as an effective tool for increasing both the organic carbon Vegetation restoration has been projected as an effective tool for increasing the organic carbon content.
Significantly higher SOCC was found in natural vegetation restoration as compared to the managed vegetation and tillage land.
Natural vegetation restoration can enhance soil organic carbon (SOC).
The transformation and conversion to planted forests took in greater SOC accumulation as compared to another land usage.
[31]- [36] Factors influencing vegetation restoration Vegetation restoration highly relates to the progress of gully erosion.
Soil organic matter influences vegetation restoration to a greater extent.
Temperature, rainfall, and wind also proved to be important factors.
Soil taken from the densely cultivated land and assessed at a given interval of time proved to be a great positive enhancement factor for vegetation restoration [37]- [42] Factors controlling soil organic carbon Various factors, especially soil, water, and crop management, impact SOC sequestration.
The carters of soil organic carbon storage are more likely to be distinct in significance about factors that are controlling for SOC The primary sources of prediction for SOC dynamics include changes in soil properties, quality, the number of carbon inputs, and the composition of the C pool.
[5] [18] [19] Soil organic carbon (SOC) decomposition rates by vegetation restoration GGP improved the content of SOC. The increased accumulation of organic carbon in the soil led to an increased rate of decomposition of SOC to maintain the balance between the atmospheric CO 2 and C pools in the soil.
The Inorganic N decreases the priming effect, while the organic N is found to have a positive priming effect. The organic inputs are found to be effective for SOC.
The decomposition rate of soil organic carbon shows a positive and significant relationship with total bacterial, microbial, actinomycetes PLFAs and soil enzyme activities. Thus, the enzymatic activity, as well as the structure of the microbial community, are found to be associated with the decomposition rate of SOC.
[20] [21] [22] [23] [24] [43] Changes in proportion of new and old soil organic carbon The incorporation of Carbon is lowered in the upper layer of soil as compared to the depth of the soil as a result of "land use for crops." The soil dynamics, as well as the land use, are found to be dependent on the depth of the soil The vegetation, as well as the characteristics of the soil, play an essential role in increasing the SOC stock, which in return also increases the efflux of CO 2 in the atmosphere.
The fertilization by carbon dioxide and warming is found to significantly impact the stocks of SOC in new soil.

Factors Influencing the Vegetation Restoration
Vegetation

Factors Controlling Soil Organic Carbon
A study by Lal [5] stated that the stock of SOC can be protected by adopting best management practices which are subjected to complex rotations, no soil disturbance, ground cover, and integrated nutrient management. Arunrat et al. [18] were of the view that the density of SOC is higher at high elevations due to low temperature and the high content of clay while chemical fertilizers increase the density of SOC, increasing crop yields. The findings of [19]

Soil Organic Carbon (SOC) Decomposition Rates by Vegetation Restoration
The decomposition rates of SOC are highly influenced by the enzymatic activity and microbial community. To determine this theme, 6 related studies were focused on. One such study stated that the C pools in deep soil are found to be more vulnerable to anthropogenic and environmental change. It also impacts the decomposition of SOC, influencing the net exchange of CO 2 between the atmosphere and the land [20]. Whereas, Luo et al. [21] were of the view that the decomposition of soil organic carbon increases by an average of 14% when inputs of new Carbon are utilized while the forest soils had a low decomposition rate (1%) due to no input of fresh Carbon while other ecosystems were found to have more decomposition rate (>24%). One study also stated that using fertilizers results in nitrogen enrichment in the soil, decreasing microbial activity due to a decrease in fungal biomass and higher accumulation rates of lignin. This decreases the rate of decomposition of SOC thus, increasing the content of SOC in the soil [22]. A study by [23], also stated that different factors, including "the mean annual temperature (MAT), the mean annual precipitation (MAP) and soil temperature, silt and clay fraction, total phosphorus and nitrate nitrogen," are found to impact the microbial community in the soil, which influences the decomposition rate of SOC. Whereas the enzymatic activity is dependent on soil nutrients. Riggs et al. [24] concluded that microbial respiration was found to be decreased by 29% of the overall organic matter due to the addition of nitrogen.  an essential role in this regard while enrichment of Nitrogen leads to increased accumulation of soil carbon. This is due to the suppression of decomposition of soil organic carbon rather than increased carbon inputs.

Changes in the Proportion of New and Old Soil Organic Carbon
This study focused on five themes that mentioned in Table 2. The main findings of these studies showed that fertilization is done by incorporating carbon compounds, the rates of SOC increase compared to the fertilization treatments provided by inorganic fertilizers and manure application is found to impact the C sequestration significantly greatly [25]. According to [26], the changes related to land use and land cover in Mediterranean Soils from 1956 to 2007 resulted in the loss of about 16 × 8 Tg SOC. However, afforestation resulted in increasing C sequestration to 8 × 62 Mg ha −1 in the topsoil. Another study showed that the median SOC stocks were found to be higher in the parkland soils when compared to soils of the urban forest. Contrary to this, the efflux of CO 2 was found to be quite similar in both parkland soils as well as soils of the urban forest [27].
However, [28] was of the view that the fraction of clay plays a crucial role in contributing to 80% of the stocks of SOC. The higher root growth in the subsoil increases SOC stocks. The findings of another study showed that the incorporation of Carbon has a recent depth of 10 cm in the soil globally and the aridity index is used to explain the allocation of SCO in deep soil layers [29]. Hopkins et al. [30] also supported the argument that high carbon input increases overall SOC, also improving the decomposition rate of SOC. Thus, it was concluded that the proportion of SOC is different when soil is fertilized by inorganic fertilizers, and incorporation of Carbon utilizing straw return or manure application and afforestation is found to be effective in increasing the soil organic carbon, whereas deforestation gradually decreases the SOC.

Conclusion
The studies concluded that vegetation restoration plays an integral role in im-

Implications of the Study
This study practically and theoretically ensures a provision of meaningful implication as it is a sound addition in reviewing literature for next researchers who would like to conduct the same study in this context. Furthermore, agricultural sectors can also get practical benefits from such sort of studies which is an opportunity for them to identify the lacking parts or those sections/aspects that need improvement or that hinder the vegetation restoration phases in the path of improving the quality of soil organic carbon along with the changes in the proportion of new and old soil organic carbon.

Limitations of the Study
As this study is comprised mostly of papers that were reviewed of experimental group study type, in future, those reviews can also be made that include papers from quantitative studies as sample and survey techniques in agricultural sectors can also be adopted to study variations in results as if the vegetation restoration's impact on soil organic carbon. Moreover, better and more insightful information can be obtained by testing the study under another context with a different methodology. A longitudinal study would be better suitable to draw results as studying and analyzing the context of soil under various seasons and different periods would help assist the researcher in exploring better aspects and results of vegetation restoration-related topics.