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Anthropogenic activities profoundly impact soil organic carbon (SOC), affecting its contribution to ecosystem services such as climate regulation. Here, we conducted a thorough review of the impacts of land-use change, land management, and climate change on SOC. Using second-order meta-analysis, we synthesized findings from 230 first-order meta-analyses comprising over 25,000 primary studies. We show that (i) land conversion for crop production leads to high SOC loss, that can be partially restored through land management practices, particularly by introducing trees and incorporating exogenous carbon in the form of biochar or organic amendments, (ii) land management practices that are implemented in forests generally result in depletion of SOC, and (iii) indirect effects of climate change, such as through wildfires, have a greater impact on SOC than direct climate change effects (e.g., from rising temperatures). The findings of our study provide strong evidence to assist decision-makers in safeguarding SOC stocks and promoting land management practices for SOC restoration. Furthermore, they serve as a crucial research roadmap, identifying areas that require attention to fill the knowledge gaps concerning the factors driving changes in SOC. Over 25,000 synthesized experiments identify effective land management practices, such as tree planting and biochar use, that partially mitigate land-use change effects. Yet, indirect climate change effects still pose a potential severe threat to SOC globally [ABSTRACT FROM AUTHOR]

Increasing soil organic carbon (SOC) in natural and cultivated ecosystems is proposed as a natural climate solution to limit global warming. SOC dynamics is driven by numerous factors such as land-use change, land management and climate change. The amount of additional carbon potentially stored in the soil is the subject of much debate in the scientific community. We present a global database compiling the results of 217 meta-analyses analyzing the effects of land management, land-use change and climate change on SOC. We report a total of 15,857 effect sizes, 6,550 directly related to soil carbon, and 9,307 related to other associated soil or plant variables. The database further synthesizes results of 13,632 unique primary studies across more than 150 countries that were used in the meta-analyses. Meta-analyses and their effect sizes and were classified by type of intervention and land use, outcomes, country and region. This database helps to understand the drivers of SOC sequestration, the associated co-benefits and potential drawbacks, and is a useful tool to guide future global climate change policies. Measurement(s) soil organic carbon Technology Type(s) systematic literature review Factor Type(s) land-use change •land management • climate change Sample Characteristic - Environment anthropogenic environment • natural environment Sample Characteristic - Location global [ABSTRACT FROM AUTHOR]

Major drivers of gains or losses in soil organic carbon (SOC) include land management, land‐use change, and climate change. Thousands of original studies have focused on these drivers of SOC change and are now compiled in a growing number of meta‐analyses. To critically assess the research efforts in this domain, we retrieved and characterized 192 meta‐analyses of SOC stocks or concentrations. These meta‐analyses comprise more than 13,200 original studies conducted from 1910 to 2020 in 150 countries. First, we show that, despite a growing number of studies over time, the geographical coverage of studies is limited. For example, the effect of land management, land‐use change, and climate change on SOC has been only occasionally studied in North and Central Africa, and in the Middle East and Central Asia. Second, the meta‐analyses investigated a limited number of land management practices, mostly mineral fertilization, organic amendments, and tillage. Third, the meta‐analyses demonstrated relatively low quality and transparency. Lastly, we discuss the mismatch between the increasing number of studies and the need for more local, reusable, and diversified knowledge on how to preserve high SOC stocks or restore depleted SOC stocks. [ABSTRACT FROM AUTHOR]