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  Carbon storage in soils

  Soils are a pivotal component in the global carbon cycle, while carbon storage in soils is a natural phenomenon involving organic carbon. Maintaining or increasing soil carbon levels is beneficial for many ecosystem services. Soil carbon is also a soil condition indicator and a key focus of several Sustainable Development Goals. This chapter describes the forms of carbon in soils, the quantification of carbon stocks and storage, the processes underlying the heterogeneous distribution of carbon stocks across the planet and their dynamics, land-use changes and practices that affect soil carbon stocks, as well as the socioeconomic benefits of soil carbon storage.
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  Effects of land clearing for agriculture on soil organic carbon stocks in drylands: a meta-analysis

  Agricultural activities have been expanding globally with the pressure to provide food security to the earth’s growing population. These agricultural activities have profoundly impacted soil organic carbon (SOC) stocks in global drylands. However, the effects of clearing natural ecosystems for cropland (CNEC) on SOC are uncertain. To improve our understanding of carbon emissions and sequestration under different land uses, it is necessary to characterize the response patterns of SOC stocks to different types of CNEC. We conducted a meta-analysis with mixed-effect model based on 873 paired observations of SOC in croplands and adjacent natural ecosystems from 159 individual studies in global drylands. Our results indicate that CNEC significantly (P < 0.05) affects SOC stocks, resulting from a combination of natural land clearing, cropland management practices (fertilizer application, crop species, cultivation duration) and the significant negative effects of initial SOC stocks. Increases in SOC stocks (in 1m depth) were found in croplands which previously natural land (deserts and shrublands) had low SOC stocks, and the increases were 278.86% (95% confidence interval, 196.43–361.29%) and 45.38% (26.53–62.23%), respectively. In contrast, SOC stocks (in 1m depth) decreased by 24.11% (18.38–29.85%) and 10.70% (1.80–19.59%) in clearing forests and grasslands for cropland, respectively. We also established the general response curves of SOC stocks change to increasing cultivation duration, which is crucial for accurately estimating regional carbon dynamics following CNEC. SOC stocks increased significantly (P < 0.05) with high long-term fertilizer consumption in cleared grasslands with low initial SOC stocks (about 27.2 M g/ha). The results derived from our meta-analysis could be used for refining the estimation of dryland carbon dynamics and developing SOC sequestration strategies to achieve the removal of CO2 from the atmosphere.
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  Effects of land clearing for agriculture on soil organic carbon stocks in drylands: a meta-analysis

  Agricultural activities have been expanding globally with the pressure to provide food security to the earth’s growing population. These agricultural activities have profoundly impacted soil organic carbon (SOC) stocks in global drylands. However, the effects of clearing natural ecosystems for cropland (CNEC) on SOC are uncertain. To improve our understanding of carbon emissions and sequestration under different land uses, it is necessary to characterize the response patterns of SOC stocks to different types of CNEC. We conducted a meta-analysis with mixed-effect model based on 873 paired observations of SOC in croplands and adjacent natural ecosystems from 159 individual studies in global drylands. Our results indicate that CNEC significantly (P < 0.05) affects SOC stocks, resulting from a combination of natural land clearing, cropland management practices (fertilizer application, crop species, cultivation duration) and the significant negative effects of initial SOC stocks. Increases in SOC stocks (in 1m depth) were found in croplands which previously natural land (deserts and shrublands) had low SOC stocks, and the increases were 278.86% (95% confidence interval, 196.43–361.29%) and 45.38% (26.53–62.23%), respectively. In contrast, SOC stocks (in 1m depth) decreased by 24.11% (18.38–29.85%) and 10.70% (1.80–19.59%) in clearing forests and grasslands for cropland, respectively. We also established the general response curves of SOC stocks change to increasing cultivation duration, which is crucial for accurately estimating regional carbon dynamics following CNEC. SOC stocks increased significantly (P < 0.05) with high long-term fertilizer consumption in cleared grasslands with low initial SOC stocks (about 27.2 M g/ha). The results derived from our meta-analysis could be used for refining the estimation of dryland carbon dynamics and developing SOC sequestration strategies to achieve the removal of CO2 from the atmosphere.
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  Carbon sequestration potential through conservation agriculture in Africa has been largely overestimated: Comment on: “Meta-analysis on carbon sequestration through conservation agriculture in Africa”

  Soil organic carbon (SOC) sequestration depends on several factors,including land use, pedo-climatic conditions, topographic position andthe initial SOC stock (Post and Kwon, 2000; Minasny et al., 2017). Atthe plot scale, a positive SOC balance is created by increasing the inputof organic matter to the soil to exceed the carbon (C) losses by miner-alization, leaching and erosion or by decreasing the rate of SOC de-composition. In Africa, agricultural soils are generally known to havepotential as a C sink due to previous SOC depletion (Vågen et al., 2005;Swanepoel et al., 2016). Two widely promoted crop managementpractices to store C in agricultural soils are conservation agriculture(CA) and agroforestry. Both practices can increase SOC through in-creased C inputs from higher biomass productivity and reduced C losses(through soil cover and reduced soil tillage), leading to a net transfer ofC from the atmosphere to the soil, thus contributing to the mitigation ofclimate change (Smith et al., 2005;Powlson et al., 2011; Griscom et al.,2017).