Soil organic carbon (SOC) is the largest component of the terrestrial biosphere carbon pool. Afforestation is an effective solution to mitigate Carbon (C) emission and sequester C into soils. However, how and to which extent afforestation influences SOC stock changes are not well understood. This study conducts a quantitative review that synthesizes 544 data points from 261 sites from 90 papers, to examine the impact of afforestation on SOC changes in three soil layers (0-20 cm, 20-40 cm and 40-60 cm). 212 data points are obtained by standardization and/ or extrapolationwith high reliability. The results indicate that stand age has significant effects on the SOC stock dynamics under different conditions of previous land use types, plant functional types, temperature or precipitation. The effect is greatest at the topsoil layer of 0-20 cm. Previous land use types significantly influence SOC accumulations, but these effects are not significant in the first 10 years or after 30 years of afforestation. Besides, afforestation on grassland seems to sequester more SOC than that of cropland in the long term. Plant functional types also significantly affect SOC dynamics, with deciduous hardwood reporting a continuous increase of SOC contents at soil depth of 0-60 cm during the whole afforestation period. On the other hand, the accumulation of SOC in evergreen hardwood and evergreen softwood start from the third decades. Higher SOC accumulation rates are observed under evergreen hardwood but no significant differences were found between deciduous hardwood and evergreen softwood for the longer period after afforestation (>20 years). Mean annual temperature and precipitation negatively affect SOC accumulation in the first two decades of afforestation, however, the effects become positive in the later years.We also found that initial SOC stocks did not play a major role in SOC sequestration. In other words, lower SOC soils could also sequestrate a significant amount of SOC after reforestation.

The sensitivity of vegetation productivity to precipitation (S ppt ) is a key metric for understanding the variations in vegetation productivity under changing precipitation and predicting future changes in ecosystem functions. However, a comprehensive assessment of S ppt over all the global land is lacking. Here, we investigated spatial patterns and temporal changes of S ppt across the global land from 2001 to 2018 with multiple streams of satellite observations. We found consistent spatial patterns of S ppt with different satellite products: S ppt was highest in dry regions while low in humid regions. Grassland and shrubland showed the highest S ppt , and evergreen needle-leaf forest and wetland showed the lowest. Temporally, S ppt showed a generally declining trend over the past two decades (p < .05), yet with clear spatial heterogeneities. The decline in S ppt was especially noticeable in North America and Europe, likely due to the increase in precipitation. In central Russia and Australia, however, S ppt showed an increasing trend. Biome-wise, most ecosystem types exhibited significant decrease in S ppt , while grassland, evergreen broadleaf forest, and mixed forest showed slight increases or non-significant changes in S ppt . Our finding of the overall decline in S ppt implies a potential stabilization mechanism for ecosystem productivity under climate change. However, the revealed S ppt increase for some regions and ecosystem types, in particular global grasslands, suggests that grasslands might be increasingly vulnerable to climatic variability with continuing global climate change.

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& Key message Different tree species have dissimilar capacities to sequester soil organic carbon (SOC). Deciduous broadleaved trees show the most stable increase in SOC stock after afforestation than other tree species, while sempervirent conifer trees show the lowest rate of SOC stock change. Sempervirent broadleaved trees show the greatest increase in SOC stock 20 years after afforestation. & Context The rate at which soil organic carbon (SOC) stock changes after afforestation varies considerably with the tree species. A better understanding of the role of tree species in SOC change dynamic is needed to evaluate the SOC sequestration potential of afforestation programs. & Aims The aim of this paper is to identify the dissimilar rates at which different tree species sequester SOC, following afforestation. & Methods We complete a meta-analysis with 544 data points from 261 sites in 90 papers. We group tree species into decidious broadleved, sempervirent broadleaved and sempervirent conifer. We use standardization and/or extrapolation methods to standardize soil depths. Statistical analysis test the main effects of tree species and their interactions with previous land use and plantation age on SOC stock change after afforestation. & Results Deciduous broadleaved trees show a stable increase in SOC stock, and are especially suited for afforestation of grassland or soils with high initial SOC. Sempervirent broadleaved afforestation results in loss of SOC stock in young stands, but greater SOC stock in mature stands. Sempervirent conifer trees show the lowest rate of SOC stock change, but are suitable for nutrient-poor soil. & Conclusion The results emphasize the importance of considering tree species when estimating SOC stock change, in particular when carbon sequestration is an objective of afforestation programs.

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