Former ecosystem type and soil organic matter regulate the effects of reforestation on soil bulk density, porosity, electrical conductivity, and texture

Reforestation is globally promoted and significantly impacts soil properties, yet a comprehensive global assessment of its effects on soil physical properties is lacking. This study aimed to assess the global patterns and drivers of reforestation impacts. We analyzed 7890 paired observations from 455 publications across the globe to quantify reforestation’s impact on soil bulk density (BD), porosity, electrical conductivity (EC), and textures. We found that (1) reforestation increased total porosity (TP), capillary porosity (CP), clay content, and silt content by 7.90%, 12.22%, 7.48%, and 6.15%, respectively, but decreased BD and EC by 3.66% and 14.29%, respectively (these effects were stronger in barelands and deserts); (2) reforestation effects were significantly affected by former ecosystem and reforestation patterns, with higher improving effects reforestation in barelands and deserts, and with different combinations of tree, shrub, and/or grass, respectively; (3) latitude, stand age, and the logarithmic response ratio (lnRR) of soil organic matter (SOM) positively influenced TP, CP, clay content, and silt content, but negatively affected BD; and (4) former ecosystem type, lnRR of SOM, stand age, latitude, and stand density are the important moderator variables, although their effects differ across BD, TP, CP, EC, sand content, and silt content. Overall, our quantitative analyses clearly show the effects of reforestation on BD, porosity, EC, and textures at a global scale. This is essential for enhancing our understanding and forecasting the effects of reforestation on soil nutrient retention, structure improvement, water retention, under global change in the future.