**4. Conclusions**

The drastic decrease in soil organic matter in semi-arid soils due to deforestation or in some areas used as deposits for mine tailings are attractive sites for soil restoration. These areas need application of organic residues to avoid subsequent soil erosion by losing soil structure and minimize high risk of pollution in adjacent areas. Application of organic wastes may provide nutrients to pioneering vegetation increasing organic content and improving soil physico‐ chemical and biological properties and thus their natural fertility.

This work provides a clear demonstration of the role of organic waste to increase or release heavy metals according to the quality of organic matter amendment. Recycling valuable components such as C and N available in semi-arid soils was supported by the potential C and N mineralization, dehydrogenase activity, and plant growth. Retention of heavy metals on the fractionation of organic matter cannot be generalized, it will depend on the chemical charac‐ terizations of organic waste and soil. Chemical characteristics of the organic waste such as the nitrogen and humified organic matter content, pH, EC, CEC, ESP (exchangeable sodium percent), and SAR (sodium absorption ratio) are important properties to consider in organic matter amendment to semi-arid soils participating on the complexity and leaching of heavy metals and nutrients in the matrix of soil. Measurements such as heavy metal fractionation, percent of heavy metal mobility, soil biochemical processes, and heavy metals accumulation in roots or translocation to plants give a global picture of the complexity of semi-arid soils amendment with organic waste, either in mine soils with heavy metal content, or when used for organic fertilization to semi-arid soils. Behavior of organic composts or organic waste as organic fertilizer in semi-arid soils will find two directions, one: as the organic matter increases heavy metal immobilization in soil and provides nutrients to plants; two: heavy metals present in organic waste or in semi-arid soils from mine sites will interact with soil microbial activities, plant growth, and heavy metal fractionation. Measurement of infiltration factor in both ecosystems containing high heavy metals and salts concentration under simulated rainfall deserve close attention. Monitoring constantly these types of ecosystems to look for the risk of heavy metal mobility after the season changes, and the potential C and N mineralization and heavy metal stabilization in plants should be a priority in semi-arid soils in process of remediation or reforestation.
