Effectiveness of the Application of Arbuscular Mycorrhiza Fungi and Organic Amendments to Improve Soil Quality and Plant Performance under Stress Conditions

Plant growth is limited in arid and/or contaminated sites due to the adverse conditions coming from heavy metal (HM) contamination and/or water stress. Moreover, soils from these areas are generally characterised by poor soil structure, low water-holding capacity, lack of organic matter and nutrient deficiency. In order to carry out a successful re-afforestation, it is necessary to improve soil quality and the ability of plants species to resist this harsh environment. The symbiosis with arbuscular mycorrhizal (AM) fungi has been proposed as one of the mechanisms of plant heavy metal tolerance and water stress avoidance. On the other hand, addition of organic amendments to the soil can reverse degradation of soil properties. Agro-waste residues such as dry olive cake (DOC) and sugar beet waste (SB) supplemented with rock phosphate (RP) can be used as organic amendments after fermentation by Aspergillus niger. The application of A. nigertreated DOC and/or SB to semi-arid soils and/or HM-contaminated soils increased aggregate stability, soil enzymatic activities, water soluble C and water soluble carbohydrates as well as nutrient availability, especially P. AM inoculation, using adapted endophytes, was more efficient with respect to increasing plant nutrition and growth as well as plant tolerance to drought or HM-stress conditions. The combined treatments involving mycorrhiza fungi inoculation and addition of the amendments into the soil can be proposed as a successful revegetation strategy for plant performance in P-deficient soils under semiarid Mediterranean conditions. The beneficial effectiveness of this symbiosis with suitable AM fungi in A. niger-treated agro-waste residue-amended soil can also be regarded as a successful biotechnological tool for reclamation of HM-contaminated soils.