Afforestation of degraded soils with Pinus halepensis Mill.: Effects of inoculation with selected microorganisms and soil amendment on plant growth, rhizospheric microbial activity and ectomycorrhizal formation

Two techniques commonly used in soil restoration: (1) inoculation with selected microbial strains and (2) soil amendment with urban waste, were evaluated for afforestation of a degraded gypsiferous soil with Pinus halepensis Miller. One year after planting, survival of pines was not affected by any of these techniques, reaching values over 97% in all cases. Inoculation with two PGPR strains previously proven to enhance Pinus spp. growth, or with an ectomycorrhizal fungus separately did not affect pine growth or the nutrient content in needles. However, significant increments of pine growth due to soil amendment were detected. Most of the nutrients analysed in needles were found in higher amounts in plants grown in non-amended soil, except nitrogen that was higher in plants grown in the amended soil. The rhizospheric microbial activity was highly increased by soil amendment whereas inoculation with the different microorganisms had no effect on this parameter. The amendment of soil with urban waste drastically reduced the percentages of ectomycorrhizas on the pine roots and affected the frequency of the described ectomycorrhizal morphotypes compared with plants grown in the non-amended soil. Our results suggest that before undertaking a strategy for soil restoration, like plant inoculations with selected microorganisms or soil amendments, the autochthonous beneficial bacterial and fungal communities in the soil should be taken into account, as well as studies for adjusting the doses of amendments, to assure the establishment and development of plants and to keep the ecological equilibrium of soil.