Establishment of Two Ectomycorrhizal Shrub Species in a Semiarid Site after in Situ Amendment with Sugar Beet, Rock Phosphate, and Aspergillus niger

A field experiment was carried out to assess the effectiveness of the addition of sugar beet, rock phosphate, and Aspergillus niger directly into the planting hole, and the mycorrhizal inoculation of seedlings with Scleroderma verrucosum, for promotion of plant growth of Cistus albidus L. and Quercus coccifera L. and enhancement of soil physicochemical, biochemical, and biological properties, in a degraded semiarid Mediterranean area. One year after planting, the available phosphorus content in the amended soils of both species was about fourfold higher than in the nonamended soil. Amendment addition increased the aggregate stability of the rhizosphere of C. albidus (by 56% with respect to control soil) while the mycorrhizal inoculation increased only the aggregate stability of the rhizosphere of Q. coccifera (by 13% with respect to control soil). Biomass C content and enzyme activities (dehydrogenase, urease, protease-BAA, acid phosphatase, and -glucosidase) of the rhizosphere of C. albidus were increased by amendment addition but not by mycorrhizal inoculation. Both treatments increased enzyme activities of the rhizosphere of Q. coccifera. The mycorrhizal inoculation of the seedlings with S. verrucosum was the most effective treatment for stimulating the growth of C. albidus (by 469% with respect to control plants) and Q. coccifera (by 74% with respect to control plants). The combined treatment, involving mycorrhizal inoculation of seedlings and addition of the amendment directly into soil, had no additive effect on the growth of either shrub species.