2.2. Experimental design
Two types of organic wastes used as organic amendments (compost from urban organic solid wastes and sewage sludge from urban wastewater) were added to two types of substrates (mudstone and topsoil), within the top 15 cm substrates, and not only into planting holes as done in some large forest restoration plans (e.g., Junta de Andalucía, 1996) and in some recent experiments on mine tailings (e.g., Bindang-Oná et al., 2021).
Moreover, woodchips from silvicultural thinning treatments in a nearby pine forest (Pinus halepensis ) were used as mulch. Three autochthonous species in seedling format, (M. tenacissima ,A. terniflora, and A. cytisoides ), were planted. These native plants are keystone elements for ecosystem restoration, providing numerous benefits such as to accelerate secondary succession and ecological recovery.
Six 5 m x 15 m plots (eighteen 5 m x 5 m subplots as repetitions) were equally split over two different substrates: a) marl outcrop with 11.6o slope gradient, and b) topsoil with 15o slope gradient. The treatments, different technosols, applied equally on both substrates, consisted in two organic amendments (compost from urban organic solid waste and sewage sludge from municipal waste-water treatment plant) plus a control without organic waste in a factorial design (3 x 2). The stored topsoil had been spread on the surface of 3 plots (9 subplots) just after finishing the mining activities, about 10 years before this experiment. The sewage sludge was added at a rate of 28 kg m-2 (total organic C = 351.5 g kg-1 and moisture = 59%). The compost was applied at 34 kg m-2 (Total organic C = 196.5 g kg-1 and moisture = 40%). The added amount was calculated in order the technosols had 2% of organic matter content. Compost and sewage sludge were incorporated and homogenized into the top 15 cm of the soil using a mechanical backhoe. The heavy metals contents in both sewage sludge and compost were below the limits established by both the EU Legislation (Official Journal of the European Union, Directives 2018/851 and 2011/92) and the Regional legislation (BOJA, 2015) for contaminated soils.
Subsequent to the application of the organic amendments, woodchip mulch was spread out on the plots with a thickness of 5 cm. Average pine chips measured 1-5 cm wide, 2-10 cm long, and 1 cm thick (Fig. 1). Finally, 75 native plants (35 M. tenacissima, 15 A. terniflora and 25A. cytisoides ) were planted in each experimental plot and some results about these plants were published in Luna et al. (2017, 2018) (Fig. 2).