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).