4.2. Effects of soil treatments on vegetation
The early establishment of spontaneous species in the restored sites had
some impact on both the restored soil and the planted vegetation. These
species (e.g., Moricandia foetida, Thymelaea hirsuta, Ulex
parviflorus, Artemisia barrilieri, Dittrichia viscosa ) quickly
colonized disturbed areas as soon as a minimal improvement in edaphic
conditions occurred, as reported in similar environments by Soliveres et
al. (2012). In some cases, the spontaneous species coverage exceeded
that of the native planted species (table 2). For this reason, it was
necessary to study the possible effects of the soil treatments on the
total plant community. It is well known that changes in edaphic
conditions in combination with the competitive characteristics of some
plant species can facilitate conditions leading to alternative
ecological outcomes among rehabilitated ecosystems (Audet et al., 2013).
Total plant cover in the treatments without organic amendments was much
lower than on the amended soils; however, pine cover percentage was
greater than on the substrate where no organic amendment had been
applied (table 2). Somehow, woodchip mulch must have improved the soil
conditions as has also been reported by other authors (Fristensky and
Grismer, 2009; Hueso-González et al., 2016; Yin et al., 2015).
Organic amendments did influence the whole plant community, which means
the external nutrient inputs can be important drivers of changes of
species diversity. After 6 years, the planted native species reached
greater cover with the sludge treatment, despite the fact that the
organic matter content in this treatment was considerably lower than
that of the compost treatment. Nevertheless there are several factors
that were able to affect the response of the planted species. On the one
hand, the level and nature of benefit of organic amendments depend on
both the quantity (carbon loading) and quality (how easily OM is
mineralized, s.l.) of the total OM applied (Bhogal et al., 2018). Higher
TOC and N values from compost with regards sewage sludge can be ascribed
to the specific chemical composition of these organic amendments (Tejada
et al., 2006). Sewage sludge has more labile organic matter and
consequently microorganisms favour its initial mineralization and
provides readily available nutrients to plants (Hahn and Quideau 2013;
Carabassa et al. 2018). On the other hand, the high values of electrical
conductivity in the compost treatment could be a handicap for this type
of vegetation at the beginning of the experiment (Scotti et al., 2016).
However, spontaneous species covered a large percentage the experimental
plots where salinity was high, up to 65% in the treatment with compost
over topsoil. These species have an opportunistic strategy,
characteristic of severely disturbed but potentially productive habitats
(Grime, 1977).
The number of pines and their cover showed a different response to the
soil fertility depending on the type of substrate: marl and topsoil
(Fig. 6). The results indicate that factors other than organic matter
content of the soil could influence, such as competition for light and
space (Hess et al., 2019). As expected, inputs of organic material
improved pine growth, reaching better height with compost treatments and
consequently higher nutrient content. The increase in the slenderness
(Fig 5) coefficient under the compost can be interpreted as an indicator
of competition for light between woody seedlings and herbaceous
vegetation (Eliason & Allen 1997; Soliveres et al. 2010).
Other authors (Barberá et al., 2005; Ortiz et al., 2012), also found
organic amendments as the most effective growth enhancers for pine
reforestation in Mediterranean degraded ecosystems. Fuentes et al.
(2010) indicated that low to moderate doses of sewage sludge, similar to
those used in this research, showed the best combination of seedling
survival and growth promoting P. halepensis establishment in dry
Mediterranean areas. Similar effects had also been reported by Roldán et
al. (1996) with 60 and 100 Mg ha-1 of urban solid
refuses in a similar environment.