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.