Improving the soil health before native plant reintroduction: a pipeline for the restoration of plant habitats.

Soils are the fundament of terrestrial ecosystems, playing a key role in habitat formation and maintenance. Soils host a huge variety of life forms (bacteria, fungi, protozoa, and animals), many of which establish close relationships with plants, the most represented taxa in soil. The interactions between plant roots and soil create favorable physical and chemical conditions, which are essential for both plant growth and reproduction, and for soil health maintenance. Such a relationship depends on the plant community composition, and environment disturbance may easily bring to the disruption of such an equilibrium. Plant conservation is tightly connected with the conservation of habitats, so the success of restoration actions largely depends on the restoration of a rich and diverse belowground life, which in turn is sustained by a healthy soil in terms of mineral nutrients, organic matter, and bearable presence of xenobiotic substances. Actions for an efficient plant conservation should thus start from the requalification of soil health. The maintenance or restoration of over-exploited soils according to ecological principles has been recognized a pillar for future agriculture and large efforts have been dedicated to developing sustainable soil and crop management practices and identifying the most appropriate and measured inputs of either organic or mineral fertilizers. The European Soil Mission defines ‘healthy’ the soil with a continued capacity to support ecosystem services and indicates the improvement of soil structure and concentration of the stable soil organic carbon as necessary to enhance habitat quality for soil biota and crops. The restoration of soil in natural ecosystems pose special challenges, because of the need to limit soil disturbance, to reduce the input of xenobiotic and to avoid or limit the introduction of alien organisms, primary plants, which could become direct competitors of the target species or change habitat conditions. We present here a pipeline of actions developed for the establishment of native trees on a degraded sandy soil, which starts from the restoration of soil health and the establishment of a ground vegetation cover prior the transplanting of target shrubs and trees. Preliminary actions should assess the soil physical, chemical, and biological properties, and the abundance and composition of the soil seed bank, in order to obtain information on nutrient availability and recruitment potential. In addition, the potential vegetation should be assessed through literature, when available, or the floristic inventory of close areas. As degraded soils generally lack in organic matter and the soil seed bank is poor or contains undesired ruderal and alien species, external inputs are often necessary. In natural ecosystems, soil amendment with manure is not to recommend because it may contain foreign seeds. Thus, better options are either litter and composted vegetation debris collected in close areas, or inert products like biochar. In case the seed bank is not sufficient for recruiting a ground cover vegetation, artificial seeding should be performed with seeds of species collected in the surroundings, preferably grass and legume mixtures, because the former reduce erosion and leaching, and the latter increase soil nitrogen through biological N2-fixation. To foster symbiosis, seeds should be mixed with the soil collected where legume species were growing. The transplanting of shrubs and finally trees should proceed once an herbaceous ground cover has established. To spare time and to overcome difficulties in seed collection, the first ground cover could be obtained also with commercial mixtures of annual species, which should, however, be overthrown before seed production, thus improving the input of organic matter into the soil, without the risk of introducing foreign seeds. This pipeline demonstrates that, because of the huge complexity of ecosystems, the conservation of plants within their natural habitats needs a multidisciplinary approach, and the cooperation with the agricultural sector is essential because of its deep know-how in the techniques for soil health improvement and plant management. Within this framework, the SHARIng-Med project (PRIMA foundation) may provide a useful tool to estimate the level of soil degradation and guide the steps of restoration, as it has the objective of developing a standardized database of soil information for the Mediterranean area which integrates data on soil and land management with environmental indicators.