Interactions between soil and plants in halophile systems: plant zonation and effects on pedogenesis

The increasing interest around the protection of coastal wetlands highlighted the scarce knowledge achieved about these systems and the consequent inability to operate efficiently during conservation and restoration programs. One crucial issue is related to saltmarshes, whose functioning appears highly complex and where a strong connection between soil and halophytes drives the definition of the main characteristics. For its complexity, this thesis work used a holistic approach with the aim to evaluate the main driving forces involved in the definition of saltmarsh functions. It focused on the effect of flooding, soil features and plant-soil feedback mechanisms. Following a flooding gradient, the soil was described using a pedological approach and plant communities were analysed though vegetation surveys. Vegetation was considered a consequence of different soil features as well as a primary factor affecting pedogenesis. In fact, some species are able to release oxygen from roots, as suggested by the red-mottles present along soil profiles, and the litter deposition contributes to soil organic carbon. This multidisciplinary approach entailed also the evaluation of the uptake ability of different plant species for the main macro-nutrients. Therefore, vegetation revealed itself a great tool also to describe soil hydric and nutritional conditions, where the nutritional status varied within plant communities. Flooding has a negative effect on soil redox status and on plant growth. Nevertheless, plant community richness seemed only indirectly influenced by flooding. A combination of multiple variables, tested in a structural equation model, returned a high influence of plants in mitigating the negative effect of flooding on soil and in shaping plant communities. Halophytes are plants extremely well-adapted to flooding involving several different strategies. Internal tissue aeration is the most common strategy based on the presence of high tissues porosity. The role of internal plant aeration was investigated in the halophytes Limonium narbonense and Sarcocornia fruticosa during submergence. Internal aeration seemed relevant only for S. fruticosa, which benefitted from snorkelling, transporting oxygen from green shoots to submerged tissues. L. narbonense flooding tolerance seemed instead more complex: the high tissue porosity did not support aeration even during partial submergence. These achievements suggested that other processes could be even more important when under water, e.g. fermentative processes supporting plant metabolism during anoxia. In addition to anoxia, saltmarsh soils suffer considerably from the accumulation of sulphides. Species like L. narbonense, able to alter the redox conditions in the rhizosphere, seemed however not capable to diminish sulphides concentrations in soils, probably due to the restricted radial area of oxygen diffusion from roots. Sulphides are toxic for plants and for this reason can interfere with zonation but, few works investigated this aspect and adequate field methods are not available, especially for the determination of the acid volatile sulphides (AVS), the most bioavailable fraction. For this reason, a quick field colorimetric procedure for a semi-quantitative determination of AVS was assessed. Sulphides evolved from acidification as H2S were trapped in a paper strip previously treated, precipitating as lead sulphide. The colour of the strip corresponded to the amount of sulphides in the sample, which was determined through comparison with reference charts. This highly reproducible method can benefit from future implementation of smart-phone apps that, through image analysis, will enable to determine sulphides concentration instantly. A deeper knowledge of these mechanisms appears nowadays essential for understanding the functioning of saltmarsh systems, especially regarding the alarming increase of the sea level and the more frequent flooding events forecasted in the next future