Influence of aqueous phase from hydrothermal carbonization (AHL) of Sargassum muticum (Phaeophyceae) on germination and growth of Phaseolus vulgaris (Fabaceae)

In the last twenty years, climate change has severely affected agricultural productivity. Socio-economic effects of loss in crops yield are expected to be exacerbated in the next future, also as a result of the forecasted increase in population growth and, consequently, in food requirement. Thus, finding and/or fine-tuning practices limiting climate-change-driven loss of crop yield, aiming at circular economy and sustainable agriculture, is very urgent. Macroalgae are often used as plant fertilizer and/or biostimulant and provide also numerous molecules used in industry, including hydrochar by thermo-chemical process called hydrothermal carbonization (HTC). In the last few years, HTC has been reconsidered as an actual alternative to process wet biomass into a value-added products. Unlike conventional dry thermochemical processes (e.g., combustion, pyrolysis, and gasification), HTC does not require an expensive or energy intensive preliminary drying step, as it directly exploits the water retained in digestate as a solvent during the process. In particular, the HTC process takes place between 180 °C and 300 °C and 10 bars and 80 bars, with residence time ranging from few minutes to several hours. The HTC process converts wet feedstock into a carbonaceous solid fraction (hydrochar) and a liquid phase termed aqueous HTC liquid (AHL). AHL phase is often discarded in carbonization process and revaluation will be an incentive to use HTC in the practical problems of resource management. Wet biomass may be derived from macroalgal species and some studies have been carried out about chemical characterization and applications on algal hydrochar but not on AHL. Macroalgae may be selected from anthropized environment, where they produce abundant biomass that negatively affects human activities and, in part, also other organisms, so they must be dredged and disposed of in some way. The aim of the present work is to use AHL deriving from a process of HTC in Sargassum muticum to probe their potential effects on plant productivity of Phaseolus vulgaris. Different HTC protocols were tested, differentiating temperatures, to obtain slightly dissimilar hydrochar. AHL obtained from all different HTC protocols were supplied to seeds of Phaseolus vulgaris and germination rate and plant growth were recorded. To exclude a mere osmotic influence, isotonic solutions were also tested in parallel. Growth as well as water relations were monitored until fruits were developed. Among results, seed germination of about 80% was recorded in all studied treatments and it could be noted that 180 °C AHL seed priming treatment induced higher productivity in P. vulgaris, in terms of higher photosynthesis rate, as supported, likely, by higher stomatal density values, together with higher shoot/root ratio, as driven by higher whole leaf area and stem biomass. Nevertheless, it can be noted that plants treated with 240 °C AHL showed the higher root dry mass value and, as a consequence, the lower root/shoot ratio values, but higher photosynthesis rate than control samples. Plant treated with 300 °C AHL showed higher leaf mass area values respect to all treated samples. Further studies need to be performed to investigate the effect of these discarded solutions in plants growing in mild water stress, to explore their possible use for triggering plant water stress resistance and obtain high productivity at low water demand. Overall, our findings suggest a promising use of AHL in agriculture in the framework of resource management and circular green economy.