Sargassum-Derived Biochar Affects Lettuce Growth and Promotes Sodium Accumulation in Plant Tissues

This study investigated biochar production from stranded Sargassum biomass and its effects on lettuce (Lactuca sativa L.) growth and foliar elemental composition. Biochars were produced by laboratory-scale pyrolysis at 400, 500, and 600 °C and using an artisanal double-drum kiln. The resulting materials were alkaline and mineral-rich, with ash contents of 24.6–42.3 wt% and pH values of 8.4–8.7. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS) revealed structural changes after pyrolysis, including the development of rough, porous surfaces. The SEM/EDS semiquantitative analysis indicated relatively high proportions of Na (4.55 wt%), K (4.31 wt%), Ca (2.46 wt%), and Mg (1.75 wt%) in the double-drum biochar, consistent with the mineral-rich composition of the original marine biomass. Lettuce responses varied with biochar application rate. Lettuce response depended strongly on biochar application rate. The 1 t ha−1 rate did not significantly affect plant growth, whereas 12 t ha−1 reduced shoot fresh mass by approximately 45% and decreased plant diameter, rosette diameter, leaf number, and leaf area. Foliar Na concentration increased from 4.95 in the control to 11.50 mg kg−1 at 12 t ha−1, while the Na/K ratio increased from 3.88 to 9.91. The highest application rate was therefore characterized by increased foliar Na and Na/K ratio together with reduced plant growth. These findings highlight the potential of Sargassum-derived biochar for biomass valorization and mineral recovery, while indicating that its high sodium content warrants careful consideration of application rates for lettuce cultivation.