Marine-derived bioproducts are increasingly proposed as sustainable soil amendments influencing nutrient availability, including nitrogen which is essential for plant nutrition and development, microbial functioning, and soil quality. The study aimed to identify which marine-derived bioproduct most effectively promoted soil nitrogen availability and to determine whether these changes would reflect in shifts in nitrogen-cycling microbial functional groups, consequently, affecting lettuce growth and functional traits. Therefore, an agricultural soil was amended at 1% (w/w) with the exopolysaccharide produced by Geobacillus thermodenitrificans B3-72 (EPS), the biosurfactant produced by Bacillus sp s7s-1 (BS), cyanobacterial biomass from Leptolyngbya sp. (BC), and macroalgal biomass from Ulva ohnoi (BM), unamended soil served as the control. Bulk-soil and Lactuca sativa-planted mesocosms were maintained for 60 days under controlled conditions. Soil nitrogen forms concentrations (total N, NH₄⁺-N, NO₃⁻-N), DNA yield, eubacteria abundance, and genes associated with N fixation (nifH), ammonia oxidation (bacterial amoB and archaeal amoA), and denitrification (nirK) were quantified. Plant shoot and roots biomasses, rosette diameter and leaf functional traits were also assessed. The results showed that, in bulk-soil mesocosms all the amendments increased soil NO₃⁻-N; BS and BC increased soil total N and NH₄⁺-N; BC increased DNA yield and total bacterial abundance; BS and BM increased nifH abundance. In planted mesocosm, the amendments reduced the increase of soil NH₄⁺-N concentrations but BS and BC further increased soil NO₃⁻-N concentrations, suggesting that the presence of Lactuca sativa L. exerted a stronger effect on nitrogen-cycling microorganisms than that observed with the only addition of amendments, largely masking treatment-specific responses. EPS produced the broadest positive response on plant growth, whereas BS resulted in the lowest shoot and root biomass and rosette diameter. Overall, EPS was the most promising amendment for plant growth, while BC provided the most balanced response across soil N status and microbial abundance.