Assessing the potential of sea-derived biopolymers and biomasses for soil quality improvement in soils with and without Lactucasativa.

Although many sustainable solutions have been tested to improve soil quality, there is very limited data about bio-based soil amendments derived from marine sources. In this study, the effect of amendment with four sea-derived biopolymers/biomasses on soil physical, chemical and biological properties was assayed at 1% w/w in soil pots with and without (bare soil) Lactuca sativa L. (lettuce). Specifically, the exopolysaccharide produced by Geobacillus thermodenitrificans (EPS), the biosurfactant from the thermophilic Bacillus licheniformis s7s-1 (BS), and biomasses of the cyanobacterium Leptolyngbya sp. (BC) and the seaweed Ulva ohnoi (BM) were used. The structural characteristics of biopolymers/biomasses were preliminarily analyzed by ATR-FTIR spectroscopy, and bioluminescence inhibition was assessed to evaluate any toxic effects. Then, fifteen indicators (WHC, BD, pH, CEC, SOC, Cext, Cmin, Ntot, NH4+-N, NO3--N, Min, Nit, Cmic, FM and R) defined based on their ability to represent soil functions, were used to compute an additive soil quality index (SQI), providing an integrated assessment of treatment effects. ATR-FTIR analysis confirmed carbohydrate-dominated structures in EPS, BC and BM, whereas lipopeptide characteristics were observed in BS. EPS produced the greatest pH rise (from 5.30 to 6.13 in bare soil and up to 7.49 under lettuce), BM increased cation exchange capacity, nitrogen mineralization and nitrification in bare soil, while BS showed higher water holding capacity, inorganic nitrogen under lettuce, and soil respiration in both conditions. Notably, all considered biopolymers/biomasses in soil without L. sativa showed an improved SQI compared to the control, whereas under lettuce presence all treatments (except BS, P < 0.01) were comparable to the control. These findings provide comprehensive evidence that application of non-toxic biopolymers derived from thermophiles and sea biomasses can differentially modulate soil properties, thereby representing a sustainable soil management practice.