Improving soil quality without creating any environmental problems is an unescapable goal of sustainable agroecosystem management, according to the United Nations 2030 Agenda for Sustainable Development. Therefore, sustainable solutions are in high demand. One of these is the use of biopolymers derived from microbes and seaweed. This paper aims to provide an overview of the sources of extraction and use of microbial (bacteria and cyanobacteria) and seaweed-based biopolymers as soil conditioners, the characteristics of biopolymer-treated soils, and their environmental concerns. A preliminary search was also carried out on the entire Scopus database on biopolymers to find out how much attention has been paid to biopolymers as biofertilizers compared to other applications of these molecules until now. Several soil quality indicators were evaluated, including soil moisture, color, structure, porosity, bulk density, temperature, aggregate stability, nutrient availability, organic matter, and microbial activity. The mechanisms involved in improving soil quality were also discussed.

Many years ago, widespread and ineffective technologies led to significant ecological contamination, primarily due to heavy metals (HMs), particularly lead (Pb) and cadmium (Cd). Plant growth in heavy metals contaminated and calcareous soil is a challenge for all the growers worldwide. Several organic and inorganic amendments are used by researchers in different types of soils. However, their impact on soil contaminated with toxic metals and alkaline in reaction is still unclear. Therefore, the current pot experiment investigates the relative effect of organic (wheat straw biochar and farmyard manure) at 10 and 20 t ha⁻¹ and inorganic (diammonium phosphate and gypsum) at 100 and 150 kg ha⁻¹ amendments on NPK availability, soil organic matter, soil properties, and maize growth under Cd and Pb stress. Highest soil pH (7.61), soil electrical conductivity (0.25 dS m⁻¹), soil organic matter (1.40%), plant height (92 cm), shoot dry weight (41 g), and root dry weight (5.5 g) were observed because of biochar application at 20 t ha⁻¹. While the highest total nitrogen (N) (0.13%), and phosphorus (P) (8.16 mg kg⁻¹) were recorded with diammonium phosphate application at 150 kg ha⁻¹, moreover, application of farmyard manure at 20 t ha⁻¹ increased the soil potassium (K) (108.14 mg kg⁻¹). Biochar application at 20 t ha⁻¹ greatly reduced the toxic metals bioavailability with a high immobilization index, that is, Cd (52%) and Pb (43.79%) lead to lower concentrations of metals in roots and shoots as well. A negative correlation was identified between soil pH, soil organic matter, and soil Cd, Pb, demonstrating the positive impact of biochar in terms of the reduction of toxic metals by enhancing pH and soil organic matter, found to be the most effective amendment for soil improvement and plant growth under Cd and Pb stress condition.

Nutrient deficiencies, low organic matter content, and a limited soil–water saturation percentage in calcareous soils hinder plant growth and crop production. To address these challenges, sustainable and green-based farming practices have been introduced. This study investigates the synergistic effects of biochar and nitrogen levels as sustainable solutions for improving soil fertility and supporting wheat growth in calcareous soils. A pot experiment assessed the effects of biochar (5-, 10-, and 15-tons ha⁻¹) and nitrogen levels (60, 90, and 120 kg ha⁻¹) on soil physicochemical properties, nutrient availability, and wheat growth. The randomized complete block design included three replicates and a control. The results highlight that the highest biochar rate (15 tons ha⁻¹) combined with the highest nitrogen level (120 kg ha⁻¹) significantly (p ≤ 0.05) improved soil physicochemical properties and nutrient status. Notably, soil pH increased by 2.8%, electrical conductivity by 29.8%, and soil organic matter by 185%, while bulk density decreased by 22.3%. Soil total nitrogen surged by 163.7%, soil–water saturation percentage by 27.2%, plant-available phosphorus by 66.8%, and plant-available potassium by 96.8%. Wheat growth parameters also showed marked improvement, with plant height up 29.7%, spike length by 20.7%, grains per spike by 41.5%, thousand-grain weight by 24.7%, grain yield by 81.3%, and biological yield by 26.5%. There was a strong positive correlation between enhanced soil properties and improved wheat growth, except for soil bulk density, which showed a negative correlation. This underscores the role of biochar in boosting soil fertility and crop productivity. A principal component analysis further validated these findings, suggesting that integrating biochar with appropriate nitrogen fertilization offers a sustainable strategy to enhance soil health, manage nutrient availability, and strengthen crop yields in calcareous soil. Biochar application combined with elevated nitrogen levels significantly enhances soil fertility and wheat productivity in semi-arid regions, offering a sustainable solution for improving calcareous soils. Future studies should explore the long-term impacts and scalability of this approach.

Soil water retention is a key factor in ecological processes regulating ecosystem stability and resilience under environmental stress. In this regard, marine-derived additives may provide sustainable strategies to enhance soil water dynamics. Here, novel biopolymers derived from thermophilic bacteria, including six exopolysaccharides (EPS1–EPS6) and four biosurfactants (BS1-BS4), and biomasses from seaweed (BM1–BM4) and marine cyanobacteria (BC1–BC2), were investigated for their wetting properties and soil water retention. Wetting properties, including reduction in contact angle (RCA) and atmospheric-air moisture uptake (AMU), were monitored for 36 h at constant temperature (30 °C). The effect on soil water retention was evaluated in terms of water loss of soil samples treated with two different concentrations (0.5 and 1% w/w) of either biopolymers or biomasses in a microcosm consisting of 10 g of soil and 10 mL of water, kept at a stable temperature of 22 °C for 200 h (until complete evaporation occurred). BC2 derived from Leptolyngbya sp. 43.3 was the best wetting agent (RCA = 39.44%), while the EPS4 produced by Bacillus horneckiae SBP3 was the best humectant agent (AMU = 179.63%). Soils amended with bacterial biopolymers (EPS4, EPS5, EPS6, BS1 and BS3), as well as biomasses derived from cyanobacteria BC2 and seaweed BM1–BM4, produced better improvement in soil water retention, with marked effects at the concentration of 1% w/w. The lipopeptide BS1 was the most effective in water loss reduction over a specific time of 96–125 h at both concentrations. These findings highlight the potential of these materials as nature-based solutions to improve soil-mediated ecosystem resilience to drought under climate change.

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, C_ext, C_min, N_tot, NH₄⁺-N, NO₃^--N, Min, Nit, C_mic, 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.