The increasing emission of greenhouse gases such as CO₂, CH₄, and N₂O from the soil has become a growing concern globally. To address this issue, biochar has emerged as an environmentally friendly soil amendment that can affect the gas permeability of soil and reduce greenhouse gas emissions. The biochar-soil-plant system exhibits a complicated interaction that promotes plant productivity and root elongation, further impacting greenhouse gas emissions. The objective of this paper is to provide a comprehensive review of the effects of biochar on soil gas permeability and consequently greenhouse gas emission in vegetated soil. The paper begins by discussing the basic characteristics of biochar and its impact on soil microstructure. It then explores the impact of biochar on the gas permeability of both non-vegetated and vegetated soil. The mechanisms through which biochar influences greenhouse gas emission are explained in terms of modified soil aeration, water holding capacity, adsorption, pH, available nutrients, and the activity of soil microbes and enzymes. The role of plants in greenhouse gas emission in biochar-amended soil is also analysed by comparing the vegetated group with the non-vegetation group. The paper includes a discussion of the various methods used to measure soil gas permeability, such as the steady-state and transient methods, as well as greenhouse gas emission measurement techniques, such as the chamber system and micrometeorological methods. Finally, future research directions are proposed to highlight the impact and corresponding mechanisms of plant roots on the biochar-induced variation of soil gas permeability and greenhouse gas emission.

Sand slope is an important part of coastal zone and islands, which is severely affected by wave erosion and causes problems such as degradation of coastal zone and reduction of island area. Enzyme-induced calcium carbonate precipitation (EICP) technology is a new reinforcement technology with environmental friendly and excellent effect, which has been widely studied in the field of geotechnical engineering in recent years. In this research, we focus on the coastal or reef sand slopes in marine environments. The EICP reinforcement of representative sand slope units and large scale flume wave thumping experimental study are conducted indoors. By analyzing the physical and mechanical properties, erosion resistance, and microstructure of EICP-reinforced sand slopes, the mechanism of EICP reinforced sand slopes is revealed, the feasibility of EICP reinforced sand slopes is confirmed, and a feasible solution for EICP reinforced sand slopes is finally obtained. Results show that: (1) EICP reinforcement effectively enhances the surface strength and erosion resistance of sand slopes. Higher calcium carbonate content in the sand slopes corresponds to greater surface strength and improved erosion resistance. When the calcium carbonate content is similar, using low-concentration reinforcement twice is more advantageous than using high-concentration reinforcement once due to its superior uniformity. (2) The intensity of waves, the angle of the sand slope, and the severity of erosion damage are interrelated. Higher wave intensity, steeper sand slope angles, and more serious erosion damage require stronger reinforcement measures. (3) Scanning Electron Microscope (SEM) image analysis reveals that the reinforcing effect of sand slopes primarily depends on the amount of calcium carbonate crystals cemented between sand particles. A higher content of calcium carbonate crystals leads to better erosion resistance in the sand slope.

In order to improve the uniformity of calcite precipitation and engineering practicability, a series of tests using bacillus megaterium (BNCC 336739) were conducted to enhance sandy clayey purple soil, with different concentration bacterial solution and cementation reagent flowing to the samples perforated in the center with different grouting speed. Based on the mineral component (XRD) and soil microstructure (SEM), cementation mechanism was analyzed. Based on measurement of CaCO₃ production and unconfined compressive strength tests, the influence law of grouting factors on CaCO₃ production amount (C), CaCO₃ uniformity (s), CaCO₃ deposition rate (P), unconfined compressive strength (UCS) and stiffness (elastic secant modulus E₅₀) were analyzed and the correlation between C, s and UCS, E₅₀ were analyzed. The results show that the uniformity can be improved by perforation grouting, and the UCS and E₅₀ of samples treated by MICP increased by 105.58% and 464.14%. The CaCO₃ induced by bacillus megaterium are 1-5 µm calcite crystal, which cemented and wrapped soil particles. The higher the concentration of bacteria solution and cementation reagent and the slower the grouting speed are, the bigger the C and the s. The C has a lower threshold of 2.5% and an upper threshold of 5%, the UCS of samples treated by MICP significantly increases with the increase of C in the interval, and the UCS growth becomes slow or even negative outside the interval. The smaller the s is, the bigger the UCS and E₅₀ are, and this effect is small when C< 4% and is significant when C> 4%. With the effect of s, the UCS and E₅₀ of sample treated by MICP increase with different speed and then reduced as the increase of C. It provides scientific reference for the application of MICP technology in purple soil area.

The effects of desiccation cracking in clay soils on geotechnical constructions are substantial. This study investigates the viability of utilizing Enzyme-induced calcite precipitation (EICP), a bio inspired approach, as a potential solution for addressing desiccation cracking in fine-grain soils. For the EICP technique, crude soybean extract is employed for the purpose of urea hydrolysis. Multiple fluid samples, including a control sample, a cementation solution containing 1 M urea, 0.675 M CaCl₂, and 4 g/L milk, along with various concentrations of enzyme solutions (3-80 g/L), were tested for the study. To evaluate the surface cracking patterns, the method involved constant monitoring and photo recording using a high-resolution camera aided by image processing software. The results showed that fine-grain soils improved from increased calcite precipitation and decreased desiccation cracking intensity when the EICP method was used. Cementation and enzyme solution with low concentrations (3 g/L and 10 g/L) had similar effects on crack remediation, suggesting a modest influence. In contrast to the sample treated with water, the crack network remained unaltered in this case. CaCO₃ precipitation within the void area kept the crack network in place even as the void thickness decreased at increasing enzyme concentrations (30 g/L, 50 g/L, and 80 g/L). Wetting and drying cycles were found to decrease the crack ratio, crack width, and crack length in the EICP-treated sample, particularly under higher concentrations of urease enzyme. Lower enzyme concentrations of 3 g/L and 10 g/L have minimal impact on crack remediation but effectively inhibit new crack formation. Furthermore, higher enzyme concentrations result in calcium carbonate precipitates, forming a soil crust and increasing surface roughness. The study aims to enhance understanding of the EICP methodology and to provide novel perspectives on potential uses for soil enhancement.

The majority of cities worldwide are grappling with the challenge of dust pollution. Recently, the application potential of enzymatically induced carbonate precipitation (EICP), a novel environmentally friendly method, for dust control has been convincingly demonstrated. However, the long-term durability of EICP treatment is consistently a significant concern, particularly in regions prone to recurrent erosion caused by rainfall. As a result, the erosion durability of the EICP-treated dust soils requires further investigation. To address this, Polyacrylic acid (PAA) was added to the cementation solution in this study as the combined PAA and EICP treatment for dust control. The results showed that the addition of PAA slightly affected urea degradation; however, the combined PAA and EICP treatment significantly improved surface strength from 300 kPa to 500 kPa, especially for the wind-erosion resistance compared with the EICP treatment alone. The surface strength of samples treated with the combined PAA and EICP still exhibited a decrease due to repeated rainfall erosion, along with a reduction in calcium carbonate (CaCO₃) contents. Nevertheless, the decreasing slopes of surface strength (k = 13.434, 14.002, or 14.186) in response to repeated rainfall for EICP-PAA-treated slopes were much smaller than those for EICP-treated samples (k = 14.271), as well as the decreasing slopes of CaCO₃ contents, which suggested the slopes with the combined treatment had significantly improved durability. By comparing the cementation effect and the influence of repeated rainfalls on treated dust samples, the EICP-PAA (50 g/L) treatment achieved better dust control effects. Overall, the combined treatment of EICP-PAA shows promising potential for effectively suppressing dust generation and enhancing erosion durability.

Biocarbonation of reactive magnesia based on microbially induced carbonate precipitation (MICP) process is a sustainable geotechnical reinforcement technology for strength development and permeability reduction. This method can be used to produce microbial restoration mortar (MRM) for the application of stone cultural relics restoration. In this paper, the influence of particle size distribution on the strength and porosity of MRM was examined. By mixing fine and coarse sandstone powder in various proportions, nine different particle size distributions were obtained to investigate the restoration performance, including the unconfined compressive strength (UCS), porosity, and color difference. The results indicate that the well-graded particle size distribution can lead to the UCS improvement and porosity reduction of MRM. The findings also imply that adding fine sandstone powder to the coarse sandstone powder can provide extra bridging contacts within the soil matrix. These bridging contacts can be easily connected by the precipitated hydrated magnesium carbonates (HMCs) minerals, consequently resulting in more effective bonding and filling within the pore matrix. The microstructural images of MRM confirm the formation of HMCs, which exhibited a dense network structure, filling out the gap and bonding the sandstone powders. Furthermore, the microbial restoration mortar showed a high weather resistance to dry-wet cycles, acid rain, and salt attack, which is attributed to better stability and strength of HMCs than the original calcic cemented minerals in sandstone.

This study explores the effects of two nucleating agents, sucrose and sorbitol, on soybean-urease induced calcium carbonate precipitation (SICP) at a crystal level. Comparative studies on the mineral composition, crystal size, surface morphologies and thermal stability of SICP samples with/without nucleating agent were investigated with high resolution XRD, SEM and synchronous thermal analyzer (STA), respectively. The results show the introductions of sorbitol or sucrose to SICP reduce the content of vaterite(114) from 10.07% to 1.81%-3.93%, indicating their effect on transforming vaterite into stabler calcite. Sorbitol can enlarge the crystals and improve the thermostability of SICP, indicating an improvement of the crystallinity of SICP. The sucrose-regulated SICP shows medium thermostability which is worse than SICP without the nucleating agent, indicating the addition of sucrose reduces the crystallinity of SICP. Sorbitol is an effective nucleating agent that can improve the behaviors all-around, while sucrose increases the calcite content of SICP but inhibits the crystallinity of SICP. This study reveals the regulations of SICP because of the introduction of sorbitol or sucrose, and provides guidance to the subsequent engineering application of SICP.