Molecular Soil Biology 2024, Vol.15, No.1, 17-27 http://bioscipublisher.com/index.php/msb 23 of these materials significantly increased enzyme activities and bacterial community diversity, which are critical indicators of soil health. Another study highlighted the role of vermicompost and humic acid fertilizers in enhancing soil structure and microbial community composition, leading to improved salt leaching and reduced nitrogen losses (Liu et al., 2020). Furthermore, research in various regions of Inner Mongolia identified keystone microbial taxa that exhibit high tolerance and resilience to saline-alkali stress, suggesting their potential in SynCom formulations for soil restoration (Zhang et al., 2021). 5.3 Criteria and metrics for evaluating the performance of SynComs in field conditions Evaluating the performance of SynComs in field conditions involves several criteria and metrics. Key indicators include soil pH, enzyme activities (such as catalase, urease, alkaline phosphatase, and cellulase), and microbial community diversity and richness. Soil aggregate stability and microstructure improvements, such as increased porosity and effective permeability, are also critical metrics, as they directly influence salt leaching and nutrient cycling (Liu et al., 2020). Additionally, the presence and abundance of keystone microbial taxa, which play disproportionate ecological roles in stress resistance and soil health, serve as important markers for the success of SynCom applications (Wang et al., 2020a). These metrics collectively provide a comprehensive assessment of the effectiveness of SynComs in restoring saline-alkali soils. 6 Case Studies 6.1 Detailed analysis of specific field trials and their outcomes Field trials across various saline-alkaline environments have demonstrated the potential of engineered synthetic microbial communities (SynComs) in restoring soil health. For instance, a study conducted in Inner Mongolia revealed that microbial assemblages in saline-alkaline soils are primarily driven by soluble salt ion components rather than salinity itself. This study identified keystone bacteria and fungi that could potentially adapt to and restore saline-alkaline soils by enhancing plant growth and soil functions (Zhang et al., 2021). Another field trial in Northern China tested the effects of bio-fertilizer and rotten straw amendments on oat productivity in saline-alkaline soils. The combined amendment treatment significantly improved oat yields, reduced soil pH, and increased soil salt content, demonstrating the effectiveness of organic amendments in such challenging environments (Lu et al., 2020). Similarly, a five-year field experiment with Miscanthus cultivation showed substantial improvements in soil fertility and reductions in soil salinity and pH, highlighting the role of beneficial bacteria in soil desalinization and nutrient cycling (Xu et al., 2021). 6.2 Success stories and lessons learned Several success stories have emerged from these field trials. The use of vermicompost and humic acid fertilizer in saline-alkaline soils improved soil aggregate stability and microstructure, leading to enhanced salt leaching and reduced nitrogen losses. This approach also increased the abundance of beneficial aerobic heterotrophs, which play a crucial role in nutrient cycling and soil health (Liu et al., 2020). In the Yellow River Delta, the combination of straw and desulfurization gypsum significantly improved soil organic carbon and microbial biomass, while reducing soil pH and exchangeable sodium percentage. This integrated strategy proved effective in enhancing soil health and microbial diversity (Liu et al., 2023). Another notable success was observed in Daqing, where the use of T. ambiguum Bieb. and microbial inoculums in a terraced landscape design led to a 94.7% removal rate of oily substances and significant reductions in soil salinity and alkalinity (Li et al., 2021). 6.3 Limitations and areas for improvement Despite these successes, there are limitations and areas for improvement in the application of SynComs for soil health restoration. One major limitation is the variability in microbial community responses to different soil types and environmental conditions.In Cd-contaminated soils, soil bacteria were more responsive to saline-alkaline stress than fungi, indicating the need for tailored microbial solutions for different soil conditions (Wang et al., 2019). Additionally, while the combination of organic amendments and microbial inoculums has shown promise, the long-term sustainability and scalability of these approaches need further investigation. More extensive multi-location trials are required to assess the consistency of these improvements across diverse environments (Xu et al., 2021). Furthermore, understanding the specific roles and interactions of microbial taxa in saline-alkaline
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