Evaluating frost-resistant substrate quality and the effects of vegetation restoration in the alpine region of Tibet, China

Frost-resistant substrate is one type of soil suitable for the slope ecological restoration in alpine regions. It is composed of natural soil, cement, organic materials, greening additives, silicon powder, and palm fiber. This study aims to quantitatively evaluate the application of the frost-resistant substrate for the slope vegetation restoration in the alpine region of Tibet, China. Field sampling, field monitoring, and indoor experiments were conducted to explore the physical, chemical, and biological properties of the frost-resistant substrate. A systematic investigation was made to explore the slope vegetation restoration from 2019 to 2023. The vegetation concrete, and natural soil were adopted under similar site conditions. Principal component analysis was carried out on the microscopic images of the three substrates. The soil quality index (SQI) was then calculated after five years of monitoring. The results revealed that the frost-resistant substrate shared significantly superior physical and chemical properties, compared with both vegetation concrete and natural soil (P<0.05). Their SQI values were ranked in the descending order of the frost-resistant substrate (0.681) > vegetation concrete (0.605) > natural soil (0.378). The cement and brown fiber improved the stability of the frost-resistant substrates. While its fertility was enhanced by the organic materials, silicon powder, and greening additives. In addition, the frost-resistant substrate also demonstrated the optimal performance in terms of the vegetation coverage, species diversity, as well as both aboveground and belowground biomass. An inverted "V" trend was observed in the SQI and vegetation restoration for the three types of soils, as the years increased. The highest value was reached in 2020. And then there was the gradual decrease. The freeze-thaw process reduced the physical and chemical properties of the soil. There was a relatively minor impact of the freeze-thaw process on the frost-resistant substrate, compared with the three types of soil. The proportion of green additives and cement in the substrate material ratio could be increased by 1% respectively, in order to improve the long-term use. Regression analysis showed that a close correlation was found between the SQI values and the four vegetation indicators. The SQI was accurately evaluated for the slope vegetation restoration. Principal component and correlation analysis reveal that the greatest contribution to the soil quality was found in the four indicators, including the water stable macroaggregate content (WAC), soil cohesion force (SC), soil organic matter (SOM), and catalase activity (CAT). Therefore, the soil quality could be evaluated and improved by the above four indicators in future studies. The frost-resistant substrate was verified in the application of slope vegetation restoration in the alpine regions. A scientific basis was also offered to optimize the frost-resistant substrate ratio in the future. In future research, the monitoring time and the application scope of the sample plot can be extended, and application experiment under different slope site conditions can be carry out to fully explore the long-term effect of the frost-resistant substrate in the slope vegetation restoration. The research results can also provide a strong reference and effective technical support for the slope ecological restoration in alpine regions.