Riparian restoration in sandy zones with alfalfa as pioneer plant in initial stage of soil and water bioengineering

As Nature-Based and Bio-based Solutions, soil and water bioengineering (SWB) provides several benefits to humans and nature. It has been widely used for erosion control, vegetation recovery and ecosystem restoration in riparian zones. Nevertheless, studies on riparian restoration in sandy zones with alfalfa as pioneer plant in initial stage of SWB are still rare, which are important for understanding the role of pioneer plants during the initial stage of SWB and also for choice of the appropriate measures. Here, three commonly applied SWB measures (vegetation geobag / VGB, grass-planting concrete block / GCB, and untreated flat area / NTF) are established in sandy riparian zones to explore the growth characteristics structural mechanics and reinforcement of pioneer plant (alfalfa) root, and also the factors influencing biomechanical properties in initial stage of SBE. Our results show that: (1)NTF demonstrated superior overall growth compared to GCB and VGB, underscoring the limitations that geobags and steep slopes impose on initial SWB vegetation establishment. In all treatments, alfalfa roots were able to penetrate soil layers below 60 cm, with NTF exhibiting the highest root biomass and diameter (NTF > GCB > VGB). GCB's higher root-to-shoot ratio may reflect a drought-resistant strategy. In contrast, VGB showed greater root length, maximum rooting depth, and a smaller crown width, indicating a focus on root growth to overcome geobag constraints (2) Differences in root growth distribution among the three treatments resulted in varying biomechanical impacts. Specifically, NTF exhibited significantly lower soil-root bond strength than GCB, while both NTF and GCB had higher maximum pull-out forces compared to VGB. Root diameter showed a significant negative correlation with Young's modulus and root tensile strength, but a positive correlation with root tensile force (p < 0.05). Although no significant correlations were found between root water content and root tensile strength, root tensile strength was positively correlated with Young's modulus. (3) In the comprehensive evaluation of root reinforcement performance, the scores ranked as VGB > GCB > NTF. Result of this study could provide valuable insights into the practical applications of SWB in sandy riparian restoration. Furthermore, it underscores the importance of pioneer plants in the fragile and critical initial stages of SWB, and also benefit both researchers and practitioners in effectively transitioning from the scientific research to practical solutions for riparian restoration.