Soil anti-scourability enhanced by herbaceous species roots in a reservoir water level fluctuation zone

Revegetation is one of the successful approaches to soil consolidation and streambank protection in reservoir water level fluctuation zones (WLFZs). However, little research has been conducted to explore the impact of herbaceous species roots on soil anti-scourability during different growth stages and under different degrees of inundation in this zone. This study sampled two typical grasslands (Hemarthria compressa grassland and Xanthium sibiricum grassland) at two elevations (172 and 165 m a.s.l.) in the water level fluctuation zone (WLFZ) in the Three Gorges Reservoir (TGR) of China to quantify the changes in soil and root properties and their effects on soil anti-scourability. A simulated scouring experiment was conducted to test the soil anti-scourability in April and August of 2018. The results showed that the discrepancy in inundation duration and predominant herbaceous species was associated with a difference in root biomass between the two grasslands. The root weight density (RWD) values in the topsoil (0-10 cm) ranged from 7.31 to 13 mg cm−3 for the Hemarthria compressa grassland, while smaller values ranging from 0.48 to 8.61 mg cm−3 were observed for the Xanthium sibiricum grassland. In addition, the root biomass of the two herbs was significantly greater at 172 m a.s.l. than that at 165 m a.s.l. in the early recovery growth period (April). Both herbs can effectively improve the soil properties; the organic matter contents of the grasslands were 128.06% to 191.99% higher than that in the bare land (CK), while the increase in the water-stable aggregate ranged from 8.21% to 18.56%. Similarly, the topsoil anti-scourability indices in both the herbaceous grasslands were larger than those in the CK. The correlation coefficients between the root length density (RLD), root surface area density (RSAD) and root volume density (RVD) of fine roots and the soil anti-scourability index were 0.501, 0.776 and 0.936, respectively. Moreover, the change in the soil anti-scourability index was more sensitive to alternations in the RLD with diameters less than 0.5 mm. Overall, the present study showed that the perennial herbaceous (H. compressa) has great potential as a countermeasure to reduce or mitigate the impact of erosion in the WLFZ of the Three Gorges Reservoir.