Low-dose natural clay Kaolin promotes the growth of submerged macrophytes and alters the rhizosphere microorganism community: Implications for lake restoration.

Sediment properties have a crucial effect on the growth and recovery of aquatic plants in lakes. Addition of various chemical substances has been proposed to reinforce the recovery of plants after a nutrient loading reduction. However, the effects of such sediment amendments on plant growth, especially those from rhizosphere microorganisms, is limited. We added Kaolin clay to sediments in different concentrations to explore its impact on the growth of Vallisneria natans and Ottelia acuminate and the concurrent shift in rhizosphere microorganisms using high-throughput sequencing technology. We found that the addition of low doses (10 % and 20 % in mass ratio) of Kaolin significantly modified sediment conditions (oxidation reduction potential and pH), with implications also for the composition, diversity, and stability of rhizosphere microorganisms. LEfSe analysis revealed that low-dose addition of Kaolin increased the abundances of functional microbial groups that benefit plant nutrient absorption and enhance plant stress resistance, such as Spirillaceae, Rhodocyclaceae, and Burkholderiales. Moreover, low doses of Kaolin significantly promoted the photosynthesis and nutrient absorption of submerged macrophytes, thereby facilitating plant growth. A structural equation model (SEM) indicated that the direct impact of Kaolin on the growth of submerged plants was relatively minor, while the indirect effect through modulation of rhizosphere microorganisms was important. Our study suggests that low doses of Kaolin may be used to promote the growth of submerged macrophytes when lakes with a high organic content in the sediment are recovering after nutrient loading reduction.