Pond-ditch circulation system (PDCS) is an efficient and low-cost technology for rural domestic wastewater treatment. Aquatic plants play important roles in the nutrient removal of the PDCS. Besides directly absorbing nitrogen (N) and phosphorus (P), aquatic plants affect nutrient removal via root exudates. However, mechanisms of root exudates regulating nutrient removal in PDCSs remain unclear.   

Researchers from the Wuhan Botanical Garden and Institute of Hydrobiology of the Chinese Academy of Sciences investigated water quality, sediment properties, root exudates, nutrients in plants, and community structure of rhizosphere microorganisms in PDCSs for rural wastewater treatment lasting two successive months. 

According to these researchers, in PDCSs, protein contents in root exudates were negatively associated with Simpson indices. The lactic acid and tartaric acid were dominant organic acids in root exudates of the PDCS, which showed opposite variation trends. The former had negative correlation with total N (TN), total P (TP), TP removal rate and sediment inorganic P (SIP), while the latter exhibited tight correlation with nutrient removal rates and most sediment properties, especially sediment TN (STN) and total organic carbon (TOC). 

Bacillus, Geobacter, and Clostridium sensu stricto 1 were the top three relative dominant genus, which might be the predominant groups in nutrient removal of PDCSs. Lactic acid, protein, and amino acids had positive effects on Geobacter; and Bacillus was significantly influenced by water content.  

In addition, the direct effects of sediment properties on nutrient removal rates were remarkably lower than their indirect effects through altering N/P contents in plants and bacterial diversity and relative abundance.  

Root exudates, such as protein, amino acids, and lactic acid, modulated rhizosphere microbial relative abundance and diversity, and then in turn influenced nutrient removal rates of the PDCS. 

Relevant results have been published in Science of the Total Environment entitled "Effects of root exudates on rhizosphere bacteria and nutrient removal in pond-ditch circulation systems (PDCSs) for rural wastewater treatment." This study was supported by grants from the National Natural Science Foundation of China.    

The possible pathways that root exudates may affect rhizosphere bacteria and nutrient removal in the PDCS (Image by MA Lin)