Mixed culture of plants improved nutrient removal in constructed wetlands: response of microbes and root exudates

Root exudates are determined by plant species configuration and affect microbial community, which in turn affect purification efficiency of constructed wetlands (CWs). However, it is not well understood how plant configuration affects CW purification efficiency through specific root exudates. Herein, four mixed culture CWs were constructed; CW-G3 with Iris pseudacorus, Iris sibirica, Juncus effusus, and Hydrocotyle vulgaris showed the optimal diversity nutrients removal efficiency (TN: 94.2%, TP: 82.9%, COD: 74.7%). Highly increased antioxidant enzymes (peroxidase and catalase) reduced photosynthesis-negative enzyme (malondialdehyde) activity of plants in CW-G3, which ensured oxygen (O-2) and organic carbon (OC) production and successfully released to rhizosphere by well-developed root aeration tissues. Further, CW-G3 enriched higher abundance of genus Saccharimonadales and Flavobacterium, which benefited nitrogen removal. Moreover, as OC, higher contents of maltose in CW-G3 (6.6 similar to 11.1-fold of that in other three CWs), as well as lauramide, choline, triethylamine and urocanic acid contributed to microbial denitrifying. Differently, higher contents of unsaturated fatty acids (linoleic acid and oleic acid) in other three CWs inhibited microbial nitrifying as inhibitors, which also proved by co-occurrent network. Thereby, plant configuration in CW-G3 provided higher O-2 and OC contents for bacteria and reduced nitrifying inhibitors, which contributed to higher purifying efficiency. The study promoted the understanding about root exudates' effects on bacteria through plant configurations and improved the purification efficiency of CWs.

Automated summary

Root exudates are influenced by plant species configuration, which in turn affect microbial community and the purification efficiency of constructed wetlands (CWs). This study constructed four CWs with different plant configurations and found that CW-G3 with diverse plant species showed the highest removal efficiency of nutrients. The well-developed root aeration tissues in CW-G3 enabled the release of oxygen and organic carbon, supporting bacteria growth and nitrogen removal. The root exudates from different plant configurations also had different effects on microbial nitrification and denitrification processes.