## 1 Introduction  
At present, a large amount of low C/N effluents are discharged into surface rivers and lakes, causing serious eutrophication of water bodies. Therefore, nitrogen removal has become an urgent problem to be solved in surface water restoration. As a composite ecosystem, constructed wetlands can efficiently remove organic matter and nutrients such as nitrogen and phosphorus in wastewater through the diverse synergistic effects of substrates, plants and microorganisms. In addition, it has been widely used in the wastewater of urban sewage treatment plants, agricultural wastewater and industrial wastewater for deep processing [1; 2], as well as storm-water control and treatment [3]. Studies have shown that the main pathways of nitrogen removal in constructed wetlands include nitrification, denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA) driven by microbial metabolic activities [4]. The organic carbon source acts as an electron donor and energy source for microbial metabolism in the denitrification process, which is one of the main limiting factors of denitrification [5]. Micro-polluted rivers and lakes with low C/N levels often lack organic carbon sources, resulting in poor microbial denitrification in constructed wetlands. Therefore, researchers usually use an external carbon source to solve this problem. Various carbon sources have been used to improve the denitrification performance of constructed wetlands. Conventional liquid low-molecular organic carbon sources such as methanol, ethanol and acetate have problems such as rapid consumption, difficult storage, high price and difficult dosage control, and do not meet the criteria of sustainable development [6]. However, new carbon sources such as biodegradable polymers have complex manufacturing processes and high operating costs, so that large-scale applications cannot be achieved in the short term [7]. Therefore, finding a new carbon source with low price, high efficiency and environmental protection has become an urgent necessity for the research and development of constructed wetlands. As a high-carbon solid carbon source, the plant carbon source has the advantages of sufficient source and low cost. It can continuously degrade and release organic substances under the action of microbial enzymes, and has the characteristics of long-term release and convenient administration after one addition, and has gradually become a research hotspot [8]. At present, most municipal sewage treatment plants in China have a high removal efficiency for organic matters in wastewater, while the removal efficiency for TN is low. China's most stringent wastewater discharge standard (GB 18918-2002, China) for municipal sewage treatment plants allows total nitrogen levels (TN 15 mg/L) that are significantly higher than the Class IV standard (TN 1.5 mg/L) requirements for surface water quality (GB 3838-2002, China). During the dry season or near the outlets of the sewage treatment plants, the C/N value of the river usually reaches 2-3. While in the rainy season or far from the outflows of the sewage treatment plants, the C/N content of the river increases slightly after the dilution of the river water [9]. [PERSON] et al. [10] showed that the wetland system denitrification process is significantly improved by using agricultural biomass materials such as wheat straw, apricot kernels and walnut shells as additional carbon sources in the treatment of nitrogen-rich wastewater with a TN concentration of 41.79 to 44.24 mg/L in constructed wetlands. [PERSON] et al. [11] used alkali-treated corn stalks as an additional carbon source to treat low C/N agricultural effluents with a TN concentration of 12 mg/L. The results showed that with a hydraulic residence time of 3 days, the TN removal rate of the constructed wetland increased by 37.2%. [PERSON] et al. [12] used _Plattanus acerifolia_ leaf litter as an additional carbon source to treat low C/N wastewater with a NO\({}_{3}\) -N concentration of 20 mg/L in a laboratory-scale vertical-flow wetland.