The global issue of ecological ditches being poor in removing nitrogen and phosphorus under cold winter temperatures has been identified. This study introduced three types of biochar (reed, rice, and corn) into ecological ditch sediments via two application methods: rhizosphere and mixed addition. The purpose was to explore how these methods affect the removal of nitrogen and phosphorus, as well as their influences on microbial communities in sediments. The results indicated that the addition of biochar to ecological ditches significantly enhanced the removal of nitrogen and phosphorus. Among the three types of biochar, the mixed addition of corn biochar yielded the greatest results, achieving removal rates of 77.1% for total nitrogen (TN), 93.3% for NH₄⁺-N, and 90.3% for total phosphorus (TP). The growth of Vallisneria natans was greatly improved by the mixed addition method, resulting in an average increase of 154%. This improvement was superior to the rhizosphere addition group, which led to a growth increase of 125%. In comparison, the control group (CK) showed a decrease of 4.8% in growth. Different methods of biochar addition resulted in changes in the physicochemical properties and stoichiometry of the plants. Microbial analyses showed that the addition of biochar reduced the diversity and abundance of the substrate microbial community.

The limited nitrogen and phosphorus concentration reduction performance of wetlands during cold weather is a global concern. To address the challenge of low nutrient removal efficiency in winter, this study introduces a hollow fiber membrane module integrated into paddy soil, enabling the in situ formation of a hydrogen-based bacterial–algal biofilm. Over a 60-day experiment at water temperatures of 6.2–15.0 °C, the biofilm-enhanced system (HS) reduced aqueous ${\mathrm{NH}}_4^{+}$-N from 25 mg/L to 0.2 mg/L by day 30 and maintained this low level through day 60, whereas the S system still had 5.5 mg/L ${\mathrm{NH}}_4^{+}$-N. Moreover, the HS system consistently maintained lower ${\mathrm{NO}}_3^{-}$-N and total phosphorus (TP) concentrations than the S system. Microbial communities were characterized using third-generation 16S full-length sequencing and 18S rRNA V4 amplicon sequencing. The biofilm was dominated by Geminocystis_PCC_6308, Rikenellaceae_RC9_gut_group, and Rhodobacter, while eukaryotic green algae, mainly Botryococcus sp., were also abundant. In the paddy soil, Terrimonas and unclassified_AKYH767 were identified as potential key taxa associated with nitrogen and phosphorus concentration reduction. Overall, the hydrogen-based bacterial–algal biofilm effectively improves the nitrogen and phosphorus concentration reduction of paddy soil wetlands, offering a promising strategy for cold-weather eutrophic water remediation.