Effects of meander restoration on microbial activities and abundances in temperate urban streams

Ecological restoration of rivers provides various ecosystem services, such as water quality improvement, biodiversity conservation, and enhancement of aesthetic functions, drawing significant interest for these reasons. In particular, many urban rivers have been degraded into a straightened form due to economic development. To address this, a widely used restoration technique involves transforming them into naturalized rivers by introducing meandering reaches. The introduction of meanders can increase the physical heterogeneity and diversity of river structures, leading to various ecologically positive effects. For example, it is widely reported that meandering reach restoration enhances invertebrate biodiversity and expands habitat diversity. However, relatively little is known about its effects on ecosystem functions, particularly microbial activity and nitrogen removal.

To bridge this critical knowledge gap, this study monitored differences in microbial enzyme activity, denitrification potential, the abundance of denitrifying microbes, bacterial abundance, and the physicochemical properties of water quality between straightened and restored meandering reaches in urban rivers. The analysis revealed that microbial enzyme activities were significantly increased in all four restored reaches studied (by 13–150 %), and denitrification enzyme activity exhibited the same trend with 15–40 % higher rates in meandering reaches. Additionally, the abundance of the nirS gene, which is involved in denitrification, also increased by 31–92 % in meandering reaches. Furthermore, the inorganic nitrogen concentration in the water column of the meandering reaches showed a sharp decline (70.2 % for nitrate and 42.1 % for ammonium on average), indicating that these areas act as nitrogen removal hotspots. Overall, our findings suggest that meandering restoration enhances microbial activity, particularly the abundance of denitrifying microbes, and plays a positive role in water quality improvement. These results provide quantitative evidence that can inform river management strategies, particularly in urban areas where nitrogen pollution is a concern, demonstrating that physical restoration can effectively enhance biogeochemical functions.