Effect of Temperature on Microorganisms and Nitrogen Removal in a Multi-Stage Surface Flow Constructed Wetland

Abstract

The effect of low temperature on microbial nitrogen metabolism in constructed wetlands has yet to be extensively investigated. In this study, we analyzed the effects of temperature changes on nitrogen-associated microorganisms and nitrogen metabolism functional genes in a multi-stage surface flow constructed wetland (MSSFCW) using metagenomic sequencing. The treatment of polluted river water in the MSSFCW, which had a mean water temperature (MWT) of ≤17 °C, resulted in a low removal efficiency (RE) for total nitrogen (TN; average RE: 23.05% at 1–17 °C) and nitrate nitrogen (NO3−-N; average RE: −2.41% at 1–17 °C). Furthermore, at a MWT of ≤11 °C, the REs were low for ammonium nitrogen (NH4+-N; average RE: 67.92% at 1–11 °C) and for chemical oxygen demand (COD; average RE: 27.45% at 1–11 °C). At 0.24 m3 m−2 d−1 influent load, the highest REs for TN (66.84%), NO3−-N (74.90%), NH4+-N (83.93%), and COD (52.97%) occurred in July and August, when water temperatures were between 26 and 28 °C. The lowest rates (TN: 11.90%, NO3−-N: −21.98%, NH4+-N: 65.47%, COD: 24.14%) occurred in the January–February period, when the water temperature was lowest (1–5 °C). A total of 25 significantly different species were detected in surface sediment, none of which were dominant species. The dominant phyla and genera at low (January) and high (July) temperatures were similar; however, microorganisms were more abundant in the low-temperature months. Our analysis indicated that the same nitrogen metabolism pathways occurred in January and July. Denitrification-associated functional genes were the most abundant; nitrification-related functional genes were the least abundant. Only nirBD displayed significantly different abundances between January and July. This paper can hopefully help researchers and managers further understand how temperature affects nitrogen removal performance in constructed wetlands.