Effects of herbicide butachlor application on the growth of periphytic biofilms and nitrogen loss in paddy systems

The application of butachlor as an herbicide in paddy fields is widely practiced,aiming to increase rice yield by directly or indirectly influencing the paddy environment.Periphytic biofilms,which form at the soil-water interface in paddy fields,are complex bioaggregates that play an important role in nitrogen (N) cycling.The objective of this study was to investigate the effect of butachlor on periphytic biofilm growth and N cycling under both light and dark conditions in the laboratory.The results revealed that butachlor application hindered the growth of periphytic biofilms and led to the dominance of Cyanobacteria as the primary prokaryotes,while inhibiting the development of eukaryotic Trebouxiophyceae.Furthermore,the application of butachlor reduced the richness and diversity of prokaryotes,but increased those of eukaryotes in periphytic biofilms.The light treatments exhibited higher total N loss because light favored periphytic biofilm growth and enhanced ammonium (NH_(4)^(+)) assimilation and nitrification.Additionally,butachlor application resulted in the increased retention of NH_(4)^(+)-N and nitrate (NO_(3)^(-))-N and an increase in N loss via denitrification.The abundances of functional genes encoding enzymes such as ammonia monooxygenase,nitrite reductase,and nitrous oxide reductase were increased by butachlor application,favoring nitrification and denitrification processes.Overall,the results suggest that butachlor application leads to an increase in total N loss mainly through denitrification in paddy systems,particularly in the presence of periphytic biofilms.Thus,the results may provide valuable insights into the changes in periphytic biofilm growth and N cycling induced by butachlor,and future studies can further explore the potential implications of these changes in paddy soils.

Spatiotemporal variation of microeukaryotic biodiversity and biotic condition associated with disinfectants after the COVID-19 lockdown in Wuhan,China

Intensive application of chlorine-based disinfectants driven by the COVID-19 pandemic was suspected to be detrimental to receiving water ecosystems,but with little field evidence.We characterized the occurrences of typical disinfectants and microeukaryotic communities in surface waters associated with three wastewater treatment plants(WWTP)-river systems one year after the lockdown in Wuhan,China.Trihalomethanes(THMs)declined from summer 2020(0.01–1.82μg/L)to summer 2021(0.01–0.95μg/L),whereas quaternary ammonium salts(QAs)increased from summer 2020(0.53–6.35 ng/L)to summer 2021(8.49–191 ng/L).Biodiversity monitoring with environmental DNA(eDNA)revealed significant temporal variation in microeukaryotic community composition.The monitored disinfectants were correlated with some eukaryotic communities as demonstrated by redundancy analysis.For example,QAs were positively related to Cryptophyta relative abundance,but negatively related to Rotifera relative abundance.A microeukaryote-based multimetric index indicated ecological impairment near the Han River WWTP outlet in 2020.Our findings indicate the influence of heavily used disinfectants on river microeukaryotic communities,and the usefulness of assessing mid-term ecological risks from disinfectants in the post COVID-19 era.