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 (NH₄⁺) assimilation and nitrification. Additionally, butachlor application resulted in the increased retention of NH₄⁺-N and nitrate (NO₃^-)-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.