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

The application of butachlor as a herbicide in paddy fields is widespread, aiming to enhance rice yield by directly or indirectly influencing the paddy environment. Periphytic biofilms, which form around the soil-water interface in paddy fields, are complex bio-aggregates that play an important role in nitrogen (N) cycling. This objective of this study was to investigate the effect of butachlor on the growth of periphytic biofilm and N cycling under both light and dark conditions in 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 the richness and diversity of eukaryotes within the periphytic biofilms. In terms of N loss, the light treatments exhibited higher total N loss because the light favored periphytic biofilm growth and enhanced NH4+ assimilation and nitrification. Additionally, butachlor application resulted in increased retention of NH4+-N and NO3—N and an increase in denitrification loss. The abundance of functional genes encoding enzymes related to archaea and bacterial aminomonooxygenase (AOA, AOB), nitrite reductase (nirK) and nitrous oxide reductase (nosZ) were stimulated by butachlor application, favoring nitrification and denitrification processes. Overall, the results suggested that butachlor application leads to an increase in total N loss through an elevation in denitrification loss in paddy systems, particularly in the presence of periphytic biofilms. Thus, the results may provide valuable insights into the changes induced by butachlor in periphytic biofilm growth and N cycling, and future studies can further explore its potential implications in paddy soils.