## 4 Discussion  
This study aimed to clarify the factors affecting the degradation of pharmaceuticals in Asian paddy fields, which are characterized with flooded and anaerobic condition. The completely mixed conditions at the state of slurry in this study would have promoted the degradation of the target compounds compared with the conditions in the real paddy fields. The results clearly indicated that aerobic condition promoted the degradation of the target pharmaceuticals except for NPX. Although exact half-life time was not calculated in this study, it is anticipated that most pharmaceuticals were more resistant to biological reactions in anaerobic rice paddy fields compared with the implications of a previous study under water unsaturated condition where relatively shorter half-life times 1-6 days for IBP, 11-18 days for CA, and soil-dependent half-life times (3-20 days) for NPX and DCF were reported in agricultural soils [15].  
This study also showed that the soils taken from rice paddy fields contained microorganisms which potentially degrade the target pharmaceuticals if aerobic environment is provided, because low residual ratios of the parent compounds were observed in aerobic condition even when the same soils were incubated. The degradation spectrum of rice paddy soils in aerobic condition was similar to that of aerobic wastewater treatment in that IBP and FEP (at least as parent compounds) are readily biodegradable. In addition, slow but steady degradation of DCF and CA observed in this study implies that the microbial community of rice paddy fields would have higher diversity compared  
Figure 6: Effect of pH on the residual ratios and solid-liquid distribution of the pharmaceuticals after 14 days of incubation under aerobic condition.  
with that of activated sludge in aerobic wastewater treatment where many studies on biological degradation have been conducted [6; 27; 28; 30; 31; 32; 35]. The decrease in the concentration of DCF and CA took place in this study without lag-time and without the acclimatization of anaerobic soil to the aerobic environments. Consequently, the higher degradation rates in aerobic condition suggest the possibility of promoted degradation of pharmaceuticals by supplying oxygen with plowing anaerobic rice fields or with drying the field during off-season for farming. Future studies are needed for the fate of transformation products in aerobic and anaerobic reactions in a real rice paddy field, because some of them are anticipated to have persistent nature [25; 28; 31; 34]. To generalize the results, the degradation experiments using pharmaceuticals other than NSAIDs by soils taken in other countries may still be needed.  
Data curation: PN.; supervision: H.T.; writing--review and editing: T.U.  
This research was partially funded by Japan Society for the Promotion of Science, category of scientific research (C), grant number 17K06622.  
The authors gratefully acknowledge [PERSON], [PERSON], and [PERSON] (Tokyo University of Technology) for their experimental works.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.