Vertical Distribution of Nitrogen Reduction Process and Its Influencing Factors at a Typical Farmland Vadose Zone-Aquifer System in Danjiang River Basin, China

It is of great significance to explore nitrate migration and transformation process in vadose zone-aquifer for the nitrate pollution control in groundwater.In this study, 12 vadose zone soil and aquifer sediment samples from different depths(0~42.5m) were collected at a typical farmland in Danjiang River Basin.Concentrations of nitrogen, antibiotic, rates of dissimilatory nitrate reduction processes, and abundance of related functional genes were analyzed.Concentrations of TOC,NO₃-N,and TP in the vadose zone soil and aquifer sediment showed significant differences.Seven of 83 investigated antibiotics were detected with concentration in the range of 0.01~115.20 and 0.01~2.50 ng/g for vadose zone soil and aquifer sediment, respectively.Besides, results showed that anaerobic ammonium oxidation(ANA) and denitrification(DEN) were the key processes for the nitrate reduction in the shallow vadose zone, with an average contribution of 53.9% and 26.4%,respectively.While the dissimilation and reduction of nitrate to ammonium(DNRA) accounted for 93.1% of total nitrate reduction rate in the deep vadose zone.In aquifer sediments, denitrification(DEN) was the most important nitrogen conversion pathway(50.5% on average).Vertical distribution of functional genes shared the same patterns as their corresponding nitrate reduction processes.Furthermore, antibiotics showed significant influence on nitrate reduction processes, in which sulfaguanidine(SGD) was positively correlated with the nitrate reduction process, while anhydroerythromycin(AETM) demonstrated significantly negative correlation.Among physiochemical parameters, only NO₃-N was positively correlated with the abundance of functional genes of DEN.This study is vital to revealing the nitrate reduction process in the vadose zone-aquifer system.