[Influence of Antecedent Dry Days on Nitrogen Removal in Bioretention Systems]

Bioretention systems have become an optimal technology during the construction of the sponge city, but its nitrogen removal performance can be affected by antecedent dry days (ADD). This study was designed to investigate the effects of different lengths of ADD (1,2,3, 5, 7, 12, and 22 d) on nitrogen removal performance using a series of laboratory-scale bioretention systems to form seven constant alternate drying-rewetting regimes. The influence mechanism was further investigated by analyzing the spatial distribution of nitrogen reductase activity and microbial community structure under different drying-rewetting regimes. The results showed that the ammonium removal efficiency was not significantly affected by ADD, while exhibiting high variation depending on the hydraulic permeability of the filler and plant growth conditions. The nitrate and total nitrogen removal efficiency decreased as the length of ADD increased form 7 d to 22 d. In addition, the spatial distribution of nitrate reductase (NaR), nitrite reductase (NiR), and hydroxylamine reductase (HyR) were affected by ADD to some extent. It was found that the soil moisture of submerged layer (SL) regulated the nitrogen processes. The nitrate dissimilatory reduction to ammonium (DNRA) can occur in the SL through secondary catalytic reduction by nitrogen reductases, thus affecting the removal of ammonium. The soil microbial community structure and its spatial distribution could be altered by ADD significantly, and the removal of multiple nitrogen species was partly affected. Thereinto, under shorter ADD values of 1, 2, 3, and 5 d, the dominant phylum was Firmicutes, a group of denitrifying microbes, and its dominant genus, Clostridium_sensu_stricto_1, also had the function of DNRA. The results of the study confirmed that ADD has a certain effect on the nitrogen removal capacity and nitrogen reductase activity, while resulting in spatial changes in the microbial community structure in the bioretention system under constant drying-rewetting conditions.